Stereotactic Radiosurgery · Spine

Spine SBRT: Clinical Practice

Dose and fractionation, cord constraints, the randomized pain evidence, toxicity, and the postoperative setting

With selection settled — the cord constraint understood and the NOMS/SINS/ESCC questions answered on the foundations page — this page covers how spine SBRT is actually prescribed: the common dose/fractionation schedules, the spinal-cord tolerance limits that bound them, what the randomized trials show for pain, the characteristic toxicities (vertebral compression fracture, pain flare, the rare radiation myelopathy), and the increasingly common postoperative/separation-surgery and reirradiation settings.

Timoteo Almeida, MD, PhD | Department of Neurological Surgery

Orientation

Spine SBRT is a balance between a dose high enough for durable tumor control and a dose to the cord low enough to avoid myelopathy. Practice has converged on a small number of schedules — single fraction, and 2- or 3-fraction regimens — each paired with a published cord-tolerance limit. The randomized evidence is genuinely split on pain: a single-fraction trial was negative against conventional RT, while a 2-fraction trial was positive, which is why fractionation choice matters and why local control, not just pain, frames the indication.

Part I

Dose, Fractionation, and the Cord Limit

1.Common schedules

Several regimens are in routine use, chosen by tumor, prior radiation, proximity to cord, and postoperative status:

Single fraction: commonly 16–24 Gy × 1. High-dose single fraction (e.g., 24 Gy) gives excellent local control but the steepest cord-tolerance challenge and higher fracture risk.
2 fractions: 24 Gy / 2 fx — the SC.24 regimen with randomized pain benefit.
3 fractions: 27 Gy / 3 fx (and related 24–30 Gy/3 fx schedules) — widely used, particularly for larger volumes or postoperative beds.
5 fractions: 30–40 Gy / 5 fx — favored when the cord or a long segment limits hypofractionation, and common postoperatively.

2.The spinal-cord constraint

The cord (or thecal sac as a surrogate) constraint is the hard boundary of every plan. Widely used limits derive from QUANTEC and the Sahgal dose-tolerance analyses; representative point-maximum (Dmax) values are approximately single fraction ≈ 12.4–14 Gy, 2 fractions ≈ 17 Gy, and 3 fractions ≈ 20–22 Gy to the thecal sac, with the postoperative/reirradiation setting using lower limits. These are institution- and protocol-specific and must be taken from the center's adopted constraint set, not memorized as universal numbers; the principle is that a small Dmax tolerance, applied to a contoured cord PRV, is what permits the ablative tumor dose just millimeters away.

The constraint is the plan The reported cord/thecal-sac limits are point-dose maxima tied to a specific contour (true cord vs thecal sac vs PRV) and a specific fractionation; they are not interchangeable across protocols. Always use the constraint set your physics and radiation-oncology team have adopted, and apply tighter limits in the reirradiation setting where cumulative cord dose governs.

Part II

Evidence: Local Control and Pain

3.Durable local control — and the split pain trials

Modern series report local control ~80–90% with spine SBRT, with the advantage most pronounced for radioresistant histologies. The randomized data on pain, however, diverge by regimen:

RTOG 0631 (single fraction): randomized single-fraction SBRT (16–18 Gy) against conventional EBRT (8 Gy × 1) for 1–3 spine metastases and found no improvement in pain response (e.g., ~40% vs ~58% at 3 months) — a negative trial for the pain endpoint.
SC.24 / TROG 17.06 (2 fractions): randomized SBRT 24 Gy / 2 fx against conventional 20 Gy / 5 fx and found a significantly higher complete pain-response rate (~35% vs ~14% at 3 months), meeting its primary endpoint.

The reconciliation is that durable local tumor control is the robust, consistent benefit of spine SBRT; the pain advantage is regimen-dependent and clearest with the dose-intense fractionated (24 Gy/2 fx) approach rather than a single moderate fraction.

The two randomized spine-SBRT pain trials (representative reported figures).
Trial	SBRT arm vs control	Pain result
RTOG 0631 (Ryu)	16–18 Gy × 1 vs 8 Gy × 1 cEBRT	No improvement in pain response (negative)
SC.24 / TROG 17.06 (Sahgal)	24 Gy / 2 fx vs 20 Gy / 5 fx cEBRT	Higher complete pain response (~35% vs ~14% at 3 mo)

Part III

Toxicity

4.Vertebral compression fracture, pain flare, myelopathy

Three toxicities define spine-SBRT counseling and follow-up:

Vertebral compression fracture (VCF) — the most clinically relevant late effect, occurring in roughly 10–15% (higher with high single-fraction dose). Risk rises with lytic lesions, pre-existing deformity/kyphosis, higher dose per fraction, and a higher SINS; recognizing fracture risk up front feeds the prophylactic-stabilization discussion.
Pain flare — a transient post-treatment increase in pain affecting a substantial minority of patients; commonly mitigated with a short peri-treatment corticosteroid (dexamethasone) course.
Radiation myelopathy — rare (well under 1% when cord constraints are respected) but catastrophic; it is the reason the cord limit is non-negotiable and is the dominant concern in reirradiation.

