A bespoke modular 3D printed pelvis phantom for validation of prostate SABR with dominant intra-prostatic lesion (DIL) boost regions

Introduction Patients with prostate cancer are increasingly receiving a boost dose to dominant intra-prostatic lesions (DILs) as part of their radiotherapy treatment. Validation of the dose to the boost region is challenging due to the small volume. The aim of this investigation was to 3D print a patient-specific phantom to validate prostate SABR boost plan. Method The patient outline and bone structure were segmented on their CT scan using the treatment planning system. A cylindrical module positioned at the location of the prostate was added and used to encapsulate dosimeters (film and pinpoint ionization chamber (PP)). Each element was printed using a RepRap X400 3D printer. The bone was printed using Stonefil filament with variable infill. The pelvis and dosimetry inserts were printed using PETG as shells and filled with water. A SABR plan was delivered to the phantom with film placed through the boost region and a PP placed at positions within the prostate and boost regions. The films were compared to the dose calculation using the FilmProQA with global gamma analysis performed using a 2%/2mm criteria and 10% threshold. The plans were recalculated on the phantom and directly compared with the original patient plan using the Verisoft software (global gamma analysis at 1%/1mm and 10% threshold). Results Outlines of the external contour and the bones were consistent between the phantom and patient CT scans. Dose comparisons between the phantom and patient plan calculation revealed 99.0±0.3% of pixels were within 1%/1mm. Prostate and boost point doses were within ±1.5% of expected. Pass rates for film were > 91% at 2%/2mm and > 98.5% at 3%/3mm. Areas of failure coincided with air gaps between the slices of the phantom. Conclusions A bespoke pelvic phantom was designed, constructed and validated to ensure the safe delivery of SABR prostate boost plans.