A Novel Proton Therapy Technique for Treatment of Postmastectomy Breast Cancer Patients with Tissue Expanders Containing High-Density Metallic Filling Ports

As part of the postsurgical reconstructive process, many postmastectomy breast cancer patients have a tissue expander in place at the time that radiation therapy is indicated. Tissue expanders often include a high-density filling port that can introduce large uncertainties in the calculation of proton dose around and specifically through the metallic filling port. Newer treatment planning techniques, coupled with pencil beam scanning proton delivery, enable greater flexibility to the treatment planner. The purpose for this study was to determine if the current planning and delivery systems can be configured to maintain appropriate target dosing while treating around the metal filling port. Such a method would eliminate the added complexity of calculations of particles passing through the metallic port. A technique using advanced proton treatment planning systems has been developed to eliminate the large uncertainties and inaccuracies in dose calculation algorithms when calculating dose through high-density, metallic filling port. The method uses 2 pencil beam proton fields that are optimized with constraints that eliminate any proton spots from treating downstream of the high-density filling ports. Areas of lower dose created by the shadowing of the filling port are brought to prescriptions doses by the complementary proton field using multifield optimization methods. Using the method described, treatment plans have been developed that demonstrate appropriate target coverage to the target areas. To minimize the sensitivities to setup errors, the target contours on specific axial images were broken up into 2 types: (1) target contours that occurred on axial slices that included the metallic port and (2) target contours that occurred on axial slices that did not. In the area where the port was present, multifield optimization was used to treat around the port. In the areas where the port was not present, each field was optimized to deliver half of the prescribed dose in a uniform manner to the target areas, independent of the other field. This segregation of optimization areas was used to increase overall plan robustness. This novel intensity modulated proton therapy planning method enables prescribed doses to be delivered to patients with a tissue expander in place, even in the presence of a high-density filling port. Sensitivities to setup error can be reduced by separating the target areas into distinct portions that are optimized using single-field uniform dose methods or multifield optimization methods depending on the relative position of the filling port.