Should we perform the quality assurance for the stereotactic planning of prostate cancer in the treatment condition configuration? An evaluation with a cylindrical diode array phantom

Abstract

Introduction The Delta4 system (ScandiDos) is dedicated for quality assurance (QA) of treatment delivery and is an independent-unit from the accelerator. The purpose of this work was to evaluate the impact of the irradiation geometry on the QA for prostate SBRT using the Delta4 device. Materials and methods The prostate and the tumor were delineated by a radiologist on CT/MRI registrations of 5 patients. As part of a protocol, a 9-coplanar fields IMRT plan was optimized following a schema of prostate SBRT (5 × 6.5 Gy) with a simultaneous integrated boost into the tumor (5 × 8Gy). All the beams were verified four times consecutively using Delta4, with the gantry at: (a) 0° (Plan B0), (b) 90° (B90), (c) 315° (B315) and (d) at their original angulations (242°, 270°, 297°, 328°, 0°, 38°, 76°, 101°, 127°), named BFbyF (field-by-field). The plans B90, B315 and BFbyF were compared with B0 using two evaluation indexes (global gamma index (GGI) and local gamma index (LGI)) as differences of: percentage of detectors with gamma Results For GGI, the average differences ± SD in ‘‘%G Conclusion Usually the IMRT plan verification is made using all beams with gantry at 0_. However, it is important to verify the irradiation plans in the exact situation as at the time of treatment. We have showed here that there is an important difference in the evaluation of a stereotactic IMRT plan as it depends of the irradiation condition. These differences could be explained by the impact of the MLC weight and the couch attenuation. Therefore, it can be recommended that the IMRT verification is made with a detector independent from the accelerator and in the exact configuration of the irradiation as at the time of the treatment (gantry angle and couch position).