Since ionizing radiation has been effectively employed in the management of carcinoma of the breast, many technics have been developed for postoperative and inoperable cases. In as much as the response to radiation of both cancer and normal tissue depends on the dose, the basic information which is fundamentally necessary regarding any technic is the exact dosage to the primary tumor, the regional lymphatic drainage areas, and the normal lung tissue. The study to be described here was done to determine the dose delivered at these significant anatomic sites by the different radiation technics which are currently used or have been used in the treatment of inoperable and postoperative carcinoma of the breast. Technics Studied The technics studied either had been or are being used at the Memorial Center (New York) . They represent two types of direct-portal technics and one type of tangential technic using x-radiation of two different energies. The details of the tangential technic with 250 kevp and 1,000 kevp x-rays have been reported by Chu et al. (1). A brief description, however, will be provided here. By using three fields, medial tangential, lateral tangential, and supraclavicular, the breast, chest wall, and internal mammary, axillary, and supraclavicular areas are irradiated en bloc. The medial and lateral fields, averaging 26 × 12 cm. each, are tangential to the surface of the chest wall. Together they irradiate the breast, the chest wall, and the axillary, internal mammary, infraclavicular, and part of the supraclavicular lymphatic areas. The edge of the medial tangential field lies on a line 2 cm. beyond the midline of the sternum on the contralateral side. The skin margin of the lateral tangential field lies along the posterior axillary line. The supraclavicular field, averaging 14 × 17 cm., is so directed that its central axis is approximately parallel to the sagittal plane of the body. It irradiates the supracla vicular and axillary lymphatic areas and contributes to the dose delivered to the infraclavicular area, the chest wall, the breast, and the internal mammary nodebearing areas. The medial border of this field passes through the sternal notch. The lateral edge of the field coincides with the anterior axillary line with the arm abducted. The anterior margin is 2 cm. anterior to the most prominent part of the breast or chest wall. The posterior margin follows the anterior border of the trapezius muscle. Bolus is used with all fields in the tangential technic. Technic I: 250-kevp Tangential: The 250-kevp tangential method was used at Memorial Center for postoperative treatment from 1949 to 1953 and is still employed for palliation of some inoperable breast carcinomas and recurrent chest wall lesions. The treatment is planned to deliver a tissue dose, calculated at the intersection of the three beam axes, of 3,500 to 4,000 r in three and one-half weeks. The factors used on the phantom in this study were: