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, MSc, PhD, FCNS  |  Departments of Neurological Surgery and Radiation Oncology

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 is on the radiobiology page; the clinical framework for moving from a disease range to a final prescription is on Dose Selection & Fractionation.

Part I

Delineation

1.Target and organ-at-risk contouring

For many intact cranial targets the GTV essentially equals the CTV, but that is not universal: postoperative cavities, infiltrative disease, and selected dural targets use indication-specific clinical margins. Any PTV margin is small and workflow-specific, commonly 0–1 mm for rigid frame-based treatment and often 1 mm (sometimes more) for validated frameless workflows, reflecting measured localization and motion uncertainty rather than microscopic 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 often imaging or registration 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. The prescription isodose percentage is a platform-dependent normalization rather than a disease dose, so report coverage with the clinically meaningful prescription and maximum dose rather than treating an isodose percentage as transferable between Gamma Knife, LINAC, and robotic plans.
  • 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 tissue volume receiving 12 Gy (V12Gy), which correlates with the risk of symptomatic radionecrosis in single-fraction cranial SRS. State explicitly whether your V12 includes or excludes the target, because published risk models pool both conventions and the HyTEC analysis reports V12 with the target volume included.
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 HyTEC series, which models both single-fraction and hypofractionated high-dose-per-fraction tolerances, 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 / chiasmPlanning goal often Dmax ≤8–10 Gy; HyTEC modeled <1% RION near 12 GyNo prior RT; use 20 Gy/3 fx or 25 Gy/5 fx as corresponding low-risk HyTEC limits when fractionating
BrainstemConstraint depends on endpoint and exposed volume; many single-fraction protocols use a point maximum in the ~12.5–15 Gy rangeDo not transfer a tumor or trigeminal-nerve dose to the brainstem; inspect volume-based constraints
Spinal cordDmax ~12–14 Gy (point)Spine SBRT uses cord PRV constraints: see spine pages
CochleaMean ~4 Gy for serviceable-hearing preservation; TG-101 lists a single-fraction cochlear point maximum of 9 GyMinimize for serviceable-hearing vestibular schwannoma
LensAs low as reasonably achievableNo single SRS threshold substitutes for minimizing dose to this radiosensitive structure
Normal brainTrack V12Gy rather than treating one cutoff as universalRisk rises continuously with V12Gy; use fractionation as target size and exposed volume increase

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:

  • End-to-end testing: each component of the chain carries its own error, so cumulative system accuracy is characterized with a phantom carrying a hidden target through the entire clinical workflow: imaging, fusion, contouring, planning, localization, and delivery. Repeat it on a defined schedule, after any system or software upgrade, and separately for each imaging and tracking modality in clinical use.
  • Machine QA: the AAPM TG-142 program for linac mechanical/dosimetric accuracy, with TG-198 as its implementation guide, plus the Winston-Lutz test, which measures coincidence of the radiation isocenter with the mechanical isocenter across gantry, couch, and collimator angles; 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 or secondary monitor-unit and dose verification per TG-219, and for modulated plans measured patient-specific QA with tolerance limits set per TG-218, recognizing that small-field SRS geometry stresses both methods.
  • 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

  • Many intact cranial targets use GTV=CTV, but postoperative and infiltrative indications use disease-specific margins; PTV margin must come from the validated localization and motion workflow.
  • Judge plans on coverage, conformity (Paddick CI = (TV_PIV)²/(TV×PIV)), gradient (GI = PIV_half/PIV), and normal-brain V12Gy.
  • Know the hierarchy, not just a mnemonic: conservative optic planning goals often sit at 8–10 Gy while HyTEC models <1% RION near 12 Gy; brainstem and cord limits depend on volume metric, contour definition, and fractionation; V12Gy is a continuous risk predictor.
  • 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. Milano MT, Grimm J, Niemierko A, et al. Single- and multifraction stereotactic radiosurgery dose/volume tolerances of the brain (HyTEC). Int J Radiat Oncol Biol Phys. 2021;110(1):68–86. PubMed
  4. Lawrence YR, Li XA, el Naqa I, et al. Radiation dose-volume effects in the brain (QUANTEC). Int J Radiat Oncol Biol Phys. 2010;76(3 Suppl):S20–S27. PubMed
  5. Cirino E, Benedict SH, Dupre PJ, et al. AAPM-RSS Medical Physics Practice Guideline 9.b: SRS-SBRT. J Appl Clin Med Phys. 2025;26(4):e14624. AAPM
  6. 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
  7. Paddick I. A simple scoring ratio to index the conformity of radiosurgical treatment plans. J Neurosurg. 2000;93(Suppl 3):219–222.
  8. Paddick I, Lippitz B. A simple dose gradient measurement tool to complement the conformity index. J Neurosurg. 2006;105(Suppl):194–201.
  9. 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.
  10. 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.