A study on dose of the junction in radiotherapy of breast cancer including SCL

A purpose of this study was to compare dose of junction between breast and SCL fields in radiation therapy by MLC located at the junction. With 6 MV of 21EX-S equipped with 120-leaf Millennium MLC, treatment plans were designed with 30 patients who underwent radiation therapy using TFT. Plan 1 where the MLC was all used at the junction. In plan 2 and plan 3, MCLs were retracted 5 mm from breast and SCL, respectively. Plan 4 with all of MLC retracted at the junction were designed. In all of the plans, collimator angle for SCL field was divided into 0° and 270°. To verify junction dose, the dose at 3cm depth of junction was compared with average value by MapCHECK. In case of the SCL field with 0° collimator angle, average value of D3cm was 4131.1, 4215.9, 4351.4, and 4423.0 cGy. In case of the SCL field with 270° collimator angle, average value of D3cm was 4044.3, 4246.7, 4291.1, and 4441.2 cGy. In plan1 and 3, change in average dose depending on collimator angle was changed more significantly than paln2 and 4. Dose measured at 3cm depth of junction was similar to treatment plan. In radiation therapy plan for breast cancer with SCL , retracting MLCs from junction between breast and SCL fields will lead to decrease effect of dose of the junction