Automatic skin flash optimization in breast and chestwall VMAT with static angle modulated ports: Effect of HU and flash margin size on plan quality and robustness

Skin flash is typically added to breast and chestwall plans to ensure robust target coverage in the presence of respiratory motion, anatomic changes, and small setup uncertainties. Adding skin flash in volumetric modulated arc therapy (VMAT) plans is an iterative and manual process. RapidArc dynamic (RAD) is a new solution that integrates a dynamic collimator and static-gantry angle modulated ports directly into arc delivery. The automatic skin flash tool (ASF) allows users to automatically add skin flash directly within the optimizer. The user must select the thickness and Hounsfield Units (HUs) of the flash region, but the optimal values are not currently known. For 13 left- and right-sided breast and chestwall patients, RAD plans were created with no skin flash and with ASF with thickness of 5 to 20 mm and HU of -500 to 0 HU. To assess plan quality, DVH metrics for planning target volume (PTV), heart, ipsilateral lung, contralateral lung, and contralateral breast were recorded. To assess plan robustness, the isocenter was shifted 5 mm, moving the target 5 mm anteriorly into the flash region. The changes in clinical target volume (CTV) D95% and D99% were recorded. A paired t-test was used to determine if changes in plan quality or robustness were statistically significant (p ≤ 0.05). The addition of ≥ 7 mm of skin flash resulted in robust plans. Varying the HU did not affect robustness. Increasing the skin flash beyond 10 mm increased PTV V105%. This increase was much larger in the 0 HU plans than in the -350 HU plans. We therefore recommend using -350 HU and 7-10 mm of skin flash for anticipated inter- and intra-fraction motion of 5 mm.