Stereotactic Radiosurgery · Special Topics
Glioma & Radiation-Related Tumors
Where radiosurgery is salvage, investigational, or off the standard path
This page is deliberately a page of caveats. Radiosurgery is built for discrete targets, and diffuse glioma is the canonical poor fit — so the role of SRS here is salvage local control at recurrence, never first-line cure, and the randomized evidence says so plainly. The page also covers the converse problem: tumors caused by prior radiation, the criteria that define them, and the reassuringly low risk that radiosurgery itself induces a neoplasm.
Orientation
Read this page as a boundary marker. Its purpose is partly to say where radiosurgery does not belong: not as an upfront boost for glioblastoma, not as a substitute for fractionated radiotherapy in infiltrative low-grade glioma. Where it does appear — focal re-irradiation of recurrent high-grade glioma, treatment of a radiation-induced meningioma — the intent and the evidence should be stated honestly.
Why Glioma Resists the Radiosurgical Paradigm
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 9305 randomized newly diagnosed GBM (< 4 cm) to standard external-beam RT + chemotherapy with or without an upfront SRS boost and found no survival benefit from the boost. Standard care remains fractionated radiotherapy with temozolomide.
Recurrent High-Grade Glioma
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 buys local control and time, not cure, and that bevacizumab both partners with it and mitigates radionecrosis — which is a real hazard in the re-irradiated brain (see the adverse radiation effects page).
Low-Grade and Circumscribed Edge Cases
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.
Radiation-Related Tumors
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.
| Scenario | Role of SRS / focal RT | Evidence anchor |
|---|---|---|
| Newly diagnosed GBM | Not indicated as upfront boost | RTOG 9305 — no survival benefit |
| Recurrent GBM, small focal | Salvage local control (hypofractionated re-RT, e.g., 35 Gy/10 fx) | RTOG 1205 — improved PFS6, not OS |
| Diffuse low-grade glioma | Not indicated; fractionated RT territory | Infiltrative target |
| Discrete pilocytic / ependymoma | Occasionally, if focal and surgically difficult | Series; individualized |
| Radiation-induced meningioma | Treatable with SRS like a sporadic meningioma | Modified Cahan criteria for diagnosis |
Key points
- Glioma is infiltrative; SRS misses the margin. RTOG 9305 showed no benefit from an upfront SRS boost in newly diagnosed GBM — standard care is fractionated RT + temozolomide.
- Recurrent GBM: RTOG 1205 (35 Gy/10 fx re-RT + bevacizumab vs bevacizumab) improved PFS6 (~54% vs ~29%) but not OS — salvage local control, not cure.
- Low-grade glioma is fractionated-RT territory; SRS is reserved for small, circumscribed, focal recurrences or discrete lesions (selected pilocytic/ependymoma).
- Radiation-induced tumors (meningioma > glioma/sarcoma) follow the modified Cahan criteria: in-field, latency > 4 years, distinct histology.
- The risk of SRS itself inducing a tumor or malignant transformation is very low — useful for consent.
References
- Tsien CI, Pugh SL, Dicker AP, et al. NRG Oncology/RTOG 1205: a randomized phase II trial of concurrent bevacizumab and reirradiation versus bevacizumab alone for recurrent glioblastoma. J Clin Oncol. 2023;41(6):1285–1295. PubMed
- Souhami L, Seiferheld W, Brachman D, et al. Randomized comparison of stereotactic radiosurgery followed by conventional radiotherapy with carmustine for glioblastoma (RTOG 9305). Int J Radiat Oncol Biol Phys. 2004;60(3):853–860. DOI
- Cahan WG, Woodard HQ, Higinbotham NL, et al. Sarcoma arising in irradiated bone: report of eleven cases (origin of the Cahan criteria). Cancer. 1948;1(1):3–29.
- Wolf A, Naylor K, Tam M, et al. Risk of radiation-associated intracranial malignancy after stereotactic radiosurgery: a retrospective, multicentre, cohort study. Lancet Oncol. 2019;20(1):159–164. DOI
Educational synthesis for neurosurgery and radiation-oncology trainees; not a treatment directive. The uses described here are largely investigational or salvage; management belongs in neuro-oncology multidisciplinary review. Glioma and radiation-induced tumor references verified against PubMed/DOI records during review.