The physics of stereotactic radiosurgery

Abstract

High-energy ionizing radiation is the tool of radiosurgery. Three major sources for radiosurgery are gamma rays from Cobalt-60 radioactive isotopes, X-ray beams from linear accelerators, and proton beams from charged particle accelerators. Both gamma and X-rays have the same physical characteristics (i.e., both are photons) and their interaction mechanism with material is stochastic. Three major photon interactions are photoelectric absorption, Compton scattering, and pair production. Protons are positively charged and continuously interact with material as they travel. While the dose deposited by photons drops off exponentially with penetration depth, the dose deposited by protons increases very slowly initially then sharply increases, reaching a maximum before rapidly dropping off to 0. Dose deposition characteristic determines fundamental treatment geometry needed. Three major photon beam delivery systems are Gamma Knife, CyberKnife, and conventional linear accelerator (LINAC). Among them, LINAC is the most popular machine and its major components are electron gun, accelerator tube, bending magnet, and treatment head. Cyberknife uses a compact linear accelerator mounted on a robot while Gamma Knife utilizes many Cobalt-60 sources. Beam size has been conventionally determined by a prefabricated circular collimator. However, use of multileaf collimator is increasing in LINAC. In recent Gamma Knife unit, automatic choice among four different collimator sizes (including zero size) is possible. Cyberknife also provides a continuously varying collimator in its latest version. A significantly large dose is delivered in radiosurgery. Therefore, the impact of inaccurate target localization or beam delivery can be disastrous. To minimize such disasters, radiosurgery quality assurance should be stringent.​