SU‐E‐T‐332: Determination of Skin Dose during Smart Arc Delivery

Purpose: Treatment Planning systems (TPS's) inaccurately quantify the dose delivered to the skin during radiation therapy, due to their inability to account for all surface dose contributing factors. The purpose of this work was to use near tissue equivalent ultra‐thin Thermo Luminescent Dosimeters to determine and compare the average skin dose for Prostate patients using the Volumetric Arc Therapy (VMAT) Delivery System with measurements taken on other delivery systems. Method and Materials: Reference beam measurements were taken at energy of 6MV for both a 10×10 open field and for an open, unmodulated arc with a 10×10 field from multiple angles. Five (n=5)_prostate patients using the VMAT delivery system were evaluated. The dose per fraction was set to 2 Gy for all patients. Two TLDs were placed at discrete locations, every 25 degrees, on the periphery of the “cheese” phantom. Results: The average skin dose for all locations of interest for the five prostate patients varied from 5.25cGy to 7.47cGy. Compared to previous work done with skin dose measurements, the VMAT delivery system delivers an overall lower skin dose than IMRT step and shoot, Helical Tomotherapy, and Serial Tomotherapy. Conclusions: The VMAT delivery method delivers lower dose to the surface than other IMRT delivery methods. The lower doses probably due to lower monitor units required for VMAT compared to static field IMRT.