Cancer does not come in shapes that are easy to treat. Tumors extend into surrounding cavities and shove aside healthy organs. They march along routes of blood vessels and creep between muscles, growing into complex three-dimensional shapes. Frequently, normal structures find themselves flanked by malignant cohorts. As described in the introductory article on page 34, selective irradiation of the 3D volume of a malignant cancer is the goal of radiation therapy. The challenge is to deliver a sufficiently fatal dose of radiation to the target volume while minimizing the damage to surrounding normal tissue.
About half of all cancer patients receive radiation therapy, often in combination with other treatments. The most common form of radiotherapy, irradiation with high-energy x rays, has for five decades used multiple exposure angles to build up the dose delivered to the target without excessively exposing the healthy tissue in the vicinity of the target. Today,...
