Radiosurgery for brain metastases: one or more small targets treated with a very high dose in one to five fractions, usually with a frameless mask and VMAT, a dedicated cone or a Gamma Knife. The main planning problems are millimeter accuracy, a steep dose fall-off around every target and the combined dose to normal brain when there are several lesions.
Radiosurgery treats one or more brain metastases with a very high dose in 1 to 5 fractions. In a single fraction, the dose steps down as the lesion gets larger: RTOG 90-05 found the maximum tolerated dose to be 24 Gy for lesions up to 2 cm, 18 Gy for 2.1–3 cm and 15 Gy for 3.1–4 cm. For larger lesions, or those near the brainstem or optic pathway, the dose is often split over 3 or 5 fractions — for example 24–30 Gy in 3 or 25–35 Gy in 5 in one current protocol — to keep a higher dose to the target while lowering the risk to normal brain.
After a metastasis is removed, the resection cavity can be treated with radiosurgery instead of whole-brain treatment, as in the N107C trial. Cavity targets are larger and less round, so they are often fractionated.
Organ limits depend on the number of fractions, so the constraint table below has a column for 1, 3 and 5.
Millimeter accuracy, from immobilization through imaging and delivery.
A frameless stereotactic mask, sometimes with a bite block, or a frame fixed to the skull.
With little or no margin around the target, any movement moves the dose off the lesion. A mask system needs imaging before treatment and a way to monitor or check the head during it; a fixed frame holds the head rigidly for single-session treatment.
Image fusion aligns an outside scan with the planning CT so the target can be drawn using information the CT doesn't show. For radiosurgery this is a thin-slice, contrast-enhanced 3D T1-weighted MRI; a T2-weighted sequence can help when a lesion has bled.
Small metastases are often only visible on contrast MRI. The scan should be recent — lesions can grow and new ones can appear between the MRI and treatment — and corrected for geometric distortion, which is larger away from the center of the scanner. The registration is a rigid match of the whole brain on landmarks such as the sulci, ventricles and vessels at the skull base (with a skull-base region of interest for skull-base tumors), and is reviewed lesion by lesion.
A CT with slices about 1 mm thick, covering the whole head.
Thin slices keep small targets and organs well defined and support accurate registration and image guidance.
kV stereoscopic imaging or cone-beam CT before treatment, with correction of rotations as well as shifts, and re-imaging after couch rotations.
When one isocenter treats several lesions, a small rotation of the head moves targets far from the isocenter by more than the margin. Non-coplanar arcs improve dose fall-off but mean moving the couch between arcs.
What each volume represents. Margins depend on your immobilization and delivery system and come only from your protocol.
Each enhancing metastasis on the contrast MRI, or the resection cavity and any residual tumor after surgery.
Contouring each lesion on the fused MRI, and checking the fusion around it, matters more than in almost any other treatment, because there may be no margin to absorb error.
The GTV itself, or a small expansion, depending on the immobilization and image guidance.
Each department sets its margin from its own measured uncertainties. A larger margin adds normal brain to the high-dose volume and raises the risk of radionecrosis.
What makes an SRS plan good goes beyond coverage.
How closely the prescription isodose matches the target, and how quickly the dose falls away from it. Concept page →
Normal-brain tolerance in radiosurgery is written as the volume receiving a moderate dose (V12 Gy for single fraction), so a steep fall-off is as important as coverage. Non-coplanar arcs help.
SRS plans are usually prescribed to an isodose line below the maximum, so the center of the target receives more than the prescription.
The hot spot should fall inside the GTV, not in normal brain. The prescription isodose chosen differs between platforms, which changes the maximum dose.
Treating multiple metastases with one isocenter or several.
Low doses from neighboring targets add up in the brain between them. Watch for dose bridging between lesions and for the brain V12 Gy of the whole plan, not of each lesion.
