Impact of high dose volumetric CT on PTV margin reduction in VMAT prostate radiotherapy

The aim of the study is to determine PTV margin for inter-observer variability in the volumetric modulated arc therapy (VMAT) prostate radiotherapy with high-dose volumetric CT (HDVCT) and conventional helical CT (CCT) for planning. Secondly to investigate the impact of geometric (PTV expansion) and dosimetric (conformity) imperfection of planning process on the PTV margin analysis. Prostate gland of ten patients were scanned with CCT and HDVCT techniques consecutively on a 320 slice volumetric CT scanner with wide field detector of 16 cm. Five radiation oncologists delineated CTV of the prostate. VMAT plans were developed with PTV margin of 4 mm and 6 mm (totaling 200 plans) and target coverage of each plan was evaluated on the target volume in agreement determined by shared voxels with three or more from 5 observers. Dosimetry on 200 VMAT plans showed that PTV margin for inter-observer variability were 6 mm and 4 mm for CCT and HDVCT techniques, respectively. It is about 3 mm smaller than our estimation from the previous study (8.8 mm and 7.3 mm) based on the inter-observer variability. This difference is mainly due to the accuracy of PTV volume expansion and limited dose conformity to guarantee target coverage. PTVs were measured 2 mm larger on average than the assigned margin. Planning iso-dose volume was found to be 2 mm larger than PTV. Regardless these limitations, enhanced image quality of HDVCT reduces PTV margin by 2 mm compared to CCT. PTV reduction of 2 mm potentially leads to 15% reduction in D30% of rectal and bladder wall maintaining the same target coverage. Inter-observer variability remains a source of systematic uncertainty. HDVCT for treatment planning demonstrated reduction of the uncertainty and the PTV margin by 2 mm. It is important to consider the over-expanded PTV volume and generous iso-dose volume after optimization in the process of radiotherapy planning in the determination of PTV margin.