CHAPTER 3 MATERIALS AND METHODS

Transcription

1 CHAPTER 3 MATERIALS AND METHODS The thesis work has been carried out to develop the indigenous 3D Radiation Field Analyzer, dosimetric and commissioning of FFF beam and clinical use of FFF beam for complex targets. All the measurement of FFF beam was done with Varian True Beam machine available at Health Care Global Enterprise, Ahmadabad. 3.1 Radiation filed Analyzer In order to develop an indigenous RFA preliminarily the work was categorically divided into three parts such as, i) Development of water phantom, telescopic column lifting assembly, three stepper motors integration with water phantom and development of hand control for manual movements of stepper motors. ii) Development of software for beam data acquisition using Microsoft Visual basic 6 software and development of data analyzing software for obtaining data as per International/National protocols. iii) Integration of dual channel electrometer, the stepper motor Linear Motion (LM) guide assembly and the data acquisition software Design of water phantom Water phantom was made with 15 mm thick acrylic material with 800 x 750 x 570 mm size. UV pasting were used to ensure no water leakage when filled with water. The larger size of the phantom was to accommodate the LM guide and stepper motor assembly. The Figure 3.1 shows the CAD diagram of indigenous RFA in all three views. 71

2 a) Complete View b) Sagital View c) Coronal View Figure 3.1 : CAD diagram of indigenous RFA. 72

3 3.1.2 Telescopic Lifting Mechanism The Lift table is a separate water phantom carriage with electrically operated lifting mechanism for the positioning of the water phantom. The carriage has two fixed and two steerable rollers with breakers. The cabinet has one compartment and two drawers for storing accessories and is furthermore equipped with a leveling frame for fine adjustment in vertical and horizontal directions. The lifting mechanism is capable of lifting 700 kgs with stroke of 40cm and the speed set at 5 mm/sec on max load (can be increased up to 15mm) intended to avoid any jerk and wobbling of water phantom during movement. The lifting assembly is provided with a 2 button pendent for operation. The load bearing frame is fitted with four heavy duty caster wheels with locking options. The phantom lifting platform has 4 degrees of adjustment to account for the leveling of floor or phantom. The overall height was kept at 1450mm for the lifting mechanism LM guide and Ball Screw Linear motion systems are unique motion systems in which linear motion is supported by rolling contact elements. These linear motion systems can best be described as a motion system which uses the rotational motion principle of a deep grooved ball bearing. The combined LM and lead screw is known as linear actuator. A lead screw also known as a power screw is a screw designed to translate turning motion into linear motion. An electric motor was mechanically connected to rotate the lead screw. The lead screw has a continuous helical thread machined on its circumference running along the length (similar to the thread on a bolt). Threaded onto the lead screw is a lead nut or ball nut with corresponding helical threads. The nut is prevented from rotating with the lead screw (typically the nut interlocks with a non-rotating part of the actuator body). Therefore, when the lead screw is rotated, the nut will be driven along the 73

4 threads. The direction of motion of the nut will depend on the direction of rotation of the lead screw. By connecting linkages to the nut, the motion can be converted to usable linear displacement. The design and developed in this study used stepper motor and high precision stainless steel moving mechanism Stepper Motor A stepper motor is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller). Conventional AC and DC motors operate on continuously applied input voltage and most often produce a continuous (steady state) rotary motion. Most commercial type motors are single phase, equipped with two lead wires, or three lead wires, the third lead normally used as a grounding lead. Current flows from one lead wire into the motor and returns through the other lead wire. Unlike these motors, a Stepper motor (also called a stepping motor) will not produce continuous motion from a continuous input voltage. It will stay at a particular position as long as the power is on. An electrical phase change is necessary to make a stepper motor move in a particular direction. This means that no single phase step motor can exist. A typical three phase stepper motor has four lead wires. Three of them represent the three phase coils, normally colored with brown, red, and in orange. The fourth lead is a common lead for current return, commonly colored with black or in white. Turning one phase on will hold the rotor in one position (called a detent position). Turning off this phase and turning on the second phase will move the rotor to rest at another detent position. This on and off current flow is called a pulse. One pulse into the motor causes the rotor to increment (move) one precise angle. This movement is repeated with each input pulse. When properly applied and controlled, the number of output steps is always equal to the number of input pulses. The widespread acceptance of the digital approach for overall control of machine and process functions has generated a high demand for 74

5 mechanical motion devises capable of delivering a discrete incremental motion of known accuracy. The replacement of less dependable (susceptible to wear) mechanical devices, such as clutches and breaks, with stepper motors, provides considerably greater reliability and consistency. A stepper motor needs to be part of a stepper motor system. A Stepper motor system is made up of 3 basic elements often combined with some form of user interface (computer, dumb terminal, PLC), This interface system is used to send commands to the system. The Controller/Indexer is a microprocessor that generates the step pulses and direction signals for the driver. The controller typically performs many other sophisticated command functions as well. Most controllers have onboard memory so they can store thousands of commands. In this study, three stepper motor for X, Y and Z positioning were used which had the speed of 40 mm/s and accuracy of ± 0.5 mm. All three stepper motor are controlled by the control console system through driver card. The motors can also be controlled locally by the Human Machine Interface (HMI) pendent which communicates through the same driver card Three - Axis controller driver Card The Galil motion controller is designed for operation with many different types of motors. The signal conditioning circuits used to control different types of motors have been built into the controller and allow the user to interface with different motors easily.when the controller is powered on with the stepper motor plugged into the linear stage on, the stepper motor will no longer be able to move. This means that the stepper motor is energized and is ready to begin motion. Using e DMC smart terminal software, create a new program named stepper control. When using a stepper motor there are specific parameters that need to be set to accurately control the motor. One of the first parameters that should be set are type of motor been connected. Using the command KSX 2 tells the controller that a stepper motor is being used. KSX 2 is the command used to tell the controller what type of motor and 2 tells the controller it is a 75

6 stepper motor. once that the type of motor is selected, the acceleration, deceleration and speed of the motor must be defined. To set the acceleration of the motor the command AC defines the acceleration rate of the motor. The deceleration of the motor is defined using the DC command and the speed of the motor is defined using the SP command. All of the commands range in value from 1 to cycles per second with 1 being the slowest speed and being the fastest. Once these parameters are set the motor needs to be provide with the movement instruction. To move the stepper motor the command PA is used which tells the controller where to move. These values range from 0 to with 0 not moving the stepper motor at all and which will move the motor to the end of the stage. Once the motor is told where to move the motor starts its motion once the BG command is entered. An example of the sample code for movement is shown below. #MOVESTEP MTX=2; 'Set the type of motor SPX=11000; 'Set the speed of the motor ACX=10000; 'Set the acceleration of the motor DCX= ; 'Set the deceleration of the motor PAX=10000 EN 'End the program Human Machine Interface (HMI) Human machine interface(hmi) is the part of the machine that handles the Humanmachine interaction. membrane switches, rubber keypads and touch screens are examples of Human Machine Interface which we can operate by seeing and touching. It provides a graphics-based visualization of an industrial control and monitoring system. An HMI typically resides in an office-based Windows computer that communicates with a specialized computer in the plant such as a programmable 76

