Comparison of Contra lateral Breast & Chest wall doses during Radiotherapy of Ca-Breast (with mastectomy) using Co-60 machine and 6 MV LINAC.

Breast tissues are known to be very radiosensitive to radiation for carcinogenesis. The radiation dose received by the contralateral breast (CLB), therefore should be of primary concern during radiotherapy of primary breast cancer. The purpose of this study was to compare CLB skin entrance dose (SED) and to verify prescribed chest wall dose delivered to Ca-breast patients with mastectomy, treated either with Co-60 teletherapy machine or 6 MV photons from linear accelerator (LINAC) with three fields SSD technique, at NORI, Islamabad. TLD-100 chips were used to measure CLB -SED and chest wall doses. Sixty patients were taken for the measurement of these doses . Chest wall doses were found to be in the range of 2.07-2.25Gy per fraction (3.5% to 12.5% higher than the prescribed dose of 2 Gy per fraction) for Co-60 machine while this dose was measured to be in the range 2.05. - 2.18 Gy (2.5% to 9% higher than the prescribed dose) from LINAC. The CLB-SEDs for patients treated with Co-60 machine were found to be in the range of 3.2 -10 Gy depending on the area of the primary breast to be treated. While these doses to the patients treated with LINAC were in the range of 2.6 -7.5 Gy. For comparison purpose fifteen patients were partially treated both with LINAC and Co-60 machines for dose measurement. For these patients, on average, CLB doses delivered by Co-60 machine were found to be 43% (range: 27 -56 %) higher than those of LINAC for patients with mastectomy. CLB doses for patients with lumpectomy were found to be higher than those with mastectomy for the same lateral separation. The use of conventional half beam block of Co-60 machine was found to be the main contributing factor for higher CLB doses.