Verification of patient specific quality assurance system for volumetric modulated arc therapy (VMAT)

Volumetric Modulated Arc Therapy (VMAT) plans and its treatment are complex radiation therapy modalities and its clinical implementation requires very accurate acceptance testing and a comprehensive quality assurance program. Failure in their periodic applications can result in errors in treatment delivery. It is therefore needed to compare the calculated dose distribution to the measured dose distribution of treatment plans to ensure accuracy and efficiency to minimize errors in planned radiation dose delivery to the patient. VMAT plans for fifteen patients with cervical cancer with pathological pelvic lymph node metastasis were included in this study. Two VMAT plans were made for each patient, one using conventional 6 MV beam with flattening filter and one with flattening filter free beam (FFF). The VMAT plans were made using two arcs, and then recalculated to give the planned dose distribution to the detectors in a Delta4 phantom. All thirty VMAT plans were irradiated on the Delta4 phantom using three Elekta linear accelerators, and planned and calculated doses to the phantom were compared. Measurements were done both before and after a routine linac optimization, and variation in radiation delivery between the linacs were studied. After machine optimization, there was an increase in the mean total gamma pass rate for both the 6 MV plans from 98.7 to 99.9% and the FFF plans also had an increase in the gamma pass rate from 91.7% to 98.4% for SB5 linac. There was significant statistical difference (p < 0.05) between the pass rates of the 6 MV plans and the FFF plans. 6 MV VMAT plans with conventional flattened beams are delivered more accurately and hence more beneficial compared to flattening filter free (FFF) VMAT plans for external radiation of cervical cancer with affected pelvic lymph nodes.