SU‐E‐T‐440: Analysis of Plan Quality with Different Penalties On Segment Width for Volumetric Modulated Arc Delivery of Stereotactic Body Radiotherapy

Purpose: To investigate the plan quality and delivery efficiency with different penalty on minimum segment width in volumetric modulated arc delivery for SBRT cases. Methods: Total of eighteen cases with tumors in lung, liver and abdomen were taken for the study. For each case, three VMAT SBRT plans were generated with different penalties on minimum segment width of 0.5, 1.0 and 1.5cm using CMS Monaco v3.1 TPS. Dose prescriptions were in the range of 7 to 20Gy per fraction. Each plan with different penalty was developed in a way to achieve same dose distribution and evaluated with dose indices(DIs), Conformity Index, Conformity Number and Gradient Index. Plan efficiency was analyzed in terms of monitor units(MUs) and delivery accuracy. The plan with 0.5cm of minimum segment width was used as a reference plan. All these plans were delivered using Elekta Beam Modulator having 4mm leaf width. The delivery accuracy was verified by measured fluence and point dose using Matrixx 2D detector array. Results: The difference between the reference and compared plan DIs were achieved within 2%. When compared with reference plan the total MUs was increased on average of 15.04±8.0% and 22.14±10.7% for 1.0cm and 1.5cm of minimum segment width plans respectively. Analysis of measured dose with TPS calculated showed a deviation of 3.1±1.9%, 2.9±1.2% and 2.7±0.8% for point dose verification and 95.9±2.8%, 96.5±2.6% and 97.8±1.6% for planar dose measurements with pass criteria of 2%/2mm for 0.5, 1.0 and 1.5cm of minimum segment width plans respectively. Conclusion: VMAT SBRT plans with increased value of minimum segment width shows a clear merit of better plan quality and delivery efficiency. Reducing the MUs by controlling redundant modulation for VMAT SBRT plans with optimal values of minimum segment width will increase the comfort for the patients with motion management system.