Role of Volumetric-Modulated Arc Therapy with Flattening Filter Free Delivery in Lung Stereotactic Body Radiotherapy

Purpose: To evaluate the role of volumetric-modulated arc therapy (VMAT) combined with flattening filter free (FFF) beam delivery in lung stereotactic body radiotherapy (SBRT), using Acuros XB algorithm. Materials and methods: Ninety-eight stage I lung cancer cases treated with SBRT were included. Retrospectively, single arc, 6 MV VMAT with FFF and flattened field (FF) beam plans were generated using the identical optimization criteria with Acuros XB dose computation algorithm. Optimization constraints included target dose ratio of 50% prescription isodose volume to target (R50%) and maximal dose at 2 cm from the target (D2cm). Study parameters, including the number of monitor units (MUs), mean target dose, conformality number, and heterogeneity index of the target, R50%, D2cm, and mean dose to normal tissues were determined and compared for the two delivery modes. A paired matched t-test was used to compare the difference. The effect of tumor, patient and treatment mode (FFF vs. FF) on the difference of MUs obtained from the two delivery systems was statistically determined using linear regression analyses. Results: FFF beams required 6.9% (range -3.4% to 15.6%) more MUs and resulted in slightly higher values for mean target dose, R50% and D2cm (all P<.0001) compared with FF beams. Conformality number, homogeneity index of target, and mean dose to the normal body were similar between the two plans. Logistic regression analysis confirmed that VMAT with FFF mode, not the tumor or patient-specific parameters, caused the increase in MUs compared with FF beam delivery (P=.043). Conclusions: In lung SBRT, using VMAT with FFF beam required 7% more MUs compared with a VMAT with FF beam plan and provided similar target coverage. The low leakage dose advantage of FFF delivery is offset by the increased number of MU. The only advantage available to the FFF beams is their estimated faster delivery.