Dosimetric Evaluation of 3D Printed Bolus Material for Electron Beam Radiation Therapy

Abstract

The past decade has witnessed a veritable explosion in the adoption of 3D printing technology across a multitude of disciplines, including healthcare. In the field of radiation oncology, the fabrication of patient-specific boluses is an attractive application of this technology due to the relatively low material costs, fast fabrication times and ability to design boluses that conform to complex surface anatomy with a high degree of fidelity. There has been relatively little investigation, though, into the manufacturing techniques used to create these boluses and their effect on the final device performance. In this study, we sought to further evaluate the impact of the orientation of the 3D printed layers that constitute each bolus on the resultant homogeneity of the delivered dose profile. Flat rectangular boluses measuring 5 x 5 x 1 cm were fabricated from polyethylene terephthalate glycol-modified (PETG) filament (density: 1.22 g/cc) using a commercially available 3D printer (Aleph Objects Inc, Loveland, CO). Boluses were fabricated with the layers of PETG oriented either parallel to or perpendicular to the short axis (z-axis) of the bolus. Each block was printed with 100% solid infill. Dose calculations were performed using a treatment planning system with a total of 200 cGy prescribed to Dmax using 9 MeV electrons. EBT3 radiochromic film measurements were obtained at 0, 2 and 10-mm below each bolus using a solid-water phantom. A high degree of concordance was observed between the treatment planning system calculations and film measurements. At a depth of 10 mm, over 92% of points were within +/- 10 cGy of the calculated values. Orientation of the layers of each bolus was not found to significantly alter the device characteristics. Evaluation of the measured dose profiles from the boluses printed with the layers oriented perpendicular to the incident electron beam compared with those oriented parallel to the beam revealed minimal heterogeneity along the X and Y axes with mean absolute differences of 0.6-1.3%. Using a combination of commercially available 3D printers and materials, we were able to produce custom boluses with measured dose distributions in close agreement to the results from the treatment planning system. We additionally demonstrated minimal heterogeneity in the dose profiles between boluses with differing 3D printed layer orientations which suggests that bolus performance is relatively resilient to changes in the selection of the extrusion axis of the device during the fabrication process. While these results are promising, additional studies to more fully characterize the effect of printing technique and materials on the performance of 3D fabricated boluses are warranted.