Dynalog data tool for IMRT plan verification

Transcription

1 Dynalog data tool for IMRT plan verification Poster No.: R-0051 Congress: 2014 CSM Type: Scientific Exhibit Authors: V. Sashin; FOOTSCRAY/AU Keywords: Computer applications, Radiation physics, Experimental, Intensity Modulated Radiotherapy (IMRT), Quality assurance DOI: /ranzcr2014/R-0051 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply RANZCR/AIR/ACPSEM's endorsement, sponsorship or recommendation of the third party, information, product or service. RANZCR/AIR/ ACPSEM is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold RANZCR/AIR/ACPSEM harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies,.ppt slideshows,.doc documents and any other multimedia files are not available in the pdf version of presentations. Page 1 of 17

2 Aim The complexity of IMRT treatments demands a comprehensive quality assurance (QA) program that includes checks and tests performed at different stages of the treatment: simulation, planning and dose delivery. An important component of this QA program is the verification of MLC leaf positions during the plan data transfer and delivery. The developed software tool performs the following tasks: Checks the data transfer of the MLC leaf positions by comparing the actual coordinates against the planned ones Estimates fluence delivery errors arising from positional and fractional MU deviations Facilitates choosing a 'good' QA point for reliable dose verification Performs several axillary functions to ensure quality and the safe delivery of IMRT plans Methods and materials At our department step-and-shoot IMRT treatment plans are produced using the 3 Pinnacle treatment planning system (TPS) (ADAC Laboratories, Milpitas CA USA). The plans are first exported into the MOSAIQ (Elekta, Stockholm Sweden) R/V system and then delivered to a patient using a Varian linear accelerator Clinac ix (Varian Medical Systems, Palo Alto CA USA) equipped with the Millennium 120 leaf MLC collimator. DynaLog data files recorded by the MLC controller during the dose delivery, in combination with the MLC files exported from the Pinnacle TPS, are well suited to achieve the above stated aims. The DynaLog data provides information on the dosimetric and mechanical status of the Clinac components, including the planned and actual MLC leaf positions. The Pinnacle MLC files contain the MLC leaf positions for each beam segment. Results This in-house developed tool was written using the Visual Basic algorithmic language, which is part of the Microsoft 2005 Visual Studio. Two independent channels of information are input into the code. Through a first data 3 channel, MLC files exported from the Pinnacle TPS are read. The files contain the planned fractional MU and positional information for all IMRT field segments. Page 2 of 17

3 Through a second data channel, DynaLog files created by a Varian Clinac during the dose delivery [1] are read. Every 50 milliseconds the MLC controller samples the dosimetric and mechanical status of the Clinac components and writes the MLC leaf coordinates for each carriage as well as for each delivered IMRT field. The DynaLog and MLC files are manually transferred into the dedicated file directories for reading. The program reads the available data in those directories and connects the data files using the patient's ID number and name. The found file associations are displayed in the 'Pinnacle & DynaLog Data Explorer' window (see Fig. 1 on page 5). The following information is presented: in the upper left half of the window is a list of available patients, in the lower part to the left is a table with the basic DynaLog data for the chosen patient (time, beam number and carriage) and to the right is the corresponding Pinnacle data (file name, beam name and gantry angle for each IMRT beam). MLC leaf position transfer verification To verify the data transfer from the Pinnacle TPS to the Varian linear accelerator, the code calculates the planned MLC leaf positions for each beam segment from the DynaLog files and compares them against the Pinnacle MLC positions. The beam segments in the DynaLog data are defined as follows: they all start when the Beam Hold-off state is off and the Beam On state switches to on. The extracted raw segment positions are further corrected into the isocentre plane using the Varian calibration file 'MLCTABLE.txt'. The results of the comparison are written into.pdf or.txt files (see Fig. 2 on page 6). The files show the statistics of the positional deviations of MLC leaves (average, standard deviation and maximum) for each IMRT beam. Similarly, deviations of the actual MLC leaf positions from the planned ones are calculated during the same code run and the results written into the same file in a similar fashion. If any leaf deviation exceeds a preset threshold value of, for example, 1mm a warning message is issued. A histogram view of various deviations is also available (see Fig. 3 on page 7). Dose delivery error calculation In principle, using a dose calculation engine, DynaLog information can be used to calculate the actual IMRT delivery dose within the patient and compare it to the planned dose, thus simultaneously verifying the calculation and the delivery [2]. As a first step to this comprehensive approach the software tool can compute the fluence distribution for each IMRT beam at the isocentre plane for use as a foundation for further more complex calculations. After convoluting this fluence distribution with Gaussian kernels of the user's chosen widths and applying a uniform 'scattered radiation' background, one can approximate the convoluted fluence to the Pinnacle planar dose. A capture screen of this comparison is shown in Fig. 4 on page 8. In the upper left panel, the DynaLog convoluted fluence map of one of the fields for a Head&Neck IMRT plan is shown. In the right panel, the planned dose calculated at a depth of 5cm and Page 3 of 17

