For instance, in one particular technique of prostateradiotherapy, a patient will be treated with 5 radiation beams of 15 MV X-rays. While the beams will point into the patient from different angles, all the beams will be pointed towards one point or centre in the prostate (so called an isocentric technique). In this way the linac can rotate around the patient who does not need to move. In fact, patient motion is a source of positioning error which a radiation therapist tries to eliminate.
By crossing multiple beams, the radiation dose delivered internally in the prostate is much higher (~70-78 Gy) than the radiation dose delivered on the entry and exit tracks of each individual beam (~40 Gy). This lower dose outside the prostate rarely leads to side effects or any detectable change.
By convention, the voltage is used to characterize X- and gamma-ray beams (in volts), whilst electron beams are characterized by their energies (in electronvolts).