Robust optimization used to mitigate setup uncertainties in vulvar patients receiving VMAT

Many radiation planning techniques have been used to increase dose to the vulvar surface when treating patients with vulvar cancer with volumetric modulated arc therapy. Target volumes near the skin surface, such as vulvar tumors, do not meet the International Commission on Radiation Units and Measurements safety guidelines. Without the needed expansions and setup uncertainties, there are concerns that treated dose to the vulvar surface varies from the planned dose. The purpose of this study was to determine if the robust optimization tool used in volumetric modulated arc therapy planning can reproducibly deliver prescription dose to the vulvar skin surface despite setup uncertainties and reduced safety margins. To further mitigate the possibilities of setup errors and dose variation to the vulvar surface, robust optimization during planning must be considered. For this case study, 5 patients with similar diagnoses and vulvar surface dose requirements were retrospectively replanned using robust optimization. An evaluation structure was created representing the vulvar surface for each patient. This structure was assigned robustness characteristics. After multiple optimizations and planning scenarios, perturbed dose representing dataset shifts were evaluated along the vulvar surface. Every patient met minimal metrics of 100% of vulvar surface volume receiving 95% of the prescription dose. An additional review of 95% of vulvar surface volume receiving 100% of the prescription dose showed similar results. Robust optimization can be used in the planning process to mitigate setup uncertainties.