8/17/2011. Implementation and Quality Assurance Considerations for Volumetric Modulated Arc Therapy (VMAT)*

Transcription

1 and Quality Assurance Considerations for Volumetric Modulated Arc Therapy * What is? Report of writing group from TETAWG under Therapy Physics Committee (TPC) of AAPM Writing Group members: James M. Galvin, Q. Jackie Wu, David M. Shepard, Richard A. Popple, Ying Xiao, Fang-Fang Yin Volumetric modulated arc therapy is an arcbased dose delivery approach that produces highly conformal dose distributions similar to those generated with static gantry intensity modulated radiation therapy (SG-IMRT) *This report is currently under review for publication Potential Advantages of VMAT is faster? VMAT produces higher quality plans?? VMAT uses fewer monitor units resulting in a lower patient total body dose?? Potential Advantages of VMAT is faster? Has conventional IMRT been optimized to reduce treatment delivery time? Does a limited field size increase the time for conventional IMRT? How does the quality of the treatment plan determine the treatment time comparison? 1

2 Potential Advantages of VMAT produces higher quality plans?? When is better best? A multiobjective perspective Mark H. Phillips and Clay Holdsworth Med Phys 38, 1635 (2011) Potential Advantages of VMAT uses fewer monitor units resulting in a lower patient total body dose?? Monitor units go up when small field size MLC require split fields Rotating the field for VMAT increases available field size During arc beam delivery the dose rate, the speed of the gantry, and the position of the MLC leaves can be varied dynamically Due to the necessary synchronization of both dose rate and gantry motion with MLC movement, it is clear that VMAT involves new and different QA steps relative to SG-IMRT It is for this reason that the Therapy Emerging Technology Assessment Working Group (TETAWG) of the Therapy Physics Committee initiated a report on VMAT 2

3 This report includes information for Acceptance Testing, Commissioning, and Routine QA for VMAT The Routine QA description includes a recommendation for patient-specific QA measurements The VMAT Report uses information from two early reports: Bedford and Warrington, "Commissioning of volumetric modulated arc therapy, Int J Radiat Oncol Biol Phys. 73, (2009) Ling, Zhang, Archambault, Bocanek, Tang, and Losasso, "Commissioning and quality assurance of RapidArc radiotherapy delivery system," Int J Radiat Oncol Biol Phys. 72, (2008) These reports touch on the different aspects mentioned in the last slide with varying emphasis The aim of the TETAWG report is to provide needed details for overall safe use of this new technology The TETAWG report follows the two-step procedure suggested in the publication by Ezzell, G. A., Galvin, J. M. et al. (2003) "Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee." Med Phys 30(8) pp : (1) a group of tests that evaluate the performance of individual components of the overall delivery system; and (2) tests that evaluate the end result of the IMRT (in this case VMAT) dose delivery. The TETAWG report deals with Acceptance Testing, Commissioning and Routine QA for MAT. This report separates, as recommended in the Ezzell paper, the testing into the components listed below Specific Component QA Follows TG142 as much as possible Describes the testing procedures End-to-end testing Includes recommendations for benchmark end-to-end testing Includes recommendations for patient-specific QA measurements 3

4 Table 1 Testing Procedures for VMAT Description Purpose of Test Test Tolerance Frequency MLC componentspecific tests Isocenter stability, through and between leaf leakage See TG 142 See TG 142 See TG 142 Dynamic dose calibration test VMAT componentspecific tests VMAT rotational accuracy test End-to-end data transfer check Interruption test End-to-end patientspecific check Absolute dose accuracy for dynamic delivery MLC calibration with dynamic prescription MLC calibration with moving window prescription Beam profile stability as function of dose rate Test of VMAT component synchronization with gantry rotation Data transfer among various systems Ability to resume treatment after interuption of beam on Verify each patient s treatment ±4% relative to static abutted field Sliding window dose test Com irradiation Slide-and-shoot field abutment test AT, Com, Annual ±5% from baseline beam profile and ±1mm from average line and for Asynchronous sliding window field abutment tests/four the gantry zero position AT, Com, Monthly cardinal gantry angles, various dose rates Field flatness/symmetry various gantry angles (four ±1% relative to baseline (defined relative cardinal angles) and dose rates (including highest to profile at gantry zero position AT, Com, Annual and lowest used for VMAT) 2 and standard dose rate) Rotational Accuracy Test ±10% local dose at periphery AT, Com, Monthly 95% of points in agreement to 3% and 3 Benchmark end-to-end test including dose measurement AT, Com, Monthly mm 98% of points in agreement to 2% and 2 AT, Com, Interrupt delivery in middle and restart mm compared to uninterrupted test Annual Apply patient s plan to generic phantom with ion Before start of all 95% points in agreement to 5% and 5 mm chamber and film new plans Field flatness and symmetry at all cardinal angles for range of dose rates (from lowest to highest available for VMAT). This test with the same tolerance level given below is listed in the AAPM TG 142 report. However, TG 142 does not include the requirement to perform the testing for the range of dose rates that will be used for VMAT delivery. It is important to point out here that the tolerance limits are in addition to the deviations allowed in TG 142. Tolerance: ±1% from reference gantry zero dose profile for standard dose rate. Note: This information is intended as an example only! Values can change during review of this report. Perform the asynchronous sliding window. (See the leaf trajectories of this test in nest slide.) The asynchronous sliding window test should be performed at four different cardinal gantry angles to test effect of gravity on MLC movement. This prescription is designed to mimic the complex MLC movement used for VMAT delivery, but does not include the gantry motion. The tolerance values are extracted from both TG 142 (the ±1 value) and the Bedford et al reference (the ±5% intensity change). Tolerance: ±5% intensity change relative to gantry zero baseline profile and ±1 mm from average abutment line for gantry zero position. Figure shows the leaf trajectories for each leaf pair for the asynchronous leaf position test. Notice that the leaves hold at positions that are separated by 2.0 cm. The entire irradiation is carried out with the beam on at a constant dose rate. The dashed line shows that the time each point in a detector (usually film) sees the unobstructed beam is constant throughout the irradiation process. However, the holding of the leaf positions at positions that are spaced by 2.0 cm tests the ability of the leading and following leaves to stop at the exact same place. T im e Foll ow ing lea f trajec to ry L ea f Position (cm ) L ead ing lea f trajec to ry 4