4 SSD of 95cm is presented. The gamma factor [3] assessment of similarity between the two maps (3%DD, 3mmDTA and TH10%) is shown in the lower left panel. Green to red colour coding corresponds to the gamma factor increasing from zero to its maximum value, about three in this case. Three profiles going along the coloured arrow lines in all three plots are shown together in the lower right panel. For this panel the left vertical axis is for the dose and the right axis is for the gamma factor. One can see that there is a good degree of similarity between the two dose maps after the convolution of the DynaLog fluence. A first approximation of the dose errors during the step-and-shoot IMRT plan delivery can be based on the fluence variations that arise from positional and fractional MU errors [4]. These fluence variations, as calculated by the software tool for one IMRT beam, are presented in Fig. 5 on page 9. The upper left panel shows a colour-coded plot of the fluence delivery errors in the isocentre plane. The red corresponds to overdose, whereas the blue reflects some degree of underdose. The magnitude of these variations is about 1.5% of the maximum beam fluence. A profile taken along the red arrow is shown in the lower right panel. To calculate the fluence variations in 3D space the code uses these isocentre maps for each beam with an inverse square law scaling and combines them together using the beam MUs, taking into account the collimator and gantry angles. The convolution with Gaussians gives us some approximation to the dose delivery errors in a uniform phantom. The projections of this 3D dose delivery error distribution are presented in Fig. 6 on page 10, colour coded in a similar manner as in Fig. 5 on page 9. In the lower right panel the profiles along the arrows for each projection are shown. It can be seen that the magnitude of dose errors is about 1% of the maximum plan dose. Also generated is a report detailing the statistics of dose errors in different "dose" ranges. A sample of such a report is shown in Fig. 7 on page 11. These dose delivery error maps, especially that shown in Fig. 5 on page 9, serve as a useful tool for elucidating significant dose deviations that are sometimes shown by our QA measurements of individual IMRT beams using the MapCHECK 2 dose array (Sun Nuclear Corporation, Melbourne FL USA). The beams with smaller MUs and a larger delivery dose rate are prone to be delivered with larger dose errors [5]. Search of QA point for ion chamber measurement When we perform the pre-treatment point dose verification of an IMRT plan using either a small ion chamber inserted onto a cylindrical phantom or the RadCalc software (LifeLine Software Inc., Austin TX USA), it is important to choose a 'good' QA point located in the area with a low dose gradient for each IMRT beam. The software code facilitates choosing this point. As an example, a convoluted fluence for a Head&Neck IMRT beam, as a grey scale map, is shown in the upper left panel of Fig. 8 on page 12. After calculating the gradient value Page 4 of 17