7 automation controller (PAC), programmable logic controller (PLC) or distributed control system (DCS). A local pendent to control to adjust the X, Y and Z movements in the LM assembly was developed using HMI. The pendent was designed with two page operation to provide all the required functions. The HMI pendent was developed with touch screen technology. The first page has the following buttons of operation (Figure 3.2 ) a) X, Y,Z forward/reverse switch b) Virtual home c) Define home d) Machine home e) Speed selection (Fast/Slow)and f) Next Button The Next button provided in the first page will take the operator to the second page, which has the following operations. a) X, Y, Z value entry for desired movements b) Start and c) Speed selection 77

8 Figure 3.2 Display of first page on the HMI pendent Dual channel Electrometer The PC Electrometer Model 1014 (Figure 3.3) is a dosimetry electrometer intended for measuring the output charge of an ion chamber or diode detector in a radiotherapy beam. The measurements obtained by the electrometer is subjected in dosimetry protocols, such as AAPM Task Group No 51 to obtained the absolute or relative results of the therapy beams. The electrometer is mounted in a small shielded case with 2 triaxial connectors (BNC or TNC, depending on the model) for connection to ion chambers or diode detectors. The ion chamber voltage bias can be adjusted to various levels at either polarity. For air density correction, there are internal temperature and pressure sensors that measure ambient conditions and an input for an external remote temperature sensor. A USB port provides power and data communication with PC software. The conductive enclosure (4cm x 10.5cm x 14cm) provides EMI shielding and LED status indicators. 78

9 The calibrated Dual channel electrometer has dose rate of 0.001pA pa, 1 fa resolution (Low Range) and 0.001nA na, 1 pa resolution (High Range). In charge mode the ranges are 0.001pC μc, 1 fc resolution (Low Range) and nc μc, 1 pc resolution (High Range). Repeatability of the electrometer is ± 0.1% (IEC requirement: ± 0.5%)(10). Three preset bias voltages of 0, ± 150, ± 300 volts can be used.the bias voltages can also be set independently to fulfill our custom needs. The electrometer was procured along with the calibration certificate. Figure 3.3: PC Electrometer Ionization chamber The ionization chambers of 0.125cc (Standard Imaging Inc. USA) is used for both reference and field measurements. The cylindrical chamber has 36mm rigid stem with inner diameter of 5.5mm. The chamber has an aluminum electrode with 1mm diameter and 5mm long. The chambers are energy independent from 30KeV to 50 MeV range. For both the chambers the calibration certificates were obtained. Figure 3.4 A18 Model cylindrical chamber (0.125cc) 79

10 The Model A18 chamber features exceptional spatial resolution for relative dosimetry scanning, and is capable of measuring small field sizes. Calibration caps constructed with the same material as the major elements of the chamber were available to provide more than adequate build-up for Cobalt-60 radiation. Its waterproof construction makes it ideal for use in water phantoms. The chamber vents through a flexible tube that surrounds the triaxial cable, ensuring the collecting volume is in pressure equilibrium with the surroundings. Features and Benefits Proven guard design yields stable, precise measurements and minimizes settling time by creating uniform field lines Shell, collector, and guard are all made of durable, long lasting Shonka conductive plastic Use of homogeneous material throughout the chamber minimizes perturbation of the beam due to the presence of the chamber and optimizes measurements Axially symmetric design of the chamber provides an uniform, isotropic response Inherent waterproof construction eliminates need for additional protective coverings Non-removable one piece stem for easy, precise positioning A matching 2.0 mm thick Cobalt-60 build-up cap of C552 Shonka airequivalent plastic is provided for air calibrations and measurements Ionization collection efficiency is always 99.9% or better Collecting volume is cc Beam Analysis Software The software for radiation beam analyzer was developed in the Microsoft Visual basic 6.0 platform. With Visual Basic 6, user will find it easier to capture and analyze 80

11 information to help them make effective decisions. Visual Basic 6 enables user of every size to rapidly create more secure, manageable, and reliable applications. The dosimetry common object module (COM) was developed to support the RFA. The dosimetry COM object was developed support the following threads and objects: Each COM object is associated with one and only one apartment. This is decided at the time the object is created at runtime. After this initial setup, the object remains in that apartment throughout its lifetime. A COM thread (i.e., a thread in which COM objects are created or COM method calls are made) is also associated with an apartment. Like COM objects, the apartment with which a thread is associated is also decided at initialization time. Each COM thread also remains in its designated apartment until it terminates. Threads and objects which belong to the same apartment are said to follow the same thread access rules. Method calls which are made inside the same apartment are performed directly without any assistance from COM. Threads and objects from different apartments are said to play by different thread access rules. Method calls made across apartments are achieved via marshalling. This requires the use of proxies and stubs. The Graph com object should designed to handle the demanding requirements of server side usage. It multi-threaded design handles multiple concurrent requests quickly and efficiently Linear and Log scales with full font support Up to 8000 profiles in one chart Up to 1,000,000 samples per profile Y scale stacking X and Y value for each data point Built-in zoom, scrollbars and pan Crosshairs and coordinates display 81

12 Boolean profiles and scales Stair Mode profiles Pass data point-by-point Pass entire arrays directly for best performance with a lot of data Read/edit/set any value in the data array Auto and manual scale modes for all scales Manually set each scale's major tick interval Set toolbar tool tips for multiple languages Date Time formatting down to ms Units support for each scale Grid for each scale Pan individual scales when no scrollbars Legend with scrolling when needed Coordinates available programmatically Null data points for breaks in a profile Global number and date time formatting Full documentation help and sample code Save chart image to bitmap or metafile Copy chart image to clipboard Export chart data to tab delimited text file Use your own toolbar Extensive property pages Flickerless display, even when resizing Error log file for detailed debugging Different software modules required by the RFA system that were developed are i. Beam Scanning module ii. Beam data analysis 1. Import other vendor file module 82

13 2. Protocol library module 3. Result display module Beam Scanning Module Beam scanning module controls the stepper motor, LM guide mechanism and the dual channel electrometer. Before collecting the radiation beam data, the electrometer can be reset for residual charges by choosing the zeroing option. The option to set the bias voltage is also provided in the window (0,-300,+300 etc,). Before the commencement of actual data collection, it is mandatory to feed the type of radiation unit, energy, field size, SSD, wedge angle, gantry angle, collimator angle etc. to the developed RFA software. The screen shot indicating these options are shown in Figure 3.5. In the same window, the provision for direct selection of PDD, In plane profile, Cross plane profile, Diagonal (Left-Right) profile and Diagonal (Right-Left) profiles can be obtained for multiple depths and multiple field sizes. All the profiles are displayed in the same live window. Additionally an option is provided to select the PDD scan direction (Top-Bottom or Bottom-Top). 83

