ATTENTION! The whole content of the lecture with all the animations can be dowloaded from:

Transcription

1 ATTENTION! The whole content of the lecture with all the animations can be dowloaded from: Size > 300 M!!! The dowloaded.zip fijle should be unpacked into an independent new folder, than please start the file named Part 1.ppt. The embedded icons will show/play the pictures or films. At the end of the presentation please continue with Part 2.ppt and than with Part 3.ppt!

2 Radiation Therapy Technical Aspects

3 Introduction Treatment modalities for cancer: Surgery Radiotherapy Chemotherapy Radiotherapy 50% of patients About 60% of all tumor patients can be considered to be potentially curable. (Malignant localized tumor, no metastatic disease) The aim: to deliver a radiation dose, to kill all tumor cells. Difficulties: OAR are located close to the tumor

4 The double goal of radiation therapy: Increase the dose to the target volume! Decrease the dose to healthy tissue! 1. better tumor control TCP Tumor Control Probability 2. decrease of side effects NTCP Normal Tissue Complication Probability These means higher probability of patient cure.

5 CONVENTIONAL CONFORMAL

6 CONVENTIONAL CONFORMAL IMRT

7 The course of radiotherapy Radiotherapy treatment chain

8 I. Patient Immobilization High TU dose, low dose at OARs Sophisticated delivery techniques very steep dose gradient between target and the organs at risk patient immobilization is a crucial issue. Setup errors underdosage in the target, General Considerations overdosage in the normal tissue. 1. Definition of Target Volumes (ICRU 50, ICRU 62) GTV (Gross Tumor Volume) the clinically evident target volume, visible in diag. images. CTV (Clinical Target Volume) GTV + margin (containing micr. population of tumor cells) PTV (Planning Target Volume) accounts for setup uncertainties, organ motion and deformation

9 The PTV should have a high probality of containing the CTV during the whole treatment IMMOBILIZATION 2. Sources of Uncertainties a. Patient setup uncertainties b. Organ motion and deformation 3. Design Requirements General intention to reduce the CTV PTV safety margins. High reproducibility. Compatible with the imaging modalities. Practical and easy to use. Comfortable for the patient.

10 Immobilization Techniques Head targets: Invasive fixation radiosurgery total dose in a single fraction Overall setup uncertainty < 1mm

11 INVASIVE FIXATION

12 Non-invasive fixation bite blocks and/or face masks mask individually fabricated for each patient thermoplastic material, polyurethane foam, selfhardening Scotch-cast bandages.

13 MASK OF SCOTCH CAST BANDAGES

14 Extracranial Targets much more challenging rotations around the body axis target position may change within the body organ motions Several possible solutions Vacuum pillows Evacuated flexible bags Self-hardening bandages thermoplastic sheets, bandages. Decreasing of movements caused by breathing Breast Treatments one of the most complicated problems fixation of opposite side mamma position of arms

15 VACUUM PILLOW

16 VACUUM PILLOW PRODUCTION

17 VACUUM FIXATION SYSTEM

18 SELF- HARDENING BANDAGES

19 ABDOMINAL COMPRESSION DEVICE

20 II. Imaging Imaging for therapy planning serves the following purposes A target volume (TU) and the organs at risk are the basis for therapy planning. 3D patient model beam directions are optimized. The dose is calculated based on CT data. DVHs can be plotted for the tumor and the organs at risk. A 3D-anatomical model is also required for positioning of the patient at the therapy device. A 3D-model is normally obtained using X-ray computed tomography (CT). Functional imaging (MRI, PET, SPECT) useful for the definition of tumor, allows the visualization of microscopic disease outside the region of highest cell density. 1. X-ray Computed Tomography (CT) 2. Magnetic Resonance Imaging (MRI) 3. Nuclear Medicine SPECT (Single Photon Emission Tomograph) PET (Positron Emission Tomograph)

21 III. Tumor Localization Before image data can be used for radiation treatment planning relevant structures have to be identified Which structures are important? How structures and volumes can be delineated? How can be combined different modalities? 1. Volume Definition Two different kinds of structures are important The target volume The organs at risk (OAR),which have to be spared. ICRU Report 50 (1993) és ICRU Report 62 (1999) (International Commission on Radiation Units and Measurements) GTV, CTV, PTV GTV Gross Tumor Volume CTV Clinical Target Vomume (GTV+margin) PTV Planning Target Vomume (CTV+margin)

