Dose Tolerances in Brain Metastasis Management

Stereotactic radiosurgery (SRS), given in a single or few fractions (2–5), is the recommended treatment for patients with a limited number of brain metastases. Both single-fraction and multi-fraction SRS are effective treatment options for patients with brain metastases, with a relative risk of neurological complications. The end points typically used for assessing radiation-induced complications in the brain are the development of brain radiation necrosis, which is associated with the presence of different degrees of neurological deficits, and neurocognitive deterioration. In general, normal tissue toxicity during SRS appears to be a function of radiation dose, volume, and proximity to eloquent sensitive brain structures. For single-fraction SRS, a clear correlation has been demonstrated between the target volume and the risk of brain necrosis, with most of these data reported by the “Quantitative Analysis of Normal Tissue Effects in the Clinic” or QUANTEC papers published in 2010 by a joint American Association of Physics in Medicine (AAPM) and American Society of Therapeutic Radiology and Oncology (ASTRO) committee. For large lesions or those in close proximity to critical structures, multi-fraction SRS is usually utilized when high-dose single-fraction SRS would result in unacceptable risks of severe neurological toxicity; however, there is little systematic reporting on normal tissue dose constraints and risk of long-term toxicity. Future prospective studies would provide robust data on normal brain constraints, including dose-volume information for different SRS schedules, evaluation of neurocognitive status through formal testing, and toxicity of combined SRS and systemic treatments. This chapter summarizes the dose tolerance limits of normal tissues for patients receiving SRS treatments to the brain.