14 Figure 3.5 A typical beam scanning module screen Before obtaining the actual profile, the detector will get aligned to the preset home position (0, 0, 0 coordinates) and then the collection of data occurs for profile. The system is programmed in such a way that on completion of each scan the system will automatically normalize the maximum reading to 100%. While collecting the in-plane or cross-plane profiles the detector will move to collect beam data by calculating the scan starting point from the fed field size and the penumbral margin information. The software is developed in such a way that the detector checks for the shortest distance from its current position to start the scan. Beam scan software has the ability to set the speed of chamber, step size and chamber measurement time and also the developed software provides the option to control or 84

15 move the chamber in X, Y, and Z direction. The screen shot of the developed software with all the above said information is provided in Figure 3.6. Figure 3.6 A typical beam scanning option module screen Beam data analysis Import other vendor file module The scan data of other commercially available RFA can be imported for analyzing in the developed software. When the scan files of other vendors are imported the data is programmed to convert those files into file extension format of the developed software which is.rba (Radiation Beam Analyser) attached to each file. This module provides option to check the scanned data of other vendors with developed software and vice-versa Protocol library Figure 3.7: A typical option menu screen 85

16 The variable option available to view the profile. Continuous beam option will provide the value of X and Y in the graph continuously and discrete option will show only the measured X and Y value. The screen shot of the developed menu option provided is shown in figure 3.7. Various standard protocol libraries were made available in the developed software for the convenience of the users such as AERB, IAEA, DIN, TG 45 etc. A typical screen indicating the selection options is presented in figure 3.8. Figure 3.8 A typical protocol selection screen Analysis parameters for profiles Beam Flatenss : The beam flatness is assessed by finding the maximum D max and minimum D min dose point values on the beam profile within the central 80% of the beam width and then using the following relationship: 86

17 a) Percentage dose difference to IEC (2007) (Eq.3.1) b) Percentage Dose Ratio ( PTW RFA User manual ) (Eq.3.2) c) Filed Ratio (PTW RFA User manual ) (Eq.3.3) d) Maximum Variation (PTW RFA User manual ) (Eq.3.4) Standard linac specifications generally require that flatness to be less than 3% when measured in a water phantom at a depth of 10 cm with an SSD of 100 cm. Beam Symmetry : The beam symmetry is usually determined at D max, which represents the most sensitive depth for assessment of this beam uniformity parameter. A typical symmetry specification is that any two dose points on a beam profile, equidistant from the central axis point, are within 2% of each other (Kutcher et.al., 1994 ). Alternately, areas under the D max beam profile on each side (left and right) of the central axis extending to the 87

18 50% dose level (normalized to 100% at the central axis point) were also determined as symmetry. Some of the definitions of symmetry are a) Maximum Dose Ratio to IEC60976 ( 2007) (Eq.3.5) D(x) is the dose at point x; x and -x are points within the flattened region, symmetrical to the central axis. Symmetry is defined as being the maximum ratio within the flattened region, multiplied with 100. b) Area Ratio (PTW RFA User manual ) (Eq.3.6) Where, a = area to the left of the central axis b = area to the right of the central axis The areas are delimited by the central axis and the 50% field limit. c) Maximum variation (PTW RFA User manual ) (Eq.3.7) D(x) is the dose at point x; x and -x are points within the flattened region, symmetrical to the central axis. Field Size at SID (PTW RFA User manual ) The field size at isocenter determined by 88

19 (Eq.3.8) Where, SID = Source Isocenter Distance in cm F = Field Size for 50% dose in cm SSD = Source Surface Distance in cm D = Profile depth in cm Analysis Parameters for Photon Depth Dose Curves Quality Index ( QI) The quality index is a measure of the quality of photon radiation. It is derived from the dose values D 100 and D 200 at depths of 100 mm and 200 mm. In the Protocol Parameters has the option to choose a calculation method: a) (PTW RFA User manual ) (Eq.3.9) b) Depth Dose Curves to DIN (1997) (Eq.3.10) c) Depth Dose Curves IAEA TRS 398 (2009) (Eq.3.11) 89

20 Analysis parameters for electron depth dose curves : R Depth of the maximum dose Practical range (R p ) : Depth at the intersection of X-ray background line and best fit line on the steep sloping section of the curve. Depth of the 50% dose ( R 50 ) : In the Protocol Parameters dialog has the option to choose a calculation method: R 50 = I 50 ( Eq.3.12) R q : position of the intersection of the tangent at the inflection point of the curve and the best fit line on the steep sloping section of the curve. Percentage dose at the surface (D s ) : IEC defines the surface dose as the dose at a depth of 0.5 mm. X-Ray Bckground X-ray background as a percentage of the maximum dose In the protocol parameters dialog has option to choose a calculation method for x-ray backround : a) X-ray background is the dose of the intersection point of calculation of Rp b) X-ray background is the dose of the last measured point Most probable energy at the phantom surface (E p0 ) : In the Protocol Parameters dialog you can choose a calculation method: Dosimetry protocol to DIN (1997) E 0 (mean) < 5: E p,0 = 2.08 * R p (Eq.3.13) E 0 (mean) 5: E p,0 = * R p (Eq.3.14) Mean energy at the phantom surface E 0 (mean) : The protocol parameters dialog has the option to choose a calculation method for obtaining E 0 (Mean), 90

21 a) Dosimetry protocol to DIN (1997) for ion dose: E 0 (mean) = * R * R 50 ² for energy dose: E 0 (mean) = * R * R 50 ² b) IPEMB dosimetry protocol for ion dose: E 0 (mean) = * R * R 50 ² for energy dose: E 0 (mean) = * R * R 50 ² c) All other dosimetry protocols E 0 (mean) = 2.33 * R 50 Where, E 0 (mean) in MeV R 50 in cm (Eq.3.15) (Eq.3.16) (Eq.3.17) (Eq.3.18) (Eq.3.19) Figure 3.9 Graphical representation of parameters. The figure 3.9 shows that the graphical representation of all electron parameter. 91

22 Result display module The result obtained by the developed software will be displayed as per the parameters selected by the uses. In single screen, the option of opening multiple profiles or PDD s, Stretching/Skewing of windows were also made available in the developed software. The displayed results may be converted in to ASCII or any other file format as required by the any commercial available TPS. A typical 6 MV photon PDD for 10 x 10 cm 2 field size obtained at 100 SSD with the display of results is shown in Figure