22 IMPORTANT STRUCTURES

23 VOLUMES in ICRU 50

24 2. Image segmentation Segmentation the process of distinguishing relevant structures/volumes from the background. 1 slice 2D segmentation More than 1 slice 3D segmetation Segmentation in radiation treatment planning _- delineating the PTV delineating the organs at risk delineating the surface contour of the body Manual segmentation Semiautomatic segmentation algorithms Automatic segmentation algorithms Two groups of segmentation algorithms Region-based approaches (find an area of similar properties) Edge detection algorithms (look for sudden changes)

25 3. Image Registration Image sequences of various modalities are used: CT, MRI, PET, SPECT A definite relation is necessary between the picture elements (pixels). Registration : methods which are able to calculate these relations (transformations) Eg. At least 3 corresp. pairs of points transformation matrix calculation correlation between the two sequencies. Manual registration: user interaction Semiautomatic registration: partly user interaction Automatic registration: do not require any user interaction Scope of transformation: global and local Geometrical properties: rigid elastic transformation Image fusion Display of registered image sequences

26 PARALLEL DISPLAY

27 SLIDING WINDOW

28 INTERACTIVE MATCHING

29 IV. 3D Treatment Plannig The goal of planning: to find the optimal treatment plan. Based on 3D model of patient anatomy. - to find the optimal beam directions. - to form the beam shape exactly to the tumor shape (to minimize the dose to healthy tissues). - to accurately calculate the physical dose distribution. The 3D patient model is based on - 3D tomographs (CT,MR,PET) - 2D slices 3D (image cube) 3D Model

30 3D patient model

31 3D navigation

32 - Contouring transformation to a 3D model (interpolation) The Radiotherapy Planning Cycle - a series of beams are applied in order to concentrate the dose on the target volume. - the beams (dose) are superimposed on the target. - the healthy tissues can be kept below tolerance lewels.

33 The 3D model

34 Dose distributions

35 THE PLANNING CYCLE Acquisition of CT(MR,PET)image sequences Definition of tumor, target volume and organs at risk Definition of treatment parameters Virtual therapy simulation Optimization Dose calculation Evaluation of dose distribution Patient treatment

36 1. Definition of Beam Directions - Spatial relations between target volume and organs at risk! - Main criterion: the target volume is enclosed completely by the beam without enclosing any organs at risk. If it is not possible, to minimize the volume of organ at risk covered by beam. Tools: Beam's Eye View (BEV) The planner views the 3D-model from the position of the radiation source. Beam s Eye View Interactive Beam s Eye View

37 Beam's Eye View

38 Beam's Eye View BEAM 1 BEAM 2 BEAM 3

39 Beam's Eye View BEAM 1 BEAM 2 BEAM 3

40 Beam's Eye View BEAM 1 BEAM 2 BEAM 3

41 Beam's Eye View

42 - Observer's View presents the 3D model from an arbitrary point of view. Observer s View Observer s View Helps to minimize that subvolume where the single beams overlap. - Spherical View Spherical View

43 Observer s View

44 Observer s View

45 Spherical View

46 2. Additional Treatment Parameters Irradiation directions and beam shapes A series of other parameters: - Selection of radiation type: fotons - generally electrons superficial tumors protons, heavy ions - energy of radiation, beam quality - beam modifying devices bloks, wedges, compensators, dynamic collimátors etc. It is possible to shape the 3D dose distribution to better match to the form of target volume.

47 3. Dose Calculation - A dose calculation algorythm calculates the expected dose distribution using the beams specified. 4. Evaluation of Treatment Plans Several alternative configurations can be compared and the most suitable one used. Qualitative Evaluation of Dose Distribution Biological Models : TCP, NTCP Forward Planning, Inverse Planning

48 Forward and Inverse Planning

49 - 3D dose distribution: isodose surfaces - Isodose distribution from slice to slice: isodose lines, color wash - Dose-Volume-Histograms DVH a simple way of displaying the 3D dose distribution. DVHs usually are displayed as cumulative histograms showing the fraction of the total volume receiving doses up to a given value.

