Stereotactic Radiosurgery · Technical Foundations

Planning, Constraints & QA

Coverage, conformity, gradient, the structures you must not hit, and the checks that keep it safe

A radiosurgery plan is judged on four things at once: does it cover the target, does the high dose stay on the target (conformity), does the dose fall off steeply outside it (gradient), and does it respect the surrounding critical structures. This page covers target and organ-at-risk delineation, the plan-quality metrics that quantify those goals, the normal-tissue constraints that bound the prescription, and the end-to-end quality assurance that makes sub-millimeter delivery trustworthy.

Timoteo Almeida, MD, PhD  |  Department of Neurological Surgery

Orientation

In conventional radiotherapy, margins and fractionation absorb a great deal of geometric and biological uncertainty. Radiosurgery removes that cushion: with little or no margin and an ablative single dose, a small error in contour, fusion, or delivery is not averaged away — it is delivered. Planning therefore optimizes a steep gradient and tight conformity, dose selection is bounded by validated normal-tissue limits, and quality assurance is built to catch sub-millimeter and dosimetric errors before they reach the patient. The radiobiologic rationale for the doses themselves is on the radiobiology page.

Part I

Delineation

1.Target and organ-at-risk contouring

For most cranial radiosurgery the GTV essentially equals the CTV — there is no routine microscopic-extension margin — and any PTV margin is minimal (commonly 0–1 mm for frame-based, up to 1–2 mm for mask-based/frameless), reflecting the residual setup and image-guidance uncertainty rather than biology. Accurate delineation depends entirely on high-quality, correctly fused imaging (thin-slice contrast MRI for most tumors, specific sequences by indication), so contouring errors are usually fusion errors — the subject of the imaging and registration page. Organs at risk — optic nerves and chiasm, brainstem, cochleae, lenses, spinal cord, and the brain itself — must be contoured just as carefully as the target, because the plan is optimized against their dose limits.

Part II

Plan-Quality Metrics

2.Coverage, conformity, gradient

Four families of metric describe a radiosurgery plan:

  • Coverage — the fraction of the target receiving the prescription dose (and the prescription isodose line, commonly the 50–80% isodose depending on platform).
  • Conformity — how tightly the prescription isodose wraps the target. The Paddick conformity index, PCI = (TV_PIV)² / (TV × PIV), where TV_PIV is the target volume covered by the prescription isodose, TV the target volume, and PIV the prescription isodose volume, ranges 0–1 with 1 ideal; it penalizes both undercoverage and spill.
  • Gradient — how fast dose falls outside the target. The gradient index, GI = PIV_half / PIV (the half-prescription isodose volume divided by the prescription isodose volume), captures the falloff that protects normal brain; a lower GI is better.
  • Hotspot/normal-tissue dose — maximum dose, and critically the volume of normal brain receiving 12 Gy (V12Gy), which correlates with the risk of symptomatic radionecrosis in single-fraction cranial SRS.
Part III

Normal-Tissue Constraints

3.The structures that bound the prescription

Constraints are compiled in QUANTEC (conventional and some SRS), AAPM TG-101, and the hypofractionated HyTEC series, and they vary by institution and by single- versus multi-fraction regimen. For program standards, use current AAPM practice guidance as well, including MPPG 9.b for linac SRS/SBRT and TG-178 for Gamma stereotactic radiosurgery QA. The single-fraction values below are widely used representative limits, not a universal protocol; they must be adapted to the indication, the volume considered, and current guidance.

Representative single-fraction SRS organ-at-risk constraints (illustrative; adapt to indication, volume metric, and institutional protocol).
StructureRepresentative single-fraction limitNote
Optic nerves / chiasmDmax ~8–10 GyRION rare < 8 Gy, rises 8–12 Gy, > 10% above 12 Gy (QUANTEC)
BrainstemDmax ~12.5–15 GyHigher point doses tolerated for small volumes / TN target
Spinal cordDmax ~12–14 Gy (point)Spine SBRT uses cord PRV constraints — see spine pages
CochleaMean ~4 Gy (hearing preservation); ≤ ~9–12 GyMinimize for serviceable-hearing vestibular schwannoma
LensDmax ~5 GyCataractogenic threshold
Normal brainV12Gy ideally < ~5–10 ccHigher V12Gy → higher symptomatic radionecrosis risk

When a target cannot meet these in a single fraction — large volume, or abutting the optic apparatus or brainstem — the answer is hypofractionation (commonly 3 or 5 fractions), which raises the tolerable target dose by spreading the normal-tissue exposure, using the corresponding HyTEC multi-fraction limits.

