The radiobiology of radiosurgery and stereotactic radiotherapy.

Radiation therapy has evolved into a complex amalgamation of treatment techniques that differ significantly according to the way the radiation is delivered to the patient and coincidentally according to the biologic effects that are observed with each technique. Although there are concepts within radiobiology that unify the field, it is not apparent that the biologic effects with one methodology of treatment resemble those of another. Radiosurgery, although initially developed in the 1950s, has become more commonly used in recent years. This treatment involves high-dose, single-fraction treatments with sharp dose gradients to small volumes of tissue. This contrasts with conventional external-beam radiotherapy which involves small-dose, multiple-fraction, broad-dose-gradient treatment to relatively large volumes of tissue. Stereotactic radiotherapy generally delivers small-dose, multiple-fraction treatments to small or intermediate volumes of tissue with a sharp dose gradient compared with conventional external-beam treatment. A discussion of these technique differences with reference to the radiobiologic implications may help elucidate the potential utility of the techniques in clinical radiotherapy.