S Gardner*, S Wang, K Snyder, C Smith, B Miller, I Chetty, Henry Ford Health System, Detroit, MI

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  S Gardner

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PO-GePV-T-68 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To evaluate the influence of plan modulation on IMRT/VMAT dose calculation sensitivity to changes in MLC parameter setting.

Methods: A total of 49 plans(24 IMRT,25 VMAT) were selected for dosimetric evaluation, with each plan utilizing TrueBeam delivery/6x mode. The cases covered a wide range of disease sites-including pelvis, thorax, brain, and HN. The Modulation Factor(MF) was defined as Plan MU divided by Rx Dose. For IMRT cases-the MF range was [169, 1439]; for VMAT cases-the MF range was [146, 628]. To eliminate effects from tissue heterogeneity, each plan was calculated on water-equivalent phantom dataset using both AAA and AcurosXB algorithms. For each algorithm, multiple DLG settings were utilized to evaluate the impact of MF on dose calculation/DLG sensitivity. For each plan, the point dose(at isocenter or other meaningful high-dose location)was used for dose sensitivity evaluation.

Results: Each plan was calculated using 3 DLG settings (0.110cm,0.135cm,and 0.160cm), with dose sensitivity defined as percent difference in dose between the highest and lowest DLG setting. The plans were grouped into 3 tiers based on MF value: Low(100500). Across all modulation tiers, the average dose sensitivity was larger for IMRT compared to VMAT. For IMRT, the average dose change was 1.49±0.59% (AAA), 1.46±0.63% (AcurosXB); for VMAT: 0.88±0.30% (AAA), 0.61±0.32% (AcurosXB). When comparing modulation tiers, dose sensitivity increased with modulation (average±st.dev. among both algorithms): 0.63±0.34%(Low-Modulation), 0.98±0.35%(Mid-Modulation), and 1.50±0.66%(High-Modulation).

Conclusion: Our study offers some insight into the behavior of IMRT/VMAT dose calculation across a spectrum of plan modulation values. Given that DLG sensitivity increases with modulation and delivery modality choice(IMRT vs. VMAT), our results suggest that plan selection during algorithm validation/commissioning should cover a useful range of modulation factors for both delivery modalities. In addition, we note that these trends hold for both algorithms(AAA and AcurosXB) tested in this study.

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