Stereotactic Radiosurgery · Technical Foundations

Dose Selection & Fractionation

From a nominal disease dose to a plan the target and normal tissue can both tolerate

A practical framework for choosing a radiosurgical starting dose, deciding when a single fraction is no longer the right tool, and translating Gamma Knife, LINAC, and robotic prescriptions without confusing platform convention for radiobiology.

Timoteo Almeida, MD, PhD | Departments of Neurological Surgery and Radiation Oncology

Orientation

A dose table is useful only if it teaches why the number moves. The diagnosis supplies a starting range; the final prescription is then constrained by target size and volume, treatment intent, proximity to critical structures, prior radiation, normal-tissue dose, and whether the target can safely wait for a fractionated course. The same nominal diagnosis can therefore lead to single-fraction Gamma Knife, hypofractionated LINAC treatment, staged radiosurgery, preoperative SRS followed by resection, surgery followed by cavity SRS, or no radiosurgery at all.

Part I

The Selection Logic

1.Start with intent, not the machine

First define what radiation is expected to accomplish. A metastasis needs durable local control before systemic progression; a benign schwannoma needs long-term growth arrest with cranial-nerve preservation; an AVM needs nidus obliteration after a latency period; a functioning pituitary tumor needs biochemical remission; and functional radiosurgery deliberately creates a lesion in a normal-appearing structure. These are different biological endpoints and should not share a single undifferentiated dose rule.

2.The seven modifiers that move the dose

Size and volume: as the target enlarges, the volume of normal tissue receiving intermediate dose rises and fractionation becomes more attractive.
Critical-structure proximity: optic apparatus, brainstem, cochlea, spinal cord, and eloquent brain may cap the deliverable single-fraction dose.
Prior radiation: prior WBRT, SRS, fractionated cranial RT, or spine RT changes normal-tissue tolerance and sometimes the expected tumor response.
Treatment intent and biology: tumor control, endocrine remission, vascular obliteration, and functional lesioning require different dose intensity.
Target stability: recent cavity change, edema, steroids, hemorrhage, or rapid tumor growth may require updated imaging and a revised plan.
Plan quality: coverage, conformity, gradient, V12Gy or analogous normal-tissue metrics, and OAR dose determine whether the nominal prescription is actually acceptable.
Systemic therapy: CNS-active targeted agents and immunotherapy change the local-control expectation and the timing question, and concurrent administration has been associated with higher rates of symptomatic radiation necrosis in retrospective series. Record what the patient is on, and when the last and next doses fall, before fixing the prescription.

Working sequence Define intent → verify current imaging → choose the evidence-based starting range → test target coverage and OAR constraints → inspect normal-tissue dose and gradient → fractionate, stage, operate, or change modality if the therapeutic ratio is poor.

3.Prescription dose is not maximum dose

For tumors and AVMs, the clinically quoted number is usually a margin or prescription dose covering the target. For trigeminal neuralgia and thalamotomy, the quoted number is conventionally a maximum dose delivered through a 4-mm shot. Thus, a trigeminal plan reported as 80 Gy is 80 Gy at the shot maximum, with the 50% isodose falling at 40 Gy. Read against a tumor prescription of 40 Gy at the margin, the two numbers look comparable and are not. Mixing these conventions in one column without labeling them creates a dangerous false comparison.

