Normal cells are better at healing themselves than cancer cells. Multiple treatment “fractions” allow normal cells a chance to repair, while the intended radiation damage accumulates in the cancer cells. Also, small imperfections in the treatment setup can exist, such as exact patient positioning, body motion, internal organ motion, and the positions of moving parts in the treatment machine. By spreading the radiation therapy over a number of fractions, those small imperfections average out and their effect is decreased.
Some types of cancer benefit from much higher dose per treatment. In that case, the physician may recommend “stereotactic” therapy in 5 or fewer fractions. In order to minimize technical imperfections, stereotactic treatments may use more devices to prevent patient motion and more imaging for accurate positioning
Answered by WGATE