23 Figure 3.10 Result display screen along with the measured depth dose profile for 6MV photon beam for 10 x 10 cm Communication Configuration The communication link of the developed 3D Radiation Field Analyser were designed as per the conceptual diagram illustrated in Figure3.11. COMPUTER CONSOLE BNC cable Figure 3.11 Conceptual diagram of the developed 3D Radiation Field Analyzer The communication ports of various electronic devices that are required to form the full 3DRFA assembly posed a major challenge while undertaking the task. The ionization chamber had BNC type connector (normally available type of connector), while the electrometer had TNC type of input. The electrometer had Type A USB port as output and Type B USB port for input. The 3D motion Controller had the RS232 output. To overcome the port problem a special device called USB over IP was 93

24 introduced in the electronics which will convert the electrometer output to RS232 output. This RS 232 output and the output from the 3D motion controller are connected to an Ethernet Hub, through which the control console system communicates with both the electrometer and motion controller uninterruptedly. The completed RFA system indigenously developed for the study purpose is show in Figure Figure 3.12 The developed 3D Radiation Field Analyzer. 94

25 Absolute dosimetery software In addition to development of software for profile and depth dose measurement an attempt was made to develop exclusive software for absolute measurements using the available dual channel electrometer. Absolute dosimetry is a technique that yields information directly on absorbed dose in Gy. This absolute dosimetric measurement is also referred to as calibration of linear accelerator( 1MU=1cGy at D max ). All further measurements in a typical machine for specified energy. The software was developed to perform calculation based on the TRS-398 protocol. The Figure 3.13 show the software screen shot of absolute measurements software. 95

26 Figure 3.13 Screen shows the absolute dosimetery software Premeasurement checks The premeasruement checks were performed on the developed RFA system as per the Task Group 106 recommendations (Das et.al., 2008). The noise, signal to noise ratio, leakage, effect of polarity, effect of sampling time, effect of dose rate, effect of step size were measured as per TG 106 recommendation Validation of the 3DRFA The premeasurement checks were performed on the developed RFA system as per TG 106 recommendations. The physical parameters of photon PDDs such as D max, D 10, D 20 and Quality Index, and the electron PDDs such as R 50, R p, E 0, E po and X-ray contamination values can be obtained instantaneously by using the developed RFA system. Also the results for profile data such as field size, Central axis deviation, Penumbra, flatness and symmetry calculated according to various protocols can be obtained for both photon and electron beams. The result of PDDs for photon beams were compared with BJR25 (Jorden et al., 1996) supplement values and the profile data were compared with TG 40 (Kutcher et al., 1994 ) recommendation FFF beam analysis using developed software The characteristic of FFF beams were analyzed using developed RFA as per AERB protocol. The symmetry of the beam is measured as per the IEC60976 (2007) as described in Section The stability of the beam FFF beams, lateral distance from central axis at 90%, 75% and 60% dose points on either side of the beam profile were measured using the developed RFA. The profile scan was acquired for 20x20 cm 2 FS, SCD 100 cm at 10 cm depth. To find the inflection point, the measured data 96

27 exported to Microsoft excel file format and determined the field size and penumbra for FFF as described in Section Commissioning and QA of FFF beam The commissioning of FFF beams has been carried out for the first time in country. The irradiation facilities and instruments used to acquire the beam data presented in chapters 4, 5, 6 are briefed in this chapter. The following dosimetric equipments were used in this study. 1. FFF linear accelerator (True Beam, Varian inc) 2. Radiation Filed Analyzer (IBA and indigenous 3. Electrometer with ion chambers ( Unidos, PTW) 4. 2D Array detection (Octavius PTW) A brief description of the above equipments is given below, TRUE BEAM LINEAR ACCELERATOR TrueBeam is the new model of medical linear accelerator from Varian Medical Systems,USA, which was cleared by the FDA in December There are two varieties available with this linac TrueBeam and TrueBeam STx. The STx model is intended for stereotactic use in addation to routinue usage of True Beam and has HD MLC. A new feature for TrueBeam is the availability of the FFF mode. The photon beams usually have a metal filter on their path to make photon fluence uniform (or flat). With proliferation of accurate treatment planning algorithms and IMRT the uniform fluence is not a concern, hence assumed to be advantageous with FFF mode. When the filter is removed from the photon beam, the intensity increases by a factor of 2 for 6 MV photons and by a factor of 4 for 10 MV photons. A typical True beam machine is shown in the Figure

28 Figure 3.14 True Beam Machine (Varian Medical Systems) IBA Radiation Filed Analyzer Radiation field analyzer (RFA-200 Scanditronix) used to measure FFF beam parameters contains water phantom, lifting table, Main Control Unit (MCU) with integrated dual channel electrometer, water reservoir and TMR set. The positional reproducibility is ± 0.1 mm and the positional accuracy is ± 0.5 mm. The approximate volume of the water phantom is 206 litres. The water phantom has the Mylar foil window of thickness 0.1 mm for lateral beam scanning. The detector holder material is made up of Polyvinylidene fluoride (PVDF) which is near to air density. Water phantom is placed on motorized double telescope lift table mechanism. The maximum table load capacity is 250 kg. The vertical travel range is mm from the finished floor level. The leveling table plate thickness is 19 mm. The vertical range for leveling frame adjustment is 20 mm and horizontal adjustment is 15 mm. The MCU has the dimension of 390 x 75 x 360 mm 3. The operating polarizing voltage is 98

29 between -400 V to +400 V. The time constant is 40 ms and it contains 14 bit Analog to Digital Converter (ADC) for optimized gain control. The maximum resolution is 0.1 pc with low range and 30 pc with high range. The leakage current is <0.5 pa with low range and < 2 pa with high range. The MCU can communicate to computer through RS232 connectivity. The RK chamber used for measurement has a measuring volume of 0.12 cc with outer diameter of 7 mm with an air cavity length of 10 mm and central electrode diameter of 1 mm. The outer wall is made up of Poly Methyl Methacrylate (PMMA) with thickness of 0.12 g / cm 2 and inner wall is graphite / epoxy with the thickness of 0.07 g / cm 2. The RK ion chamber detector used for measurements is shown in Figure Figure 3.15: RK Chamber (0.12cc) used for measurement PTW UNIDOS E electrometer with ion chambers The UNIDOS is high-precision reference class electrometer that significantly fulfills the recommendations of International Electrotechnical Commission (IEC 60731). The UNIDOS E electrometer used in our study is shown Figure

30 Figure 3.16: UNIDOS E electrometer The electrometer has a large and high contrast graphic electro luminescent display with wide viewing angle for complete and comprehensive display for all measured values, selected chamber and correction factors in the main screen. It is capable of displaying the measured dose, dose rate, current, charge, average rate and dose per monitor unit are all measured and displayed simultaneously. The maximum operating bias voltage is ± 400 V, programmable in steps of ±50 V. The mains operating power supply is V, 50 / 60 Hz, the battery operational is optional. UNIDOS E can be connected to personal computer through bidirectional Recommended Standard 232 (RS-232) port. The leakage through electrometer is < ±1 fa. The electrometer has a linearity of < ±0.5% in the whole range. The FC65-G Farmer chamber is a water proof vented ion chamber suitable for electron and photon beam dosimetry. It s sensitive volume is 0.6 cc. The outer electrode wall material is graphite and the inner electrode is made up of aluminum. The recommended polarizing voltage is +400 volt and the leakage < 4 x A. 100

