Tissue Dose in Irradiation of the Breast

Irradiation of the breast is carried out by a great variety of technics in different radiotherapy centers. The very number of such methods indicates that no one can be accepted as more effective or more widely applicable than another. Because of variations in stage of the disease and radiosensitivity of the tumor, comparison of results is impossible for any but large series of cases. There is, however, one factor which may be directly compared, i.e., the tissue dose. When carcinoma of the breast is treated by radiotherapy, radiation is usually delivered through two or more portals tangential to the chest wall. Such a method might be recorded as in Figure 1. The determination of tissue dose delivered in this manner is difficult for two reasons. (1) Standard depth dose tables are not intended for tangential beams directed to irregular objects. They are designed for the estimation of a dose delivered when the portal is applied directly to the flat surface of a bulk of tissue large enough to provide the maximum scattered radiation which contributes importantly to tissue dose. (2) The error in attempting to duplicate the position and direction of the tangential beam from day to day during the course of treatment may be appreciable. These objections may be overcome by the use of two devices which are employed in some centers. Bolus material may be used to build up irregular parts to a bulk of uniform density. Rice, or rice flour and sodium bicarbonate (2), in small bags is commonly used. We have used rubber balloons filled with water (Fig. 2A). Water is the ideal unit density material, and rubber balloons are convenient and durable and mold readily to each other and to irregular skin surfaces. For beam direction, boxes or frames have been employed (1). A design we have adopted (Fig. 2B) has two plastic end pieces for contact with the end of the treatment cone, and elastic side walls for adaptation to the body contour. A 10 × 15-cm. cone directly applied to the plastic end piece, first on one side and then on the other, will assure directly opposed beams at a constant separation of 16 cm. If the breast is thus enclosed and the box filled with bolus material, standard depth dose data become applicable. There is an objection to the expression of tumor dose as a single value. This is usually given for the point of intersection of the central rays of two or more beams, a point which receives neither the maximum nor the minimum radiation delivered throughout the irradiated tissue. A knowledge of the maximum dose is desirable to avoid over-irradiation. A knowledge of the minimum dose is necessary, since an inadequate dose in the periphery of the irradiated field may be responsible for unsuccessful treatment. It is a surgical principle to resect a wide margin of normal tissue because of possible microscopic direct extension or lymphatic spread of cancer.