151 - Improving Access to Conformal Bolus Using Sheet Bolus and Piece-Wise Flat Geometry

Purpose/Objective(s): Inadequate surface dose compromises local control, especially when air gaps reduce bolus effectiveness. While 3D-printed bolus improves conformity, it requires specialized equipment or expensive out-sourcing. Tissue equivalent sheet bolus (TESB) is low-cost, reusable, and readily available, but application to curved anatomy introduces air gaps and setup variability. We developed and tested a rapid, low-cost protocol to fabricate 3D conformal bolus from TESB using piece-wise flat geometry with the goal of improving geometric accuracy, reproducibility, workflow efficiency, and access.

Materials/Methods:

Curved anatomy can be categorized as spherical (+; breast apex, nose) or hyperbolic curvature (–; inframammary fold, neck). TESB on + curvature results in folds at the base; – curvature, gaps at the periphery. Straight radial cuts reduce excess at the base but concentrate curvature into a cone-point at the peak, forming an air gap in the treatment region.

Our protocol redistributes the excess from the + to – curvature using multiple curved cuts. A breast phantom was used to clinically test the protocol. Phantom anatomy was measured directly and traced onto a disposable underpad for flat-plane measurements. Five cuts were evenly spaced and drawn with a gentle bend to the marked peak forming five concave triangles. After removal, the remaining edges were joined to create a 3D template, then reproduced using TESB, adhesive, and tape. CT scans of TESB and custom bolus on the phantom were performed and compared to an ideal virtual bolus of same thickness. Geometric consistency was analyzed using Dice Similarity Coefficient (DSC) and Hausdorff distances (HD) in 3D Slicer. Apex, medial, and lateral point doses were compared between the unmodified TESB and custom bolus for a 40.05 Gy tangential breast plan. Time and material cost were compared to the departmental third-party 3D print workflow.

Results: Compared to the virtual bolus, the TESB had a mean HD of 6.2 mm (95% 25.7 mm) and DSC of 0.43. The custom bolus achieved a mean HD of 1.2 mm (95% 4.1 mm) and DSC 0.75 representing a 56% relative reduction in segmentation error and marked improvement in surface conformity. Apex surface dose agreed within 0.5 Gy, while medial and lateral surface doses were greater for the custom bolus by 1.5 and 1.3 Gy, respectively. Fabrication was 36 minutes and used <$5 -$150 (reusing vs. new TESB). Third-party 3D-printing required =1 day turnaround and hundreds to >$1,000 per case (based on size).

Conclusion: This low-cost, rapid protocol greatly improves TESB surface conformity while reducing fabrication time and cost. Reducing air-gaps without specialized infrastructure can improve skin surface dose while expanding access to customized conformal bolus, particularly in clinics without technical or financial access to 3D-printing resources. Further evaluation of dosimetric impact and clinical reproducibility is underway.