31 D Array detector (PTW Octavius system) The PTW seven29 2D-Array consists of 729 vented plane-parallel ionization chambers with a 0.6 g/cm 2 graphite wall arranged in a 27 x 27 matrix covering an area of 27 x 27 cm 2. The 2D array detector used in our study is shown in figure Figure 3.17: Schematic view of PTW Seven29 2D array Each single chamber is air-filled with a cross section of 5 mm x 5 mm and height of 5 mm. The chambers are separated from each other by 5 mm. The distance between the centers of adjacent chambers is 10 mm. The 2D array surrounding material is made up of polymethyl methycrylate (PMMA). Figure 3.18 PTW Seven29 2D- ARRAY inserted in a Octavious phantom. 101

32 The measuring system consists of the chamber array itself, which also accommodates part of the electronic devices, the array interface, and a data acquisition board for the personal computer. A dedicated phantom for the QA of rotational treatments focusing primarily on the use of the Seven29 2D ion chamber array, called Octavius was used during measurements. Octavius is made of polystyrene (physical density 1.04 g/cm 3, relative electron density of 1.00), and is 32 cm wide and has a length of 32 cm. A 30 x 30 x 2.2 cm 3 central cavity allows the user to insert the 2D ion chamber array into the phantom as in figure The position of the cavity is such that when the 2D array is inserted, the plane through the middle of the ion chambers goes through the center of the phantom. The measurement ranges for 2D array as specified by manufacturer are 200 mgy 1000 Gy and 500 mgy min -1 to 8 Gy min -1. The 2D array is calibrated for absolute dosimetry in a Co - 60 photon beam. An on-site cross calibration factor correcting for the quality of the beam can be measured and used by the detector acquisition software. The 2D array was calibrated using a cross-calibration procedure. In this procedure a known dose was delivered and the response of the central detector was used to calculate a cross-calibration factor. This factor was applied to the entire matrix. For planar measurements, the 2D array was set up at an effective depth of 5 cm in solid water (Gammex Inc., Middleton, WI), and with 10 cm solid water backscatter Beam quality specification Beam quality specification parameters is considered important in determining quality of a particular beam and are machine specific. Both based on both air kerma standards and absorbed dose to water standards, have recommended the Tissue-Phantom Ratio (TPR 20,10 ), as a specifier of the quality of a high-energy photon beam. TPR 20,10 is defined as the ratio of absorbed doses in water on the central axis of the beam at the depths of 20cm and 10cm, obtained with a constant SCD of 100cm and a 10 x10 cm 2 field size at the position of the detector. The parameter TPR 20,10 is a measure of the 102

33 effective attenuation coefficient describing the approximately exponential decrease of a photon depth-dose curve beyond the D max, and more importantly, it is independent of the electron contamination in the incident beam. The use of dose ratios for specifying photon beam quality, the nominal accelerator potential was the parameter most commonly used in photon beam dosimetry. Measured ionization charge or current or absorbed-dose ratios were first used as a beam quality index in the dosimetry recommendations of the Nordic Association of Clinical Physicists (NACP). Other beam quality specifiers have been proposed for photon beam dosimetry which are, in most cases, related to the depth of maximum absorbed dose and can, therefore, be affected by the electron contamination at this depth. In addition, the use of ionization distributions measured with thimble-type ionization chambers is problematic, as the displacement of phantom material by the detector has to be taken into account to convert ionization into dose distributions. This is avoided if planeparallel ionization chambers are used, but these are not commonly used in photon beam dosimetry Quality Index The purpose of measuring quality index is to ensure that radiation energy has not changed significantly. By measuring tissue-phantom ratio (TPR) it is possible to assess the photon beam quality. Three exposures (100 MU each) are made with gantry angle 0, SCD 100 cm and FS 10 x 10 cm 2 using a calibrated 0.6cc ionization chamber positioned at the isocentre at depths of 10 and 20 cm in a water phantom. The ionization ratio at the depth of 20cm to that of 10cm is known as quality index. The quality index is dependent on beam energy and it increases linearly with photon beam energy. The measurement was done for both FF and FFF photons (6 and 10MV). 103

34 Percentage Depth Dose at 10 cm (%DD 10 ) The percentage depth dose value at 10cm in a 10x10cm 2 photon beam with a Source to Surface Distance (SSD ) of 100cm, % DD 10 is considered as one of the beam quality indicator and is endorsed in absolute dose measurement in AAPM TG51 protocol. In this study, we have analyzed %DD 10 for both FF and FFF of 6 and 10MV photon beams using 0.6cc cylindrical chamber Depth of dose maximum (D max ) Depth of dose maximum (D max ) depends on the beam energy and beam field size. Nominal values for D max ranges from 0 to 5 cm (orthovoltage Xray beams to 25MV photon beams). This study presents D max for a reference FS 10x10cm 2 at SSD 100cm, for both FF and FFF with 6 and 10MV photon beams Beam characteristic When defining reference parameters of profiles in photon fields, such as flatness, symmetry, stability and penumbra is an important parameter of influence since the dose distribution is affected by lateral variations in the beam quality. In megavoltage photon beams, most of these parameters are commonly specified at 10 cm depth, corresponding to a typical treatment depth. Moreover, FF are optimized for this depth and, when measuring profiles at shallow depth, e.g., D max, profiles typically exhibit horns and beam flatness is suboptimal. Without the FF, the lateral dose profiles differ significantly from the typical flat profiles from conventional linacs that have a FF. The peak in the profile intuitively associated with FFF beams is pronounced only for medium to large field sizes and depends on photon beam energy. The higher the energy, the more pronounced the peak. In order to quantify the magnitude of the peak of non-flat profiles, the lateral distance from central axis at 90%, 75% and 60% dose 104

35 points on either side of the beam profile were recorded. Penumbra requires some special consideration because the conventional definition based on 80% 20% dose values cannot be applied to FFF beams. In our study, the penumbra is determined using the inflection point of the profile. For the determination of symmetry, stability of the beam, field size and penumbra, dose profiles were scanned for a field size (FS) of 20x20 cm 2, 10 cm depth and SSD of 100 cm Output Factor ( S c,p ) The output factor is defined as the ratio of the absorbed dose for the used collimator setting to the absorbed dose for the reference or normalization field size for the same MU. S c,p were measured with a PTW ionization chamber (0.125cc) and RFA water phantom. The field size measured from 5 X 5 cm 2 to 40 X 40 cm 2 and the value were normalized to the 10 x10 cm 2. The measurement was done for both FF and FFF with 100 cm SSD at D max Scatter Factor ( S c ) The collimator scatter factor (S c ) is defined as the ratio of the output in air for a given field to that for a reference field (e.g., 10 10cm 2 ). S c may be measured with an ion chamber with a buildup cap of size large enough to provide maximum dose buildup for the given energy beam. S c were determined with a PTW ionization chamber (type 31003, volume cc) and an in-house polystyrene mini-phantom. Scatter factors for field sizes (5 X 5 cm 2 to 40 X 40 cm 2 ) were normalized to the 10 X 10 cm 2 reference field for FF and FFF beams. The measurement was done for both FF and FFF with 100cm SCD at 10cm depth for different field sizes. 105

