Impact of uncertainties in prostate radiotherapy treatment delivery: A simulation study

The purpose of this study was to model and assess the impact of systematic and random uncertainties in patient positioning and treatment delivery in prostate bed Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) performed with and without an EndoRectal Balloon (ERB) insertion. CT and contour data from four post-prostatectomy patients with and without an ERB insertion were considered for the study [1]. A step and shoot IMRT plan and single arc VMAT plan using a 6 MV photon beam model for an Elekta-Synergy accelerator was generated on both data sets (ERB and NOERB) using the Pinnacle planning system. A total of 960 plans were generated. 560 plans were generated with random uncertainties introduced. The magnitude of these uncertainties was a number chosen randomly within the range of ± 2 mm in steps of 0.5 mm for MLC and jaw positions and ±1, in steps of 0.5o for gantry and collimator angles. Treatment isocentre positions were also simulated by introducing random uncertainties within the range of ± 3 mm to each of the original isocentre co-ordinates. 384 plans were generated with systematic uncertainties introduced for MLC open/close and MLC shift within the range of ± 3 mm, in steps of 1 mm for the jaw and MLCs and ±3o, in steps of 1o for the collimator and gantry. An additional 16 baseline plans were calculated with no uncertainties introduced. Clinical endpoints were assessed by calculating the Tumour Control Probability and Normal Tissue Complication Probability for the plan with systematic uncertainties. The random uncertainty simulated plans were recalculated and the impact of random uncertainties on IMRT plans with ERB and NOERB was studied by assessing the dose change in D95 for PTV and CTV.