50 3D isodose distributions

51 2D isodose distributions

52 Dose-Volume- Histograms

53 V. Patient Positioning Substancial role in radiation therapy: Fixation and immobilization of patient Absolute positioning at the irradiation device. 1. Step: Definition of patient-fixed coordinates. 2. Step: Image acquisition.during planning target point is calculated in patient-fixed coordinates. 3. Step: Patient positioning at the irradiation device, immediately before treatment.(to move the target point to the isocenter)

54 Patient-fixed coordinate system

55 Patient-fixed coordinate system

56 Image acquisition and target point coordinates

57 Positioning at the irradiation device

58 Positioning at the irradiation device

59 2. Portal Films and Electronic Portal Imaging Films: provide information for repositioning of patients, from fractions to fractions. Electronic portal imaging devices (EPID): real time images, time efficient patient positioning. Types:Fluoroscopic systems, scintillation screen camera. Ionization chamber arrays, e.g.256 x 256. Patient positioning based on external markers and anatomical points. (Final control.) Comparison of : Simulator images Port images DRR Port images

60 The Beamview system

61 X-ray images and electronic ports

62 VII. The Treatment A. Treatment modalities 1. Linear Accelerators (Linacs) Basic idea : to accelerate electrons in the field of electromagnetic wave travelling in a waveguide. Principle of an elementary linac: X-ray tube A high voltage of several MVs means a big insulation problem or a great tube size. Instead of high-voltage a series of smaller voltage are applied.(these fields are produced by microwaves)

63 Linac

64 Increase of electron speed with energy

65 Concept of an elementary accelerator

66 Microwave cavities electron oscillations in the wall, acceleration of electrons in the cavities. Travelling-wave Accelerator: a series of microwave cavities of a length equal to onequarter wavelength. for a 10 MeV electron beam 125 cm length at higher energies standing wave waveguide

67 Principle of electron acceleration

68 Travelling wave acceleration

69 Travelling wave acceleration

70 Gyorsító cső metszete

71 Standing wave accelerator: the RF energy reflected at both ends creating a standing wave. The length of each cavities equal one-quarter wavelength. Half of the cavities have zero field all times, they can be moved off the beam axis.(shortened standing wave tube) 2. Accelerator Major Subsystems structure - gantry. RF source (magnetron or klystron), modulator, circulator, waveguides, electron gun, AFC system, cooling system, gas system, vacuum system, treatment head: bending magnet, target, primary collimmator, flattening filter, monitor chamber, secondary collimator.

72 Generation of a standing wave

73 Standing wave acceleration

74 Shortened standing wave tube

75 Shortened standing wave tube

76 Shortened tube

77 3. Multi-Leaf-Collimators (MLCs) In clinical radiotherapy it is often necessary to produce irregular shaped fields. Two possibilities: beam shaping with blocks use of multi-leaf-collimators (MLCs) Integrated MLCs: medium size and large fields (up to 40x40 cm 2 ) Accessory MLCs: e.g.for stereotactic conformal therapy, micro-mlcs, small fields (10x10 cm2)

78 Integrated MLC

79 Accessory type micro-mlc

80 Accessory type micro-mlc

81 Accessory type micro-mlc

82 Beam shaping with micro-mlc

83 Important features Maximum field size 40x40 cm 2, 10x10 cm 2 Leaf resolution (leaf width) 1 cm, 2-3 mm Maximum overtravel How far a leaf can be moved over the midline Operating modes Static: Dynamic: Focusing properties and penumbra

84 MLC in static mode

85 MLC in dynamic mode

86 B. Treatment Procedures (conformal therapy) 1. Convencional (classical) conformal RT The basic problem: - PDD is an exponential decreasing function of depth. The dose is higher close to the surface than at the depth of tumor. The solution: - using more fields - to tailor the beams to the shape of target volume. the conformity of dose distribution can be increased. Conformity: - a 3D dose distribution should follow the tumor shape while sparing the OARs.

87 Four-field treatment technic

88 How can be conformity increased? Higher number of beams Optimization of beam directions Optimization of beam energy (photons)) Application of a MLC. Smaller leaf width. More than one target point. Moving bean irradiations. The Limits of Conventional Conformal RT. Conformal and homogeneous dose distribution cannot be obtained in all cases. -Difficult to find good directions. -Beam overlap in the case of high number of beams.

