Effect of Minimum Segment Width on SRT/SBRT Volumetric Arc Therapy Plans for Flattening Filter Free Beams

Background: Volumetric arc therapy (VMAT) based stereotactic radiotherapy (SRT) or stereotactic body radiation therapy (SBRT) is a highly advanced radiation therapy technique that uses intensity-modulated radiation beams delivered in multiple arcs. After optimization, different segments of small sizes and shapes are created in an arc that will influence the indices like homogeneity index (HI), conformity index (CI), gradient index (GI), number of segments (NOS) which in turn will increase or decrease the total treatment time in terms of monitor units (MUs). The dose calculation algorithm faces difficulty in predicting the accurate dose for these small segments because of the lack of charged particle equilibrium (CPE) and requires precise modeling of lateral electron scatter. The segmentation parameter minimum segment width (MSW) can control the generation of these small-sized segments. It can also affect the quality and deliverability of a VMAT plan. Methods: This retrospective study includes 33 patients with lung, liver, and brain tumors (11 patients for each site) treated with the SRT/SBRT technique using a 6 MV flattening filter-free (FFF) beam. Four different plans with MSW 0.5 cm, 1 cm, 1.5 cm, and 2 cm were created by medical physicist using the Monaco treatment planning system (TPS) version 5.11.03. Results: A statistically significant reduction in MU (P= 0.01 for brain, P= 0.005 for lung) and NOS (P=0.034 for brain, P=0.011 for lung) was observed for brain and lung cases in plans with MSW 1 cm. For liver cases, along with MU and NOS (P= 0.029 & 0.013 respectively), the paired t-test shows a statistically significant difference (P= 0,046, 0.019 & 0.009) in the GI for intergroup comparison between two plans at different MSW. Improved GI in the case of plans with narrower segments (MSW 0.5 cm and 1 cm) corresponding to sharp dose fall-off compared to plans with broader segments (MSW 1.5 cm and 2 cm). There is no statistical difference in other parameters including global maximum dose and target coverage for plans at different MSW for all cases. Conclusion: VMAT plans for SRT/SBRT generated with an MSW of 1.0 cm demonstrated comparable dose distributions to plans with MSWs of 0.5 cm with fewer MUs for brain, liver, and lung cases treated with 6 MV FFF beams.