Customized Bolus for Orbital Tumors Created with 3D Photogrammetry and Rapid Prototyping

Abstract

Children with orbital tumors, such as retinoblastoma or sarcoma, are often treated with radiotherapy. There are many techniques, including en face electron beams, conformal and intensity-modulated radiation therapy. For tumors close to the skin, bolus is overlaid to avoid underdosing the superficial portions of the target. If bolus is separated from the skin by an air gap, this may lead to treatment inaccuracy, which is magnified by larger gaps and smaller field sizes. Commercial tissue equivalent bolus is difficult to conform around the eyes and bridge of the nose, and custom impression material requires experience to create a uniform bolus without air cavities. We describe a technique with 3D photogrammetry at consultation to rapidly prototype a custom bolus prior to simulation. Children with orbital tumors, such as retinoblastoma or sarcoma, are often treated with radiotherapy. There are many techniques, including en face electron beams, conformal and intensity-modulated radiation therapy. For tumors close to the skin, bolus is overlaid to avoid underdosing the superficial portions of the target. If bolus is separated from the skin by an air gap, this may lead to treatment inaccuracy, which is magnified by larger gaps and smaller field sizes. Commercial tissue equivalent bolus is difficult to conform around the eyes and bridge of the nose, and custom impression material requires experience to create a uniform bolus without air cavities. We describe a technique with 3D photogrammetry at consultation to rapidly prototype a custom bolus prior to simulation. The 3D printed bolus was manufactured within a day after completing the STL file. The conformality of the 3D bolus to the phantom was compared to SuperFlab and the VPS bolus with computed tomography. SuperFlab was the least conformal and the 3D bolus was intermediate. Ridges on the surface of the 3D bolus from manufacturing increased the bolus-phantom gap. The VPS bolus material was the most malleable and conformal, but was not uniformly thick and had some internal air voids. Photogrammetry is an old technique to obtain patient contours, which we modernized to create a 3D file for rapid prototyping. The resulting bolus was created quickly and fit with excellent conformality, but was the most expensive. Over time, as rapid prototyping innovates, this method will become easier, more efficient, and increasingly available with a greater choice of materials and vendors. Until then, this process is useful now for a few small and irregular sites in the head and neck region, but routine and widespread application will not benefit most patients.