SU‐E‐T‐367: Dosimetric Comparison of VMAT and S&S IMRT Regarding the Dose Delivered to the Skin in Different Cancer Sites

Purpose: The risk for radiation induced skin complications can limit the effectiveness of radiation therapy when large areas of skin are involved in the treatment. Therefore, the accurate determination of skin dose may be critical in preventing post‐therapy side effects. The purpose of this study is to quantify the skin dose received by the Volumetric Modulated Arc Therapy (VMAT) and a 7 field MLC‐based IMRT step and shoot (S&S) delivery modalities using the same dose prescription. Methods: 5 prostate, 5 head‐and‐neck, and 5 brain cancer patients were used in this study to quantify skin dose for the examined radiation modalities. 30 treatment plans were produced (15 VMAT and 15 IMRT S&S) and for each patient the skin dose was measured using ultra‐thin TLDs. Doses were measured at 11 points on the surface of a cheese phantom at 25 degree intervals using ultra‐thin lithium‐fluoride TLDs with surface dimensions of 0.3cm × 0.3cm and a thickness of 0.01cm, comparable to the thickness of the skin. For each patient, the corresponding doses were also calculated using the Pinnacle treatment planning system. Results: A 36% increase in skin dose was measured on the brain patients using VMAT vs. IMRT S&S while a 38% decrease in skin dose was measured on the H&N patients using VMAT vs. IMRT S&S. The prostate patients showed just a 12% increase in skin dose using VMAT vs. IMRT S&S. The skin doses from Pinnacle showed small differences between the two modalities. The absolute differences of the dose values between the plan and the measurements were within an order of magnitude of the measured doses. Conclusion: The skin dose differences between the VMAT vs. IMRT S&S modalities were found to be significant. However, the direction of the differences was found to be treatment site specific and its magnitude patient specific.