36 Surface dose The skin dose associated with radiotherapy may be of interest for clinical evaluation or useful in investigating the risk of late effects. The skin is at risk during radiotherapy for such effects as erythema, desquamation, and necrosis. The surface dose measured for collimator settings of 10x10cm 2 and 20x20cm 2 and compared with the corresponding nominal flat beam energy. The measurement done using PTW parallel plate chamber (PPC40) at 0.5 mm depth with SSD at 100 cm Symmetry of the beam Symmetry is defined as the maximum ratio within the flattened region, multiplied with 100 (IEC (2007)). The dose profile was acquired for a field size 20x20cm 2 at 100 SCD, 10 cm depth using IBA RFA The measurements were repeated over a period of 9 months to find its stability Stability of the beam To quantify the stability of FFF beams, lateral distance from central axis at 90%, 75% and 60% dose points on either side of the beam profile were recorded. The measurements were repeated over a period of 9 months to find its consistency. The profile scan was acquired for 20x20 cm 2 FS, SCD 100 cm at 10 cm depth. The graphical representation quantifying the stability of FFF beam is shown figure

37 Figure 3.19 : Diagram showing lateral distances at 90%, 75% and 60% dose points on the beam profile Field Size FFF high energy photon beams have radial intensity distribution with high intensity in the center and progressively falling pattern towards the edge. This is due to the forward moving nature of high energy photons. In general, field size for FF photon beams are defined at 50% of intensity level along the central axis. In FFF beam the 107

38 50% intensity level occurs at high gradient region (sharply descending part) of the beam profile. Field size for FFF beams does not follow standard definition similar to FF beams. The geometrical field size was defined by collimator setting and radiation field size was defined through lateral separation between inflection points (IP) along the central axis. IP is a point, where the progression of dose deposition changes its direction geometrically from positive to negative or vice versa. In this study, a simple physical concept for obtaining IP of FFF beams is proposed. IP calculated with the new concept was compared with Akima spline interpolation method. The measurement profile were acquired for collimator field size of 20x20 cm 2 with 100cm SCD at 10 cm depth. A scatter plot in Microsoft Excel sheet was created with acquired profile as shown in Figure In this method, normalized profile is plotted against off-axis distances to determine the inflection point. Figure 3.20: Graphical representation of the starting point (S) and end point (E). 108

39 The location of starting point (S) and ending point (E) of high gradient region of the beam profile is described (Figure 3.20). The separation between S and E is the height (h) of the high gradient region of the beam profile. The mid position of the IP is located at h/2 from either location (S or E). To verify our approximation method with mathematical inflection point, we used Akima Spline Interpolation (ASI) method (InterReg Kroll software, Germany version 2.2.0). It is a special spline which is stable to the outliers. In this study, cubic splines method was not used since it can oscillate in the neighborhood of an outlier. Figure 3.21 shows the advantage of ASI method. Figure 3.21: Comparison of Akima Spline and Cubit spline interpolation 109

40 The cubic spline with boundary conditions is indicated in green-color. On the intervals which are next to the outlier, the spline noticeably deviates from the given function - because of the outlier. ASI is indicated in red-color. Compared to the cubic spline, the ASI is less affected by the outliers. The dose profile (20x20 cm 2 FS, 100 SAD, 10 cm depth) was imported to InterReg software to determine the inflection point. Field size for a FFF photon beam is the lateral distance between left and right inflection points. The graphical representation of inflection point determined by the InterReg software shown in figure Figure 3.22 : Inflection point determined by the InterReg software 110

41 Beam Penumbra Penumbra requires some special consideration because the conventional definition based on 80% 20% dose values cannot be applied to FFF beams. In our study, the penumbra determined from inflection point of the profile. To determine penumbra, dose value at IP (Mathematical method and manual approximation method) was taken as Reference Dose Value (RDV). Points P a and P b located at 1.6 and 0.4 times of RDV respectively were identified. The separation between P a and P b on either side of the profile were measured as the penumbra. The penumbra was indicated along central axis for 6FFF. The measurement profile acquired for field size 20x20cm 2 at 100cm SAD at 10 cm depth. The graphical representation of determination of penumbra is shown in figure Figure 3.23: Determination of Penumbra. 111

42 3.2.7 Absolute dosimetry The International Atomic Energy Agency (IAEA) is an agency in the United Nations family concerned with the nuclear field. It strives to monitor nuclear technologies and promote nuclear safety. One of these recommendations is the Technical Report Series #398 ( TRS ) that is concerned with dose determination based on absorbed dose to water. According to IAEA TRS-398, the absorbed dose to water measured with an ionization chamber is Dw,Q 0 = MQ0 ND,w,Q0 (Eq.3.20) In the above formula,m Q0 is the reading of the dosimeter at a standards laboratory, and N D,w,Q0 is the calibration coefficient obtained for the ionization chamber with respect to the dosimeter at a standard laboratory at reference beam quality Q 0. The calibration factor is determined in units of μgy/nc. However, equation is only true if conditions are identical to the conditions at the standards laboratory. This is generally not the case, which means that some corrections have to be done to compensate for the differences. The IAEA has a set of factors for the calibration of ionization chambers. Beam quality factor k Q,Q0 : The beam quality factor k Q,Q0 is a factor to correct for difference in the ionization chamber response to the beam quality factor of the user beam Q and reference beam Q 0. It is defined as (Eq.3.21) 112

43 For high energy photons with a beam quality Q,K Q,Q0 is specified by the Tissue- Phantom Ratio TPR 20,10 which can be found using the below equation (Eq.3.22) where the beam energy E and field size S are held constant. Using a 10 x 10 cm 2 field size and a constant SAD of 100 cm, the absorbed dose is measured at a depth of 10 g/cm 2 and then at a depth of 20 g/cm 2. From this calculated value of the TPR 20,10 a corresponding beam quality factor K Q,Q0 can be looked up in the table available in the report. Typically the value for K Q,Q0 ranges from 0.96 to Atmospheric factor K TP : The gas in the ionization chamber is subject to change with varying temperature and pressure. Therefore an atmospheric factor K TP, given by below equation, has to be taken into account if atmospheric conditions are different from the conditions at the time of calibration. (Eq.3.23) where P and T are the pressure and temperature in the chamber at the time of measurement, while T 0 and P 0 are the reference conditions of the chamber. The reference conditions are T 0 = 20 0 C and P 0 = kpa. 113

