Glioma & Radiation-Related Tumors

1.An infiltrative target in a focal-dose tool

Radiosurgery is built for discrete, well-circumscribed targets. Diffuse gliomas are the opposite: they infiltrate beyond any enhancing margin, so a conformal high-dose shell inevitably misses the cells that matter. This mismatch is why SRS is not standard for newly diagnosed glioma. The point was tested directly: RTOG 93-05 randomized newly diagnosed GBM (≤ 4 cm) to conventional radiotherapy plus carmustine, with or without an upfront SRS boost of 15–24 Gy assigned by lesion size, and found no survival benefit from the boost (median survival 13.5 vs 13.6 months), with no difference in quality of life or cognitive decline. The trial predates the Stupp regimen, so its chemotherapy backbone was carmustine rather than temozolomide, but its conclusion about the boost has not been overturned. Standard care today is fractionated radiotherapy with concurrent and adjuvant temozolomide.

2.Radiosurgery and re-irradiation as salvage

At recurrence, focal re-irradiation (including SRS/SRT for small, focal recurrences) is used as salvage, individualized rather than standard. The key randomized data come from NRG Oncology/RTOG 1205, which tested hypofractionated re-irradiation (35 Gy in 10 fractions) plus bevacizumab versus bevacizumab alone for recurrent GBM: re-irradiation improved 6-month progression-free survival (~54% vs ~29%) but did not improve overall survival (median ~10.1 vs 9.7 months), and was well tolerated. The reading is that focal re-irradiation can buy local control and time, not cure. Bevacizumab may reduce edema and symptomatic treatment effect, but it does not eliminate the risk of radionecrosis in a re-irradiated brain (see the adverse radiation effects page).

3.Where focal radiation occasionally fits

Diffuse low-grade gliomas are likewise infiltrative and are treated with fractionated radiotherapy (with chemotherapy) when RT is indicated, not single-fraction SRS. Radiosurgery is occasionally considered for small, well-circumscribed, focal nodular recurrences and for more discrete lesions such as selected pilocytic astrocytomas or ependymomas in surgically difficult locations: the common thread being a genuinely focal, marginated target rather than an infiltrating one.

4.Tumors caused by prior radiation, and the safety of SRS itself

Years to decades after cranial irradiation, a radiation-induced neoplasm may arise in the prior field: most commonly a meningioma, less often a glioma or sarcoma. The modified Cahan criteria define the diagnosis: the tumor (a) arises within the previously irradiated field, (b) after a sufficient latency (preferably > 4 years), and (c) is histologically distinct from the original irradiated tumor (with histologic confirmation). Radiation-induced meningiomas can themselves be treated with radiosurgery. Reassuringly, the risk of SRS itself inducing a new tumor or driving malignant transformation is very low, large single-fraction series with decades of follow-up estimate it at a small fraction of a percent, a point worth conveying in consent.