Healthy tissue sparing
Indeed, some forms of radiation, specifically x-rays or photons, do penetrate through your entire body and will deposit some dose along their entire path of travel. However, the design of common linear accelerators is such that they can rotate around a single point where the radiation from different directions will add up together to collect significant dose. Tissues away from this common point will only see a small fraction of the radiation paths and collect minimal dose. This is similar to how a magnifying glass can focus the sun’s rays down to a single point and cause a quick burn on your hand while putting your hand out into the direct sun will simply feel a little bit warm.
Answered by WGATE