The linear accelerator provides very precise, uniform irradiation for stereotactic radiosurgery of brain tumors. Importantly, this device allows “fractionation” of treatment that allows the safe administration of a higher dose of radiation than can be given with the machines using multiple cobalt sources. The linear accelerator produces radiation having a higher energy than that produced by the cobalt-source machine. Further, the collimators or beam-shaping devices can be larger for the linear accelerators, resulting in much greater uniformity of dose for the larger tumors.
The cobalt source machines are also very precise. However, because the frame has to be bolted on to the patient’s head with metal bolts, fractionation of treatment is not possible. Further, the cobalt source machines have smaller collimators that may render larger tumors more difficult to treat with a homogeneous dose of radiation.
The proton radiosurgery derives its advantage from the so-called “Bragg peak” that describes deposition of radiation dose from proton beams. As the protons in the beam slow down in tissue, they give up (deposit) disproportionately more radiation per unit of travel. Just before the protons stop, they give up almost all their energy, resulting in a “peak” at that depth in tissue. The depth can be precisely defined by the energy imparted to the proton beam by the cyclotron that produces the beam. Proton beam therapy is useful for many skull base tumors and vascular malformations of the brain.
The Peacock system uses “inverse” treatment planning to make a very conformal distribution of the radiation dose in the tumor. It works in a way similar to a CT scanner to precisely determine the amount (weight) for each of many small beams that irradiate the target. This system also allows fractionation.