The first sign of breast cancer often is a breast lump or an abnormal mammogram. Breast cancer stages range from early, curable breast cancer to metastatic breast cancer, with a variety of breast cancer

--3-dimensional breast radiotherapy is the most prevalent techniques, carried out in all radiotherapy centers of the world including Iran. The objective of this study is to investigate the dose rate supplied to the opposite breast during the radiotherapy of breast cancer using the designing method of 3-dimensional therapy. 32 female patients suffering from breast cancer participated in this study. All of them were the candidate of breast radiotherapy. For each patient, a therapeutically designing was specified including. After the approval by physician (radiotherapy oncologist) the patient treatment was performed. Three TLD were used for measuring the intake dose of opposite breast for each patient. Then the doses obtained by TLD was examined. Results indicated that the average of absorptive dose calculated in the internal part of the opposite breast was by TLD (2/12) and had a significant increase (p<0.05) as compared to the above surroundings and nipple center. In addition, the dose of opposite breast in the patients treated with medial wedge in tangent field, increased significantly (p<0.05) compared to the patients without wedge in the tangent field. Conclusively, the use of medial wedge in tangent field causes an increase in the dose absorbed by opposite breast. As the radiation received by opposite non-cancerous breast can put the breast at risk, it is of importance to devise the techniques to reduce the dose of radiation received by opposite breast. Keywords---Breast Cancer, Opposite Breast, Radiotherapy, Risk.