89 Micro-MLC

90 Tumor and OARs

91 Multifield radiation

92 2. Intensity Modulated Radiation Therapy (IMRT) Solution: IMRT-technique. Basis: generation of intensity modulated fields and to treat with these fields. (Fig. Shows a beam arrangement with seven IMFs). How to deliver these fields? Step-and-Shoot technique (superposition of irregurarly shaped and partial overlapping field components.) Sliding Window technique, or dynamic MLC (independently moving leaves during radiation) Physical Compensators (absorbing material)

93 The principle of IMRT

94 Intensity modulated fields

95 Step-and-Shoot technique

96 Dynamic MLC technique

97 a, Step-and-Shoot technique (static, Bortfeld-Boyer technique) Generally an IMF is the superposition of irregularly shaped and partial overlapping field components. Terms: intensity map, channel, intensity level, field component (subfield, segment) Close-in technique Sweep technique Close-in: leaves are moving in both directions Sweep: leaves are moving in one direction only

98 Step-and-Shoot Close-in technique

99 Technical terms in IMRT

100 Step-and-Shoot Close-in technique

101 Step-and-Shoot Close-in technique One leaf pair

102 Step-and-Shoot Sweep technique

103 Step-and-Shoot Sweep technique

104 Step-and-Shoot Sweep technique One leaf pair

105 Step-and-Shoot Close-in and Sweep technique

106 b, The Dynamic Technique An analog-limiting case of the sweep technique, called sliding window technique. Leaf position accuracy is very important. + Shorter treatment time High complexity No problems with low dose fields Verification too. c, Physical compensators Compensator: an absorbing material with variable thickness. The prescribed intensity is produced by the thickness of the matter.

107 Dynamic IMRT technique

108 Small positioning error large dose error

109 Physical compensator

110 Physical compensator

111 Some specifics: - Every intensity map individual compensator (labour intensive) - Divergence - layers - High spatial resolution. - Faster, than the step-and-shoot. (treatament time) - IMFs without MLC. - Mold material.

112 VII. Clinical Radiation Dosimetry 1. Principles Definition of Dose: the absorbed dose is the energy absorbed in the dm mass element divided by the dm. (Gy Gray) Clinical dosimetry, water absorbed dose to water Radiation types and fields: wave or particle Two types of radiations play a major role in radiology. - photons: X-rays or gamma rays, with energies in the range of kev and higher. Zero rest mass. - electrons: they have a rest mass and a negative charge. Electron states or nucleus transition (beta-rays)

113 Radiation field: a part of the space where rays or particles are moving. Flux density: the number of particles which cross through a small perpendicular plane per unit time. Energy Transfer by Photons and Electrons Photons: - photoelectric effect - Compton effect - Pair production These interaction processes release secondary electrons, which again interact with the matter. Electrons: - collisions with the atoms or electrons. - radiative processes (Bremsstrahlung production) Inelastic collisions with the electrons in the atomic shell lead to excitation and ionization of the atom.

114 Photoelectric effect

115 Compton effect

116 Pair production

117 2. Measurement of Dose A variety of physical and chemical effects can be used. Ionization in gas ionization chamber proportional counter Geiger-Müller counter Ionization in solid semiconductor crystal Luminescence TLD Chemical effects photografic film chemical dosimeter Thermal effects calorimeter

118 Absolute Measurement Farmer-type ionization chamber + water phantom 1. Positioning 2. Connection 3. Measurement 4. Calculation 5. Corrections 1. Positioning 2. Connection 3. Measurement 4. Calculation N D,w calibration factor, SSDL ref. cond. 5. Corrections k = k ρ k s k p k Q e.g. Relative dose measurement

119 Chamber positioning in water phantom

120 Connection to the electrometer

121 Measurement with the electrometer

122 Control source for density correction

123 Air density correction

124 3. Phantoms A measurement of absorbed dose is performed within an absorbing medium called phantom. Standard phantoms Water phantom: TBA (Therapy Beam Analyzer) Anatomical phantoms: Alderson-Rando phantom IMRT phantoms 4. Dose verification A dose verification test is required to guarantee, that the radiation applied to the patient is exactly the same as simulated and calculated by the computer. Steps of the verification: a, Virtual treatment of an appropriate phantom (plan transfer to the phantom, calculation, dose distr.) b, Irradiation of the phantom measurement c, Comparison of the calc. and meas. results.

125 Standard phantom

126 Water phantom

127 Alderson-Rando phantom

128 IMRT phantom

129 Comparison of plan and measurement