Minimizing the Risk of Acute Skin Toxicity Due to Hip Prosthesis Avoidance in Prostate Radiation Therapy

Limiting the entrance dose through hip prostheses to improve dosimetric accuracy can result in unfavorable skin toxicity. We propose a volumetric modulated arc therapy solution that strikes a better balance between dose accuracy and skin dosimetry. Our current planning strategy limits the entrance dose through hip prostheses using stringent optimization objectives on an avoidance structure. Avoidance efficiency is evaluated by recalculating the plan with prosthesis density set at 20 g/cc, and evaluating the loss of target coverage from increased attenuation. We require this loss to be ≤5% of the original values. This approach has resulted in an uncommon skin toxicity for a prostate-bed patient with bilateral hip prostheses. Thus, the dosimetric tradeoffs between skin dose and prosthesis avoidance were investigated by incrementally reducing prosthesis avoidance to achieve maximum skin doses (D_max) between 30 and 50 Gy. When prosthesis avoidance is prioritized, the skin dose increases and the target dose coverage and conformity decrease. A large degradation in target coverage for plans with the lowest skin D_max of 30 to 35 Gy indicates that a significant proportion of the target dose arises from beams entering the prostheses. The plan with a skin D_max of 40 Gy provides a better compromise between skin and prosthesis entrance doses, with a <20% reduction in target coverage at an increased prosthesis density of 20 g/cm³. Skin dose needs to be considered when using prosthesis avoidance planning strategies. Allowing for a minimal dose through the prosthesis may be required to restrict skin dose and reduce the risk of toxicity.