Purpose: To evaluate the sensitivity of IMRT/VMAT dose calculation to changes in dosimetric leaf gap(DLG) and MLC-transmission for a suite of test plans.
Methods: A set of 5 test shapes were used to generate IMRT and VMAT plans for dose calculation sensitivity analysis. Three test shapes were selected from the TG-119 report: CShape, H&N, and Prostate. Additional test shapes were used as well: H&N_2, which contains an SIB volume, and Pelvis, which mimics prostate with pelvic lymph node irradiation. IMRT and VMAT plans were generated in Eclipse using AAAv15.6 algorithm for both 6 MV and 10 MV. Each plan was calculated on phantom image set for each DLG and MLC-transmission setting evaluated (3 settings were used for each energy). The sensitivity for each parameter was taken to be the difference in point dose relative to the baseline dose calculation. The sensitivity was also compared to the modulation factor (MU per Gy delivered for each plan).
Results: For 6MV, the mean dose sensitivity (normalized to 1 mm change in DLG) was 2.4% (VMAT) and 4.1% (IMRT). For 10 MV, DLG sensitivity was 2.0% (VMAT) and 5.0% (IMRT). For 6 MV, the mean dose sensitivity (normalized to 1% absolute change in MLC-transmission) was 1.4% (VMAT) and 3.1% (IMRT). For 10 MV, MLC-transmission sensitivity was 1.3% (VMAT) and 3.4% (IMRT). The dose sensitivity (for both DLG and MLC-transmission) was largest for the plans with largest modulation factor-CShape and Pelvis IMRT plans.
Conclusion: For both 6 and 10 MV, IMRT planning exhibited higher sensitivity to changes in MLC parameters – DLG and MLC-transmission. Further, the plan modulation factor (defined as the MU/Gy) appears to be correlated with the plan sensitivity to changes in MLC parameters. The sensitivity analysis in this study is a potential reference for physicists when tweaking MLC parameters during machine commissioning.