5 (c) Sliding window prescription with leaf pauses, dose profile parallel to the direction of leaf motion. (Copied from Bedford, JL and Warrington, AP Int. J. Radiation Oncology Biol. Phys., Vol. 73, No. 2, pp , 2009) Perform the rotational accuracy test described in section 4.3. See also the leaf trajectories for this test in Figure 2. This test is designed to guarantee synchrony of all (including gantry movement) dynamically varying parameters during VMAT delivery. Tolerances: ±10% local dose at periphery Figure illustrates the irradiation of a transaxially placed film with a rectangular field that tracks the center axis of the cylinder as the gantry rotates. The rotational isocenter is placed near the periphery of the phantom so that the aperture must sweep from side-to-side as the VMAT delivery proceeds. Fig. 3. Rotation tests on the Beam Modulator unit. (a) Radial dose profile, normalized to the center. (Copied from Bedford, JL and Warrington, AP Int. J. Radiation Oncology Biol. Phys., Vol. 73, No. 2, pp , 2009) 5

6 There are other ways for performing this test! Fig. 3.The line of interest lies at 6 cm from the center. (b) A 640 MU symmetric (central axis) prescription. (c) 640 MU asymmetric (off-axis) prescription. Isodoses are normalized to approximately 6 cm off-axis and are in intervals of 4%. (Copied from Bedford, JL and Warrington, AP Int. J. Radiation Oncology Biol. Phys., Vol. 73, No. 2, pp , 2009) The TETAWG report highlights this method because it does not depend on the availability of log files to verify synchronization The reference from Ling et al given in an earlier slide describes an slternative approach for this testing There are other ways for performing this test! There are many devices available for this type of testing for VMAT These devices will not necessarily give you the same information They must provide equivalent information End-to-end test for benchmark cases (AAPM Task Group 119) Tolerance: 95% of points in agreement to 4% and 4 mm 6

7 Perform patient-specific end-to-end QA measurements prior to the start of treatment and for any plan change Tolerance: 95% points in agreement to 5% and 5 mm Treatment interruption test. Use benchmark end-to-end test that includes measurement of dose distribution and absolute dose at a point. Interrupt beam in middle of delivery and continue treatment to completion. The tolerance levels set for this test are for comparison to test results for the same prescription when an interruption was not introduced. Thus, the overall deviation from the calculated dose distribution is on the order of 6% and 6 mm. Tolerance: 98% of points in agreement to 2% and 2 mm compare with reference uninterrupted delivery Table 1 Testing Procedures for VMAT Description Purpose of Test Test Tolerance Frequency Conclusions MLC componentspecific tests Dynamic dose calibration test VMAT componentspecific tests VMAT rotational accuracy test End-to-end data transfer check Interruption test Isocenter stability, through and between leaf leakage Absolute dose accuracy for dynamic delivery MLC calibration with dynamic prescription MLC calibration with moving window prescription Beam profile stability as function of dose rate Test of VMAT component synchronization with gantry rotation Data transfer among various systems Ability to resume treatment after interuption of beam on See TG 142 See TG 142 See TG 142 ±4% relative to static abutted field Sliding window dose test Com irradiation Slide-and-shoot field abutment test AT, Com, Annual ±5% from baseline beam profile and ±1mm from average line and for Asynchronous sliding window field abutment tests/four the gantry zero position AT, Com, Monthly cardinal gantry angles, various dose rates Field flatness/symmetry various gantry angles (four ±1% relative to baseline (defined relative cardinal angles) and dose rates (including highest to profile at gantry zero position AT, Com, Annual and lowest used for VMAT) 2 and standard dose rate) Rotational Accuracy Test ±10% local dose at periphery AT, Com, Monthly 95% of points in agreement to 3% and 3 Benchmark end-to-end test including dose measurement AT, Com, Monthly mm 98% of points in agreement to 2% and 2 AT, Com, Interrupt delivery in middle and restart mm compared to uninterrupted test Annual The synchronization of subsystems and beam parameters during VMAT delivery requires extra QA steps and procedures relative to standard IMRT dose delivery The TETAWG report on VMAT Acceptance Testing, Commissioning and Routine QA describes the additional tests needed for this new treatment modality End-to-end patientspecific check Verify each patient s treatment Apply patient s plan to generic phantom with ion chamber and film 95% points in agreement to 5% and 5 mm Before start of all new plans Note: This information is intended as an example only! Values can change during review of this report. 7

8 Thank You 8