Production of patient-specific electron beam aperture cut-outs using a low-cost, multi-purpose 3D printer

Electron beam collimators for non-standard field sizes and shapes are typically fabricated using Styrofoam molds to cast the aperture cut-out. These molds are often produced using a dedicated foam cutter, which may be expensive and only serves a single purpose. An increasing number of radiotherapy departments, however, has a 3D printer on-site, to create a wide range of custom-made treatment auxiliaries, such as bolus and dosimetry phantoms. The 3D printer can also be used to produce patient-specific aperture cut-outs, as elaborated in this note. Open-source programming language was used to automatically generate the mold's shape in a generic digital file format readable by 3D printer software. The geometric mold model has the patient's identification number integrated and is to be mounted on a uniquely fitting, reusable positioning device, which can be 3D printed as well. This assembly likewise fits uniquely onto the applicator tray, ensuring correct and error-free alignment of the mold during casting of the aperture. For dosimetric verification, two aperture cut-outs were cast, one using a conventionally cut Styrofoam mold and one using a 3D printed mold. Using these cut-outs, the clinical plan was delivered onto a phantom, for which the transversal dose distributions were measured at 2 cm depth using radiochromic film and compared using gamma-index analysis. An agreement score of 99.9% between the measured 2D dose distributions was found in the (10%-80%) dose region, using 1% (local) dose-difference and 1.0 mm distance-to-agreement acceptance criteria. The workflow using 3D printing has been clinically implemented and is in routine use at the author's institute for all patient-specific electron beam aperture cut-outs. It allows for a standardized, cost-effective, and operator-friendly workflow without the need for dedicated equipment. In addition, it offers possibilities to increase safety and quality of the process including patient identification and methods for accurate mold alignment.