Part II

Intracranial Tumors

4.Representative starting ranges

Representative literature ranges for orientation. Final dose follows current disease guidance, the adopted institutional constraint set, and multidisciplinary plan review.
Target	Representative dose	Dominant modifier	When the table stops being enough
Intact metastasis <2 cm	20–24 Gy / 1 fx	Location, prior RT, V12Gy, systemic context	OAR proximity or excessive normal-brain exposure
Intact metastasis 2–4 cm	15–18 Gy / 1 fx or 27 Gy / 3 fx to 30–32.5 Gy / 5 fx	Volume, symptoms, edema, eloquence. The 15–18 Gy figures follow the RTOG 90-05 maximum tolerated doses of 18 Gy for 21–30 mm and 15 Gy for 31–40 mm, established in previously irradiated patients and best read as a ceiling reference rather than a de novo prescription	Consider surgery for mass effect; favor fractionation as volume rises
Postoperative cavity	Size-adapted single fraction, or commonly 24–27 Gy / 3 fx to 30–32.5 Gy / 5 fx	Cavity size, dural contact, timing, brain exposure	Large, irregular, perioptic, or brainstem-adjacent cavities
Vestibular schwannoma	12–13 Gy margin	Baseline hearing, cochlear dose, Koos grade, brainstem contact	Large tumor, mass effect, hydrocephalus, or need for decompression
Other cranial-nerve schwannoma	~11–14 Gy margin	Nerve of origin, brainstem, cochlea, optic apparatus	Dumbbell extension, major mass effect, or uncertain diagnosis
Meningioma, presumed WHO grade I	~12–16 Gy margin, often 13–15 Gy	Optic proximity, edema, volume, grade confidence	Perioptic disease, rapid growth, marked edema, or suspected higher grade
Pituitary, nonfunctioning	~14–16 Gy margin	Optic dose and long-term gland/stalk exposure	Chiasmatic compression or inability to meet optic tolerance
Pituitary, functioning	~18–30 Gy margin	Secretory subtype, optic dose, normal gland/stalk	Insufficient optic separation or need for urgent endocrine/visual control
Craniopharyngioma	~12–15 Gy in selected single-fraction cases	Optic apparatus, cyst dynamics, prior surgery/RT	Perioptic or changing cystic targets generally require another strategy
Hemangioblastoma	~15–18 Gy margin	Solid nodule, cyst, volume, VHL burden	Symptomatic cyst or mass effect requiring surgery
Glomus / paraganglioma	~13–16 Gy margin	Cranial nerves, temporal bone, secretory/multifocal disease	Obtain endocrine and genetic assessment when appropriate
Chordoma / chondrosarcoma	No routine standalone single-fraction dose	High total dose, brainstem/optic proximity, prior RT	Usually resection plus high-dose fractionated or particle RT; SRS is selective boost/salvage
Diffuse glioma	No routine upfront SRS dose	Infiltrative biology and prior chemoradiation	Focal reirradiation is individualized salvage, often fractionated

Do not let the lookup table create an indication A listed dose does not mean radiosurgery is the correct treatment. Chordoma, diffuse glioma, large symptomatic metastasis, higher-grade meningioma, and a compressive sellar lesion are examples where surgery, fractionated radiation, systemic therapy, or another modality may be the real first decision.

Part III

Vascular & Functional Targets

5.Targets whose endpoint is not tumor shrinkage

Vascular and functional prescriptions use different endpoints and, for functional targets, a different dose-reporting convention.
Indication	Representative dose	Convention	Selection constraint
Brain AVM	~16–25 Gy, often 18–21 Gy	Nidus margin dose	Volume, eloquence, latency hemorrhage risk, and likelihood of obliteration
Cavernous malformation	~12–13 Gy	Lesion margin dose	Selected surgically inaccessible lesions with prior symptomatic hemorrhage; evidence remains limited
Trigeminal neuralgia	~80–90 Gy	Maximum dose, usually one 4-mm shot	Target along the nerve, brainstem exposure, prior procedure, relief-versus-numbness trade
Vim thalamotomy	~130–140 Gy (reported series span 130–150 Gy)	Maximum dose, usually one 4-mm shot	Unilateral treatment; delayed irreversible lesion; reserve for an appropriate non-DBS/non-FUS niche

Part IV

When to Fractionate

6.Fractionation is a therapeutic-ratio decision

Fractionation is not a compromise reserved for weak plans. It is the rational choice when a tumoricidal single-fraction dose cannot be delivered without excessive exposure to normal brain, optic pathways, brainstem, cochlea, or spinal cord. It is commonly favored for larger metastases and cavities, perioptic benign tumors, long or postoperative spine targets, and selected reirradiation cases. The prescription must use fraction-specific evidence and constraints; BED conversion does not make schedules clinically interchangeable. The comparison is now under randomized test: NRG-BN013 randomizes intact metastases 1 to 3 cm between single-fraction and 3-fraction SRS and is accruing, Alliance A071801 compares postoperative single-fraction with fractionated cavity SRS, and NRG-BN012 compares preoperative with postoperative SRS. Until these read out, the schedules below rest on retrospective and single-arm data.

Common fractionation triggers and representative schedules. These are patterns, not automatic prescriptions.
Scenario	Representative approach	Reason
Large intact metastasis / cavity	24–27 Gy / 3 fx or 30–32.5 Gy / 5 fx	Preserve control while reducing normal-brain injury compared with a low single-fraction dose
Perioptic benign target	Commonly 21 Gy / 3 fx or 25 Gy / 5 fx, histology-dependent	Meet optic tolerance while maintaining an effective target dose
Spine metastasis	16–24 Gy / 1 fx; 24 Gy / 2 fx; 27 Gy / 3 fx; 30–40 Gy / 5 fx	Choose by cord/thecal-sac proximity, epidural disease, prior RT, volume, and fracture risk
Reirradiation	Usually individualized multi-fraction treatment	Cumulative tolerance depends on prior dose, interval, anatomy, and recovery assumptions