44 Ion recombination factor k s : Some of the ion pairs created inside the chamber volume recombine before they are registered by the electrodes. The ion recombination factor accounts for this loss (Eq.3.24) Where M 1 and M 2 are the electrometer readouts at V 1 and V 2 voltage respectively. Constants a 0, a 1 and a 2 depend on the value of V 1, V 2 and are retrieved from a table provided in the report as calculated by Weinhous et al. (1984). Typical values of K s range from The ion recombination correction factor, K s, is defined to account for incomplete collection of charges and it is a function of dose per pulse in a linear accelerator. Dose per pulse in the unit of monitor units per pulse (MU/pulse) is dose rate (MU/min) divided by pulse rate (pulse/min). Since pulse rate of the linear accelerator for the same nominal energy does not change, Pion becomes a function of the dose rate of the photon beams. For the FFF X-rays, dose rate increases substantially and hence K s of the FFF photons may be different from the conventional flattened photons. Therefore, ion recombination in the Farmer, PinPoint, or parallel-plate ion chambers may vary in the FFF beams. The purpose of this study is to evaluate the ion recombination for typical thimble and plane-parallel chambers in the FFF photon radiation to facilitate the quality assurance procedure and accurate dose calibrations for the FFF X-rays. Polarity factor K p : This effect is mostly negligible for photon beams, but can b e prominent for charged particle beams. It is given by (Eq.3.25) 114

45 Where M+ and M- are electrometer readings obtained at positive and negative polarity, respectively. M is the electrometer reading at the routinely used polarity. The polarity correction was not taken into account for the photon measurements in this thesis as its effect is assumed to be negligible in photon beams. In our study, Evaluation of effect for the Farmer, PinPoint, or parallel-plate ion chambers. Perturbation factors : The ionization chamber perturbs the beam due to the cavity inside the chamber, the chamber wall and the waterproof sleeve. For high energy photon beams in a water phantom, all of these effects are assumed to b e accounted for in the kq,q 0 factor Beam modeling with AAA for FFF Before a treatment unit can be put in use, it s beam characteristics need to be modeled accurately in the TPS so that the beam can be reproduced for treatment planning purposes. The beam model should be general enough to cover all relevant field shaping techniques and treatment methods, such as 3D-CRT and IMRT. It may also allow for the possibility to use different dose calculation algorithms, e.g. pencil kernels and point kernels, or even direct Monte Carlo simulation. For the eclipse planning system the beam data, including profiles, depth dose curves and output factors in air and water are collected and modeled for FFF beam.in theory, the dosimetric characteristic of unflattened beams facilitates dose calculation and might improve the dose calculation accuracy of advanced algorithms. IMRT, using aperture based segments with subsequent segment weighting, will benefit from the rather flat output factor variation with field size. 115

46 Analytical Anisotropic Algorithm ( AAA ) The AAA is also convolution based, with the dose from each pencil beam (beamlet) being calculated through a convolution. The beamlet energy fluence is separated into components from primary photons, extrafocal photons, and contaminant electrons originating mainly in the flattening filter, ion chamber, collimating jaws, and air. The dose contribution Dβ(x,y,z) from beamlet β is modeled through convolution of its fluence Φ and energy deposition density function I(z,ρ) with scatter kernel K(x,y,z,ρ), that defines the lateral dose scattering in the phantom: (Eq.3.27) Each contributing function (fluence, energy deposition density function, and scatter kernel) is defined separately for each of the energy fluence components. Functions representing the energy fluence components and the primary and scatter kernels are expressed analytically, and the convolution integral over the beamlet dimensions has also been solved analytically.hence the algorithm is termed "analytical." The feature of the AAA that distinguishes it from the PBC is that the scatter kernels are density dependent and are evaluated in multiple directions laterally from the beamlet. In addition, the photon scatter is convolved with a density-scaled kernel along the beamlet direction to more accurately reproduce the dose at the border of heterogeneities. The total dose D(x,y,z) deposited at a point by a therapeutic beam is calculated as superposition of beamlet contributions D(x,y,z)β Measurements data The algorithm was commissioned for clinical use of 6,10,15 MV photon beams and for 6FFF and 10FFF beams. Commissioning was done by measuring beam data as required by Eclipse treatment planning system for AAA dose calculation algorithm. 116

47 Machine specific beam data at reference condition were acquired using RFA and other dosimetric detectors as recommended. Beam data requirements includes PDD, open beam profiles along cross plane for different field sizes. Smallest field size used for measurement is 3x3 cm 2 and the largest being 40x40cm 2. Moreover, output factors were also measured by placing the detector at central axis at reference depth. Additionally, Diagonal profile for the maximum field size 40x40cm 2 were also acquired. PDD are measured along central axis up to the depth of 30 cm in water. Open beam profiles are acquired at five different depths ( D max, 5,10,20 and 30cm ) and the scan limit is 35mm beyond 50% dose laterally. Output factors are measured in SSD 95cm with detector placed at 5cm depth in water deeper than the electron contamination range. Reference dose in Gy for reference MU at the calibration depth at 10x10 cm 2 were measured. Measurements of profile & PDD for Add-on materials includes MLC transmission factor for each photon energy, dosimetric leaf gap for modeling of the rounded leaf edge transmission were also performed Data Analysis The analysis of was performed over five regions, as shown in figure These regions are defined below. Build down (δ1): dose deviation on central axis beyond the depth of maximum dose (D max ) Build-up and penumbra (δ2): dose deviation on central axis before the depth of D max and in penumbra (where dose gradient is larger than 3% per mm) Off-axis (δ3): dose deviation in the inner field at off-axis points and beyond D max 117

48 Tail (δ4): dose deviation in region outside the geometrical beam edges (less than 7% of the central axis dose) The percentage difference between measurements and calculations for profile and PDD is given by (Eq.3.28) where D calc and D meas are the locally calculated and measured doses (absolute for depth dose curves and relative for profiles). In region δ4, the deviation was determined with respect to central axis dose as follows, (Eq.3.29) Figure 3.24 : Data analysis for Beam modeling parameters 118

49 3.2.9 Patients specific QA After patients plan is approved, The plan needs to be verified. This is usually done through either a composite plan through a field related approach. The patients plan was transferred to PTW Versoft. The PTW octavius phantom with seven29 array dector setup as per PTW manual procedure. The senven29 ionization chambers can sometimes require a warm-up time and/or dose before irradiation Dose linearity The dose linearity test was performed by irradiating the detector with 100 MU using a cm 2 field for 6 FFF and 10 FFF under reference conditions. The dose linearity was evaluated by measuring the array output for deliveries of 10,25,50,75, 100,125,150,175,200,250,300,350,400,450 and 500 MUs for 6 FFF and 10 FFF photons beams Dose rate response The dose linearity test was performed by irradiating the detector with 100 MU using a 10 10cm 2 field for 6 FFF and 10 FFF under reference conditions. The dose rate response was evaluated by measuring the array output for deliveries of 10,25,50,75, 100,125,150,175,200,250,300,350,400,450and 500 for 400 MU/min and 1400MU/min for 6FFF and 2400 MU/min for 10FFF Gamma analysis 2D Five patients were used to assess the delivery of treatment plans under typical clinical conditions. Head and neck IMRT plans using 6 FFF were created with 7 Field. Each plan was recalculated on a CT dataset of the Octavius phantom using an initial grid 119