5 at each point and choosing its maximum (or average) value, a so-called dose 2D gradient map is generated, shown in the lower left panel of the figure as a colour plot. The lower (<=2%/mm), median (>2%/mm but <=3%/mm), high (>3%/mm but <=4%/mm) and higher (>4%/mm) gradient areas are depicted in green, yellow, brown and red, respectively. Placing the ion chamber within the green area guarantees that the measurement is not skewed by the dose gradient. The dose gradient map can also include an averaging effect of a finite chamber size when the IC Integration 'Apply' button is activated (button should be green). Using the dose 2D gradient maps for each individual beam of an IMRT plan, the software tool can scale them according to the inverse square law into the 3D space and thus calculate a so-called dose 3D gradient map, each point of which is assigned with a maximum (or average) dose gradient value from the corresponding dose 2D gradient map value. As an example, the three projection planes (transverse, sagittal and coronal) of the dose 3D gradient map are shown in Fig. 9 on page 13 as colour plots, in a similar manner to Fig. 8 on page 12. Scanning through these planes one can choose a 'good' point in 3D space with a minimal dose gradient for all beams of the IMRT plan simultaneously. Axillary functions The code also checks various critical constraints imposed on the MU numbers of IMRT plans. The constraints that can be checked are: Minimum MU number for any beam segment Minimum MU number of any beam Minimum number of beam segments with minimum MUs in any beam The code uses the Pinnacle MLC files and the entered MUs for each beam to automatically calculate the segmental MUs and perform the above mentioned checks. A message about a successful check is shown in Fig. 10 on page 14. The tool can also present the distribution of segmental MUs for each IMRT beam or the total plan as a histogram, an example of which is shown in Fig. 11 on page 15. The program can also precisely calculate the delivery times for each beam of an IMRT plan. An example with the recommended times for a Varian Clinac is shown in Fig. 12 on page 16. The MOSAIQ R/V system used at our department sometimes calculates remarkably overestimated times, especially for larger field sizes, and therefore the accurate estimation of these times is an important factor in the safe delivery of an IMRT plan. Images for this section: Page 5 of 17

6 Fig. 1: Main window and Data Explorer window for the DynaLog Data Analyser Tool Page 6 of 17

7 Fig. 2: MLC leaf position performance report for IMRT step-and-shoot delivery Page 7 of 17

8 Fig. 3: Histogram of MLC leaf position deviations for IMRT step-and-shoot delivery Page 8 of 17

9 Fig. 4: Convoluted fluence map for an IMRT beam at the isocentre plane and its gamma factor comparison to a Pinnacle planar dose (ssd = 95cm;depth = 5 cm) Page 9 of 17

10 Fig. 5: Fluence delivery error plot for an IMRT beam at the isocentre plane and projection profile Page 10 of 17

11 Fig. 6: Transverse, sagittal and coronal projections of convoluted fluence errors in 3D space for an IMRT plan delivery and projection profiles Page 11 of 17

12 Fig. 7: Dose delivery error report for an IMRT plan for different "dose" ranges Page 12 of 17

13 Fig. 8: Convoluted fluence map and its 2D gradient map for an IMRT H&N beam Page 13 of 17

14 Fig. 9: Transverse, sagittal and coronal projections of a dose 3D gradient map for all beams of an IMRT H&N plan and projection profiles Page 14 of 17

15 Fig. 10: Message about a successful check of the MU number constraints for an IMRT plan Fig. 11: Histogram of the segmental MU numbers for an IMRT plan Page 15 of 17

16 Fig. 12: Report of the calculated delivery and recommended times for the Varian linear accelerator Page 16 of 17

17 Conclusion The software tool analysing DynaLog and Pinnacle MLC data was developed in-house to facilitate the IMRT QA program at our department. It has been successfully used as an important aiding tool during the pre-treatment plan QA verification for about 200 patients. Personal information References [1] Varian Medical Systems, DynaLog File Viewer Reference Guide (2003). [2] Wei Luo et al., "Monte Carlo based IMRT dose verification using MLC log files and R/ V outputs", Med. Phys. 33, (2006). [3] DA Low et al., "A technique for the quantitative evaluation of dose distributions", Med. Phys. 25, (1998). [4] Gary A. Ezzell and Suzanne Chungbin, "The overshoot phenomenon in step-andshoot IMRT delivery", J. Appl. Clin. Med. Phys. 2, (2001). [5] Antony M. Stell et al., "An extensive log-file analysis of step-and-shoot intensity modulated radiation therapy segment delivery errors", Med. Phys. 31, (2004). Page 17 of 17