A technic for the treatment of the lymph nodes along the internal mammary vessels in breast cancer has been developed in which radioactive sources are inserted into the internal mammary vessels. Usually the internal mammary artery is chosen. At the conclusion of the radical mastectomy but before closure of the skin flaps, a small transverse incision is made in the first intercostal space on the side of the operation. The internal mammary vessels are exposed without entering the pleural cavity, and the internal mammary artery is ligated as close as possible to the first rib. With a knife, a small slit is made in the artery just below the ligature. A small nylon tube, whose leading end has previously been closed by heat, is introduced through the slit into the artery. The tube can usually be threaded down for approximately 15 to 25 cm., to a point below the level of the xiphoid process. A ligature is then tied around the artery directly below the slit to prevent back bleeding. The end of the nylon tube outs...