SU‐F‐T‐317: Skin Exposure Outside the Treatment Field During Treatment of Breast Cancer with Tri‐Co‐60 MR‐IGRT System

Abstract

Purpose:To investigate exposure outside the treatment field when treating breast cancer with tri‐Co‐60 magnetic resonance (MR) image guided radiation therapy (IGRT) system.Methods:A total of 7 patients who treated with accelerated partial breast irradiation (APBI) technique were selected prospectively for this study (prescription dose = 38.5 Gy in 10 fractions). Every patient treated with two plans, one was an initial plan and the other was an adaptive plan generated after finishing 5 fractions (a total of 14 plans). Every plan was calculated with and without magnetic field in the treatment planning system. The EBT3 films were attached on the front and the back of 1 cm bolus, and then it was placed on the patient body vertically to cover patient's jaw and shoulder. After measurements, the maximum point dose and the mean dose of whole area of EBT3 film were acquired.Results:In the treatment plan with magnetic field, low dose stream outside the patient body was observed, almost reaching the patient's jaw or shoulder, while it was not observed without magnetic field. The average values of the measured maximum and mean doses at the front of bolus were 30.1 ± 11.1 cGy (7.8% of the daily dose) and 14.7 ± 3.3 cGy (3.8%), respectively. At the back of bolus, those values were 6.0 ± 1.9 cGy (1.6%) and 5.1 ± 1.6 cGy (1.3%), respectively. The largest maximum dose at the front was 54.2 cGy (14.1%) while it was 20.7 cGy (5.4%) at the back. The average decrease of the maximum dose by the bolus was 24.0 ± 11.0 cGy.Conclusion:Due to magnetic field, dose stream outside the patient body can be generated during breast cancer treatment with the tri‐Co‐60 MR‐IGRT system. Since this dose stream irradiated skin outside the treatment field, it should be shielded.This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1C1A1A01054192).