7.Normal-tissue dose closes the loop

The target dose is only half the prescription. For cranial SRS, inspect the volume of uninvolved brain receiving intermediate dose, especially V12Gy in one fraction and the corresponding fractionated metrics. As a rough calibration, HyTEC associates a brain-minus-target V12Gy of about 5 cc, 10 cc, and above 15 cc with roughly 10%, 15%, and 20% risk of symptomatic radiation necrosis after single-fraction SRS for brain metastases, and many centers plan to keep single-fraction V12Gy at or under 10 cc; for hypofractionated courses the usual metrics are brain-plus-target V20Gy in 3 fractions and V24Gy in 5 fractions, commonly held under about 20 cc. For benign skull-base targets, optic and cochlear dose may dominate. For spine SBRT, the exact structure and metric matter: true cord, thecal sac, and planning-risk volume constraints are not interchangeable. Prior irradiation requires a separate cumulative-dose analysis rather than simple reuse of de novo limits.

See also Use the Quick Reference for rapid retrieval, Planning, Constraints & QA for OAR metrics and plan evaluation, and the corresponding disease page for outcomes and indication-specific evidence.

Part V

Platform Translation

8.Keep the clinical dose; translate the convention

Gamma Knife plans often prescribe a margin dose to a lower isodose surface and carry a deliberate intratarget hotspot. LINAC plans may normalize near a higher percentage, and robotic plans use their own optimization conventions. The prescription-isodose percentage is therefore not a portable biological quantity. Cross-platform comparison should report the prescription dose, target coverage, Dmax and hotspot location, conformity, gradient, treatment volume, normal-brain dose, and relevant OAR metrics.

Key points

The diagnosis provides a starting range; size/volume, OARs, prior RT, intent, target stability, and plan quality determine the final prescription.
Small intact metastases commonly receive 20–24 Gy in one fraction; larger lesions increasingly favor 3–5 fractions or surgery when mass effect is present.
Benign tumors need long-term control with functional preservation, so cochlear, optic, brainstem, gland, and cranial-nerve dose often matter more than dose escalation.
AVM dose is a nidus margin dose; TN and Vim thalamotomy are reported as maximum lesioning doses.
Do not turn chordoma, diffuse glioma, cavernous malformation, or a large compressive target into routine SRS merely because a historical dose range exists.
Fractionation is the correct answer when it improves the therapeutic ratio, not a failure to perform single-fraction radiosurgery.
Prescription-isodose percentage is platform-specific; compare clinically meaningful dose, coverage, gradient, normal-tissue exposure, and OAR metrics.

References

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Ladbury C, Pennock M, Yilmaz T, et al. Stereotactic radiosurgery in the management of brain metastases: a case-based Radiosurgery Society practice guideline. Adv Radiat Oncol. 2024;9(3):101402. PubMed
Shaw E, Scott C, Souhami L, et al. Single-dose radiosurgical treatment of recurrent previously irradiated primary brain tumors and brain metastases: final report of RTOG 90-05. Int J Radiat Oncol Biol Phys. 2000;47(2):291–298. PubMed
Milano MT, Grimm J, Niemierko A, et al. Single- and multifraction stereotactic radiosurgery dose/volume tolerances of the brain. Int J Radiat Oncol Biol Phys. 2021;110(1):68–86. PubMed
Tsao MN, Sahgal A, Xu W, et al. Stereotactic radiosurgery for vestibular schwannoma: International Stereotactic Radiosurgery Society practice guideline. J Radiosurg SBRT. 2017;5(1):5–24. PMC
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Kotecha R, Sahgal A, Rubens M, et al. Stereotactic radiosurgery for non-functioning pituitary adenomas: meta-analysis and ISRS practice opinion. Neuro Oncol. 2020;22(3):318–332. PMC
Tuleasca C, Regis J, Sahgal A, et al. Stereotactic radiosurgery for trigeminal neuralgia: an ISRS systematic review and practice guideline. J Neurosurg. 2019;130(3):733–757. PubMed
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Milano MT, Grimm J, Soltys SG, et al. Single- and multifraction stereotactic radiosurgery dose tolerances of the optic pathways (HyTEC). Int J Radiat Oncol Biol Phys. 2021;110(1):87–99. PubMed
Sahgal A, Chang JH, Ma L, et al. Spinal cord dose tolerance to stereotactic body radiation therapy (HyTEC). Int J Radiat Oncol Biol Phys. 2021;110(1):124–136. PubMed
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Educational synthesis for neurosurgery and radiation-oncology trainees; dose ranges are representative starting points, not treatment directives. Final prescription and fractionation require current disease guidance, the adopted institutional constraint set, and multidisciplinary plan review.