Evaluation of the Skin Dose Increase with the Use of a Novel Immobilization Device in Prone Breast Radiotherapy

<h3>Purpose/Objective(s)</h3> This planning study investigates the effects of a new, 3D printed, immobilization device designed for prone breast radiotherapy in Partial Breast Irradiation (PBI). The device, with a 5mm thickness, allows skin dose as calculated by the treatment planning system to match closely with delivered doses. In this study, we evaluated the extreme case in which the full breast would be irradiated and the effects of planning techniques and beam energy on the dose to the skin. <h3>Materials/Methods</h3> Utilizing an anthropomorphic phantom (RANDO), four configurations of breast setups were examined: without a support device (BrOnly), with a cup featuring mesh (MeshC) or full design (SolidC), and with a complete immobilization device (Shell). In total 16 plans were created for a PTV cropped at 3mm from the skin, using 6 and 10 MV photon beams, employing tangential IMRT (2F) and VMAT (VM) techniques. The dose prescription was set to 40.05 Gy in 15 fractions. Dose to the skin was analyzed from dose-volume metrics for a structure of 3mm thickness located between the PTV and the phantom surface ("skin3mm") and a full-body 5mm thick structure extending within the PTV ("SkinBody5mm"). <h3>Results</h3> All plans achieved correct PTV coverage. 10MV plans exhibited superior skin sparing compared to 6MV plans for Skin3mm. This structure was fully covered by the 50% isodose (20Gy), with variations observed only when analyzing doses to the SkinBody5mm structure. Despite a PTV cropped at 3mm from the skin and the use of the shell the volume of structure SkinBody5mm receiving 20Gy was below 218cc when using VMAT. According to recent literature, this is safe for moderate-severe late toxicities. Doses to skin3mm were up to 40Gy to more than 1.5cc only when using full cup design, see table. <h3>Conclusion</h3> The novel immobilization device created for PBI exhibited good skin dose control with VMAT even when using the largest target possible, i.e., the whole breast. As previously shown, this MR compatible device ensures a reproducible setup for treatment, which can allow optimization of PBI protocols, accurate delivery of planned doses, while minimizing late toxicity to organs at risk, including the skin.