Development of an in vivo technique for dose verification at the prone breast board / skin interface

Due to the limited height of commercial prone breast boards, large or pendulous breasts may contact the base layer of the board during simulation and throughout the course of treatment. Our clinic has historically identified and marked this region of contact to ensure reproducible setup. However, this situation may result in unwanted hotspots where the breast rests atop the board due to electron scatter. In this study, we performed in-vivo dosimetric measurements to evaluate the surface dose in regions of contact with the immobilization device. The average dose and hotspot were identified and evaluated to determine whether plan modifications were necessary to avoid excess skin toxicity at the skin/breast board interface. The film method results were validated against a commissioned in vivo OSLD dosimetry system. Radiochromic film measurements agreed with OSLD readings (n = 18) overall within 1%, σ = 6.4%, with one deviation of >10%. Pertinent information for the physician includes the average, maximum, and minimum doses received at the film interface. Future readings will not require OSLD verification. Physicians now have access to additional spatial data to correlate skin toxicity with doses delivered at the skin/breast board interface. This new technique is now an established procedure at our clinic, and can inform future efforts to model enhanced methods to calculate the dosimetric effects from the prone breast board in the treatment planning system.