Click an organ for its anatomy, why it matters and contouring resources. Superscripts link to the study behind each value.
| Organ | 1 fx15–24 Gy | 3 fx24–30 Gy | 5 fx25–35 Gy |
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| BrainBrain including all targets, summed over the course. Under 20 cc: <10% risk of necrosis or edema. |
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| BrainstemMidbrain and pons; the medulla is contoured separately. A metastasis in the brainstem may justify higher doses. |
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| Optic Chiasm & Optic Nerves |
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| Globes (Eyes)Listed as the orbits in the source. |
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| Lens |
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| Cochlea1 and 3 fx goals come from a hearing-preservation trial in vestibular schwannoma. |
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| Spinal CordDefined as the whole bony canal. Matters for lesions low in the posterior fossa. |
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Values quoted from HyTEC brain (Milano 2021) and eviQ 3253 (last reviewed 21 August 2025; review due 31 December 2029), checked by DoseLab on 2026-09-25. Verify against your protocol, institutional policy and physician directive before clinical use.
Search all CNS / brain constraints in the Constraint Finder →
ASTRO clinical practice guideline on radiotherapy for intact and resected brain metastases: when to use SRS, when to consider surgery, and hippocampal avoidance with memantine for patients with a good prognosis who receive whole-brain treatment. Citation checked against Crossref.
RTOG 90-05 dose-escalation study of single-fraction radiosurgery for previously irradiated brain tumors and metastases. It defined the maximum tolerated single dose by tumor diameter — 24 Gy up to 20 mm, 18 Gy for 21–30 mm and 15 Gy for 31–40 mm — which still shapes size-based SRS dose selection. Citation checked against Crossref.
Phase 3 trial (NCCTG N0574) in patients with 1–3 brain metastases: adding whole-brain radiotherapy to radiosurgery caused more cognitive decline at 3 months without improving survival. Citation checked against Crossref.
Phase 3 trial (NCCTG N107C/CEC.3) after resection of a brain metastasis: cavity SRS preserved cognition better than whole-brain radiotherapy, with no difference in survival. Citation checked against Crossref.
The HyTEC brain report. From pooled published data it models the risk of radionecrosis after single-fraction and fractionated radiosurgery. For single-fraction SRS of brain metastases, V12 Gy of 5, 10 or more than 15 cm³ (brain including the targets) was linked to about 10%, 15% and 20% risk of symptomatic radionecrosis; for 3 and 5 fractions, V20 Gy or V24 Gy under 20 cm³ was linked to under 10% risk of any necrosis or edema. Numbers read from the PubMed abstract; citation checked against Crossref.
UK consensus on dose–volume constraints for SABR of oligometastases, primary lung cancer and hepatocellular carcinoma. Citation checked against Crossref. DoseLab read the tables in the authors' accepted manuscript (UCL Discovery and White Rose Research Online): Table 6 (intracranial, 1, 3 and 5 fractions) for the brain pages, and Tables 2, 3 and 5 (thoracic, abdominal and spinal) for the liver SBRT page. Each table gives optimal and mandatory limits by fraction count; the liver limits apply to patients with, at worst, Child-Pugh A6 liver disease. The kidney limit is written as a "cold" constraint — at least 200 cc must receive no more than the stated dose.
An editorial history of the UT Southwestern SABR constraint tables — often called the “Timmerman tables” — covering how they were developed and revised, and their relationship to AAPM TG-101 and NRG/RTOG protocols. Tables 2–11 separate serial tissues (listed with a volume, a volume max and a max point dose) from parallel tissues (listed with a critical volume and a critical-volume dose max). In Tables 2–7 a footnote defines “point” as ≤0.035 cm³; Tables 8–11 add a contouring-instructions column for each structure.
Endorsed eviQ protocol for stereotactic radiosurgery of intact brain metastases. It gives single-fraction, 3-fraction and 5-fraction dose options by lesion size, and an organs-at-risk table with a separate column for each fraction count, each value credited to its primary source (HyTEC, the UK SABR consensus, Timmerman 2022 and others). DoseLab's SRS constraint table reproduces selected values from it. A companion protocol (eviQ 3572) covers post-operative cavity SRS. Check the live page for the current version.