Use of Surface Scanner to Create Prior CT 3D-Printed Bolus to Increase Efficiency for Radiation Therapy Treatment of Patients with Immobilization Masks

Abstract

3D-printed (3DP) boluses created from computed tomography (CT) data allows optimal bolus shaping and assigning correct Hounsfield Unit (HU), but requires a second CT simulation when masks are used to later fit the bolus under it. The use of surface scanner to produce 3DP bolus pre-CT simulation could solve this issue, reduce CT radiation dose to the patient and improve efficiency. In this study we investigate the use of surface scanner images to produce 3DP boluses. The head of a RANDO® phantom was used to produce boluses in the orbital region either form CT data, (bolusCT) or from surface scanner images (bolusS). A wax bolus was also handmade. The CT and the surface scanner (HandySCAN™, Creaform) resolution were 0.9*0.9*0.8mm3 and 0.05 mm, respectively. Several 3D-printing techniques and materials were tested. To quantify the bolus fitting on the RANDO® each bolus, once printed and inspected, was applied on the phantom to acquire CT images for air gap analysis. The air gap present at the contact surface was then measured at different points (n=30). Using the CT images, HU profiles were traced perpendicular to the RANDO® surface to measure the HU depths corresponding to the air gap. Using known air gaps, created with water equivalent slabs, HU values were correlated to distances. The Wilcoxon signed-rank test and the Mann-Whitney test were used and p- values smaller than 0.05 were considered statistically significant. The bolusS reproduced properly the shape and thickness like bolusCTs, but visually showed a better fitting on the RANDO®. This translated into smaller mean (SD) air gaps compared to bolusCT and wax bolus (p<0.0001), -130 (88) HU, -384(135) HU and, -469 (219) HU, respectively. This difference was independent of the material or 3DP method used (see Table). No statistical difference was seen when comparing the air gaps between the wax bolus and the bolusCT (p =0.0645). Predefined air gaps of 2 mm, 1 mm and 0.6 mm corresponded to mean (SD) HU of -876(5), -586 (4) and -463(3) respectively, implying that mean gaps for all boluses were smaller than 0.6 mm. Nevertheless the maximum air gaps (see table) were: ≈ 2 mm for the wax bolus, 1-2 mm for bolusCT and, < 0.6 mm for bolusS. A surface scanner was successfully used on a phantom to create 3DP bolus and showed superior fitting compared to 3DP bolus using CT data. Regardless of the 3D printing method, the printer precision or material used, the air gap of a bolusS created from a high resolution surface scanner was always inferior to 0.6 mm.Abstract 3561MaterialsMean HUStandard deviation HUMaximum Gap Depth HUBolusCTClear Resin-295.2558.03-627.13Digital ABS-387.21148.80-590.52PLA-390.84102.98-591.35All-383.74134.59-627.13bolussClear Resin-45.1451.79-167.15ABS-191.1354.98-313.66PLA-155.0077.00-294.26All-130.3187.81-313.66Wax-469.42219.49-860.36Abbreviations: ABS=Acrylonitrile butadiene styrene; PLA=Polylactic acid; HU=Hounsfield Units. Open table in a new tab