Stereotactic Radiosurgery · Reference
Plan-Quality Metrics, Prognostic Scores & Constraint References
The formulas, scores, and consensus documents used at the planning station and in clinic
This page collects the quantitative tools of radiosurgical practice in one place: the plan-quality indices (conformity, gradient, coverage) used to judge a plan, the dose-modeling concepts (BED/EQD2) and their ablative-dose caveats, the prognostic scores that frame the clinical decision (GPA for brain metastases; NOMS/SINS/ESCC for spine), and the consensus constraint and contouring documents that practice rests on.
Orientation
A radiosurgery plan is judged by numbers and the clinical decision is framed by scores. Knowing what each metric measures — and its failure mode — is what separates mechanical plan evaluation from real quality control. The same applies to prognostic scores: they organize the conversation but do not replace judgment.
Plan-Quality Metrics
1.Conformity, gradient, and coverage
Three families of metrics describe how well a plan wraps dose around the target and falls off outside it:
- Paddick conformity index (CI): rewards conformity while penalizing both under-coverage and spill. Paddick CI = (TV_PIV)² / (TV × PIV) where TV = target volume, PIV = prescription isodose volume, TV_PIV = target volume covered by the prescription isodose. CI → 1 is ideal. (The older RTOG CI = PIV / TV is simpler but does not penalize a prescription volume that misses the target.)
- Gradient index (GI): how fast dose falls off outside the prescription — the key to sparing normal brain. GI = PIV_half / PIV the volume of half the prescription isodose divided by the prescription isodose volume; lower is steeper/better.
- Coverage and homogeneity: coverage = fraction of target receiving the prescription; homogeneity index describes dose uniformity within the target (note that SRS often deliberately accepts a hot, inhomogeneous interior).
2.Dose modeling: BED and EQD2 (with a caveat)
Biologically effective dose and the 2-Gy-equivalent dose let schedules be compared, but at ablative fraction sizes the underlying linear-quadratic model is contested:
BED = n × d × [1 + d / (α/β)] EQD2 = BED / [1 + 2 / (α/β)]where n = number of fractions, d = dose per fraction. These are reliable in the conventional range but should be used cautiously above the SRS dose-per-fraction threshold, where the model may overestimate effect — the reason empirical trial doses (RTOG 90-05, HyTEC) guide SRS prescribing rather than pure BED extrapolation.
Prognostic and Decision Scores
3.Brain metastases: the GPA
The Graded Prognostic Assessment (GPA) and its diagnosis-specific and molecular updates (ds-GPA, and disease-specific GPAs incorporating, e.g., EGFR/ALK in lung, HER2 in breast, BRAF in melanoma) estimate survival from factors such as age, performance status, number of metastases, extracranial disease, and tumor-specific molecular markers. Its role is to match treatment intensity to expected survival — aggressive local therapy and cognition-sparing strategies make sense in longer-prognosis groups, comfort-weighted approaches in the shortest.
4.Spine: NOMS, SINS, ESCC
For spinal disease the relevant scores are the NOMS framework (Neurologic/Oncologic/Mechanical/Systemic), the SINS (Spinal Instability Neoplastic Score), and the Bilsky ESCC epidural compression grade — developed in detail on the spine foundations page. Together they decide SBRT versus separation surgery versus stabilization versus conventional RT.
Constraint and Contouring References
5.The documents practice rests on
| Document | Scope |
|---|---|
| AAPM MPPG 9.b | Current minimum physics practice guidance for linac-based SRS/SBRT programs |
| AAPM TG-178 | Gamma stereotactic radiosurgery calibration, dosimetry, mechanical QA, and treatment-process QA |
| AAPM TG-101 | SBRT physics, prescribing, quality assurance, normal-tissue constraints |
| AAPM TG-142 / TG-135 | Linac QA; CyberKnife (robotic) QA |
| QUANTEC | Normal-tissue dose-response (conventional baseline; cord, optic, brainstem) |
| HyTEC | Hypofractionated/SRS-specific tumor-control and normal-tissue dose-response |
| RTOG 90-05 | Single-fraction maximum tolerated dose by lesion size |
| ISRS consensus / consortium contouring | Cranial and spine target-volume delineation guidelines |
| Cox et al. (spine consortium) | Spine SBRT target-volume consensus |
Key points
- Paddick CI = (TV_PIV)²/(TV×PIV) penalizes both miss and spill; RTOG CI = PIV/TV does not. Gradient index = PIV_half/PIV measures fall-off (lower = steeper).
- BED/EQD2 compare schedules but are unreliable at ablative fraction size — lean on empirical trial doses (RTOG 90-05, HyTEC), not LQ extrapolation.
- GPA (and molecular ds-GPA) matches treatment intensity to expected survival in brain metastases.
- Spine decisions run on NOMS + SINS + ESCC together.
- MPPG 9.b, TG-178, TG-101, QUANTEC, HyTEC, RTOG 90-05, and consensus contouring guidelines are the canonical references — always use the current adopted version.
References
- Paddick I. A simple scoring ratio to index the conformity of radiosurgical treatment plans. J Neurosurg. 2000;93(Suppl 3):219–222. PubMed
- 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. PubMed
- 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
- 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
- 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
- Sperduto PW, Mesko S, Li J, et al. Survival in patients with brain metastases: summary report on the updated diagnosis-specific GPA and definition of the eligibility quotient. J Clin Oncol. 2020;38(32):3773–3784. PubMed
- Cox BW, Spratt DE, Lovelock M, et al. International spine radiosurgery consortium consensus guidelines for target volume definition. Int J Radiat Oncol Biol Phys. 2012;83(5):e597–e605. PubMed
Educational reference for neurosurgery and radiation-oncology trainees; not a treatment directive. Formulas and document scopes are summarized; apply the current adopted version of each constraint/contouring reference. Citation anchors verified during review.