Part IV

Quality Assurance & Safety

4.End-to-end accuracy

Because a radiosurgery error is delivered rather than averaged out, QA is layered:

  • Machine QA — the AAPM TG-142 program for linac mechanical/dosimetric accuracy, plus the Winston-Lutz test of radiation-isocenter coincidence; cobalt and robotic systems have analogous source/targeting checks.
  • Program-level standards — AAPM MPPG 9.b for linac SRS/SBRT minimum physics practice, TG-178 for Gamma stereotactic radiosurgery, TG-101 for SBRT foundations/constraints, and small-field dosimetry per TG-155 (see the physics page).
  • Patient-specific QA — independent/secondary monitor-unit or dose verification and, for modulated plans, measured patient-specific QA.
  • Process safeguards — image-fusion verification, an explicit target/side time-out, plan review and chart rounds, and attention to MRI geometric distortion, which can move a target by clinically meaningful distances if uncorrected.
The fusion is the most dangerous step The commonest serious radiosurgery error is not a machine fault but a mis-registration or MRI distortion that places the contour — and therefore the dose — in the wrong place. Verify the fusion on multiple planes and landmarks, account for MR distortion, and treat the target/side time-out as non-negotiable. Sub-millimeter delivery onto a mislocalized target is precisely wrong.

Key points

  • Cranial SRS uses GTV=CTV with minimal/zero PTV margin (0–1 mm frame, 1–2 mm mask) — there is no fractionation cushion, so contour/fusion accuracy is paramount.
  • Judge plans on coverage, conformity (Paddick CI = (TV_PIV)²/(TV×PIV)), gradient (GI = PIV_half/PIV), and normal-brain V12Gy.
  • Know representative single-fraction limits: optic ~8–10 Gy, brainstem ~12.5–15 Gy, cord ~12–14 Gy, cochlea mean ~4 Gy, lens ~5 Gy, brain V12Gy < ~5–10 cc — adapt per HyTEC/TG-101 and indication.
  • If single-fraction constraints cannot be met (large or eloquent target), hypofractionate (3–5 fx) with multi-fraction limits.
  • QA is layered: TG-142 + Winston-Lutz machine checks, MPPG 9.b/TG-178 program standards, patient-specific MU/dose verification, and process safeguards (fusion check, target/side time-out, MR-distortion awareness).
  • The most dangerous error is mis-registration/MR distortion, not machine failure — verify the fusion.

References

  1. 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 protocol 90-05. Int J Radiat Oncol Biol Phys. 2000;47(2):291–298. PubMed
  2. Benedict SH, Yenice KM, Followill D, et al. Stereotactic body radiation therapy: the report of AAPM Task Group 101. Med Phys. 2010;37(8):4078–4101.
  3. Cirino E, Benedict SH, Dupre PJ, et al. AAPM-RSS Medical Physics Practice Guideline 9.b: SRS-SBRT. J Appl Clin Med Phys. 2025. AAPM
  4. Petti PL, Rivard MJ, Alvarez PE, et al. Recommendations on the practice of calibration, dosimetry, and quality assurance for gamma stereotactic radiosurgery: report of AAPM Task Group 178. Med Phys. 2021. AAPM
  5. Paddick I. A simple scoring ratio to index the conformity of radiosurgical treatment plans. J Neurosurg. 2000;93(Suppl 3):219–222.
  6. Paddick I, Lippitz B. A simple dose gradient measurement tool to complement the conformity index. J Neurosurg. 2006;105(Suppl):194–201.
  7. Milano MT, Grimm J, Soltys SG, et al. (HyTEC). Single- and multifraction stereotactic radiosurgery dose tolerances of the optic pathways. Int J Radiat Oncol Biol Phys. 2021;110(1):87–99.
  8. Klein EE, Hanley J, Bayouth J, et al. Task Group 142 report: quality assurance of medical accelerators. Med Phys. 2009;36(9):4197–4212.

Educational synthesis for neurosurgery and radiation-oncology trainees. Constraint values are representative and must be adapted to indication, volume metric, fractionation, and institutional protocol; they are not a treatment directive. QA and constraint references verified during review.