Verification of Patient Specific Quality Assurance System for Volumetric Modulated Arc Therapy (VMAT)

Abstract

Background: Volumetric Modulated Arc Therapy (VMAT) plans and its treatment are complex modalities and its clinical implementation requires very accurate acceptance testing and a very comprehensive quality assurance program. Failure in their periodic applications can result in errors in treatment delivery. There is therefore the need to compare the calculated dose distribution to the measured dose distribution of treatment plans to ensure accuracy, efficiency and minimize errors in planned radiation dose delivery to the patient. Methods: 15 patients with pathological pelvic lymph node metastasis were included in this study. Measurements were done using both conventional 6 MV beam with flattening filter and flattening filter free beam (FFF) for all 30 VMAT plans. Each patient thus had a 6 MV plan and an FFF plan. The treatment plans were made using two arcs and then delivered to the delta4 phantom. All 30 VMAT plans were replicated on the Delta4 phantom using three Elekta linear accelerators. Results: After machine optimization, there was an increase in the mean total gamma pass rate for 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%. There was significant statistical difference between the pass rates of the 6 MV plans and the FFF plans (p = 0.000488). The total monitor units (MUs) for the FFF plans were significantly greater than the 6 MV plans (p = 6.1x10-5). Conclusion: 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.