50 spacing of 2.5 mm and then 5 mm. On import into the planning system inhomogeneity corrections were removed to ensure that no errors were introduced by variations of Hounsfield units of a static detector/phantom. Following delivery, each plan was compared to the calculated plans with a 2D global gamma criterion of 3%/3 mm with a 10% minimum threshold. 3.3 Clinical use of FFF beam in complex treatment Medulloblastoma is a fast growing tumor of the cerebellum (posterior fossa) that controls stability, posture, and complex motor functions such as verbal communication and swallowing. About 400 new patients, primarily children, were diagnosed with medulloblastoma in the US every year, slightly more often in males than in females. It is the most common brain tumor in children aged four and younger and the second most common brain tumor in children aged 5 14 years. Subsequent to surgery, medulloblastoma is usually treated with CSI. Although radiotherapy had proven successful, investigators are still looking for new ways to mitigate the potential side effects of this treatment. Treatment related late complications are usually hearing disability, declined cognition, cardiomyopathy, cataract formation, retarded growth, endocrine dysfunction, and second malignancies. Clinicians consider using techniques such as IMRT and RA that aim to converge beams of radiation directly at the tumor eventually improving the long term complications free survival. However, radiotherapy (RT) planning, delivery, and junction dose verification remain exigent for craniospinal irradiation (CSI) in medulloblastoma patients. Hence investigating the emerging new RT techniques such as FFF in IMRT and RA on the basis of dose volume parameters was encouraged to reduce the normal tissue complications. Conventional two-dimensional planning for CSI involved field shaping using bony landmarks in X-ray radiographs; later it evolved into CT simulation techniques. Geometrical field matching was generally followed in such techniques without computing any dose volume data for the tumor and normal tissues. Modified treatment 120

51 planning methods were adapted to get better tumor coverage, dose homogeneity, and conformity. The matching of cranial and spinal fields still poses a problem in adult patients with larger spinal lengths since it usually exceeds allowable maximum field size. Helical tomotherapy allows treatment to large cylindrical volumes ( cm 2 ) that was compromised with the longer BOT. It raises concerns about intrafraction motion and whole-body integral doses. When the FF was removed from the linear accelerators head, a marked increase in dose rate up to 1400 MU/min for 6MV and 2400MU/min for 10MV beams is possible. The higher dose rate could make treatment delivery more accurate, by giving the patient less time to move between setup and treatment completion. This might be particularly helpful in CSI, where the tissues are far more mobile than in the cranium. There is no dosimetric comparison between flattened and unflattened photon beams for CSI. The aim of this study is to determine the feasibility of using FFF beams in IMRT and RA for CSI in medulloblastoma patients and to dosimetrically compare it with 3DCRT, IMRT with static segments (6X SMLC), IMRT with dynamic segments (6X DMLC), Rapid Arc therapy (6X RA) with FFF IMRT (6F DMLC), and Rapid Arc therapy (6F RA). The Eclipse Planning System was used in this study for treatment planning Target contouring Patients were CT scanned from the vertex to coccyx in prone position using immobilization device (Orfit Industries n.v., Belgium) on multislice CT scanner (GE Healthcare, USA). Axial images of 3mm slice thickness were exported to Mimvista contouring station (MIMsoftware Inc, USA) where the target volumes (PTV Brain, PTV Spine) and normal structures were delineated by radiation oncologists as per the recommended guidelines. PTV spine included the entire spinal canal, including cerebrospinal extension to spinal ganglia. OARs such as eyes, thyroid, heart, lungs, esophagus, liver, and kidney were outlined in the axial CT sections. Treatment planning was performed in Eclipse (Version 11.0; Varian Associates, Palo Alto, CA, 121

52 USA) treatment planning system (TPS). It is configured for both true beam millennium 120 multileaf collimator (MLC) and Siemens ARTISTE 160 MLC treatment units. The range of patients spine length varied from cm to cm (median length: 33.4 cm). A maximum field size of cm 2 can be possible with the 120 millennium MLC and 160 MLC Artiste machines. AAA was the dose calculation algorithm used for inverse optimization. We used the CT data set of five randomly selected medulloblastoma patients (median age:10 yrs), previously treated with conventional IMRT were used for this retrospective study Dose Planning Conventional 3DCRT plan, 6X_IMRT, 6F_IMRT, 6X_RA, and 6F_RA were iterated which resulted in six plans for each patient. The total dose prescribed was 28.80Gy in 16 fractions with 1.8Gy per fraction. An evaluation criterion of 98% of the PTV receiving 100% of the prescription dose and 107% maximum dose was followed as per our institution protocol. Normal tissue sparing was considered as important as the tumor coverage Conformal Photon Beams (3DCRT) The 3DCRT for CSI comprised three separate treatment plans such as 3d_Brain, 3d_Spine1, and 3d_Spine2. For the whole brain irradiation, 6MV photon beam was collimated in such a way that the spine field s divergence can be easily matched. Spine 1 comprised the region between 2 nd cervical vertebra, 10 th thoracic vertebra and whereas spine 2 was between 11 th thoracic vertebra and 5 th lumbar vertebra. Spinal cord treatments were planned with two oblique beam portals 330 and 30. The 25 enhanced dynamic wedges were used to avoid high-dose regions falling beneath the skin and to improve dose coverage at larger depths. For the three plans, depth from 122

53 skin where the maximum possible coverage achieved was taken as the reference point for dose normalization. Plans were summed up in evaluation mode of the TPS to analyze the junction dose. The sagittal view of the 3DCRT beam arrangement is shown in Figure Figure 3.25 : Beam arrangements for 3DCRT, IMRT, and RA Intensity Modulated Radiation Therapy (IMRT) Planning IMRT confines the radiation dose more precisely to target alone. This is achieved by modulating or controlling the radiation beam intensity in multiple beamlets. It also allows higher radiation doses to be focused on regions within the tumor while minimizing the dose to surrounding OARs. IMRT delivery methods using conventional MLCs can be realized in several ways: (1) step-and-shoot static IMRT using multiple MLC shapes and (2) dynamic IMRT with fixed gantry and moving MLC leaves. For CSI, jagged junction or intensity feathering technique was used to plan IMRT and RA plans. In this technique, 6MV photon beams with same 123