Stereotactic Radiosurgery in the Management of Brain Metastasis

Michael L. Smith, M.D.; John Y. K. Lee, M.D.

Disclosures

Neurosurg Focus. 2007;22(3) 

In This Article

Radioresistance and Radiosensitivity to SRS

Traditional concepts of radioresistance and radiosensitivity to fractionated external beam radiation may not correlate with the response of brain metastasis to SRS. For example, brain metastases from melanomas and sarcomas have traditionally been considered radioresistant based on their response to WBRT (3 Gy per fraction). In contrast, Mehta et al.[30] evaluated volumetric response rates based on the histological characteristics of the lesions. They found complete response to treatment in 100% of lymphomas, 67% of melanomas and sarcomas, 50% of non-small cell lung cancers, 33% of breast cancers, and 11% of renal cell carcinomas. Again, tumor stabilization or shrinkage noted radiologically did not correlate with clinical outcome. Patients with more radioresistant tumor types often fared better after SRS than those with radiosensitive types. In a multiinstitutional review, patients with melanomas, breast cancer, and renal cell carcinomas treated with SRS survived longer than patients with other lesion types.[18] In a review of studies assessing radiological regression and local control, Boyd and colleagues[7] noted that traditionally radiosensitive tumors did show more complete radiographic regression than the radioresistant tumors. However, clinically relevant local control rates were as good or better for the "radioresistant" types as the "radiosensitive" types.[7]

The deviation of SRS responses from the traditional definition of radiation resistance may have to do with the different mechanism of killing cells compared with fractionated methods.[34,37] This may be due to a different impact on tumor vasculature.[23] A single high dose of radiation delivered by SRS can provide local control for tumors that are resistant to standard radiation therapy.

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