Part IV

Postoperative and Reirradiation Settings

5.Separation surgery, postop SBRT, and reirradiation

For high-grade epidural compression, the contemporary paradigm is separation surgery — limited posterior decompression that creates a few millimeters of cord-to-tumor gap and stabilizes the spine — followed by postoperative SBRT to the residual tumor. This combination achieves high local control while keeping the ablative dose off the cord, and has largely supplanted aggressive en-bloc resection for metastatic compression in suitable patients. Postoperative target delineation follows consensus guidelines, hardware artifact is managed at planning, and fractionated schedules (e.g., 3–5 fractions) are common in the postoperative bed. Reirradiation of a previously irradiated segment is feasible with SBRT but governed by cumulative cord dose, demanding the tightest constraints and careful accounting of prior treatment.

Separation surgery + SBRT, in one line Decompress just enough to create a cord-to-tumor gap and stabilize — then deliver ablative SBRT to the residual tumor. The surgery buys the dose distribution; the SBRT buys the durable local control.

Part V

Landmark Trials & Open Controversies

6.Two randomized trials that point in opposite directions

Spine SBRT is defined by two phase III trials whose apparently conflicting pain results are the central teaching point of the field.

The defining randomized evidence in spine stereotactic body radiotherapy.
Trial	Comparison	Result
RTOG 0631 (Ryu, JAMA Oncol 2023)	Single-fraction SSRS 16–18 Gy vs conventional EBRT 8 Gy × 1	Negative — no improvement in the primary 3-month pain-response endpoint
SC.24 / TROG 17.06 (Sahgal, Lancet Oncol 2021)	SBRT 24 Gy / 2 fx vs conventional EBRT 20 Gy / 5 fx	Positive — complete pain response ~35% vs ~14% at 3 months, durable at 6 months
Sahgal et al. (cord tolerance)	Pooled thecal-sac dose / myelopathy modeling	The constraint framework underlying single- and multi-fraction limits
Separation surgery + SBRT (NOMS / Laufer)	Hybrid decompression then SBRT for high-grade ESCC	Durable local control without debulking to the cord

Open controversies:

Why 0631 and SC.24 diverge. The most plausible reconciliation is dose intensity and fractionation: SC.24's 24 Gy in 2 fractions delivers a substantially higher biologically effective dose than a single 16–18 Gy fraction, with a different endpoint and population. The lesson is not that spine SBRT fails, but that an adequately ablative regimen is what separates it from conventional radiotherapy.
Single-fraction versus multi-fraction. Single-fraction SBRT maximizes biologically effective dose and convenience but raises vertebral-compression-fracture risk; multi-fraction regimens trade some dose intensity for a gentler toxicity profile, especially near the cord.
Vertebral compression fracture. VCF is the signature late toxicity — more frequent after high single-fraction doses and in lytic disease with a high SINS — and a real factor in choosing dose, fractionation, and whether to stabilize first.
Postoperative and separation-surgery SBRT. The optimal dose, fractionation, and timing after separation surgery, and selection for the hybrid approach versus SBRT alone, remain actively studied.

Key points

Common schedules: 16–24 Gy × 1, 24 Gy/2 fx, 27 Gy/3 fx, 30–40 Gy/5 fx — chosen by tumor, cord proximity, prior RT, and postop status.
Cord/thecal-sac Dmax limits (roughly 12.4–14 Gy single fx, ~17 Gy/2 fx, ~20–22 Gy/3 fx) are protocol-specific and tighter on reirradiation — use your center's adopted set.
Local control is consistently ~80–90%; the pain benefit is regimen-dependent — RTOG 0631 (single fraction) was negative, SC.24 (24 Gy/2 fx) was positive.
Counsel on VCF (~10–15%, worse with lytic/deformity/high single-fraction dose/high SINS), pain flare (steroid prophylaxis), and rare radiation myelopathy.
For high-grade epidural compression: separation surgery + postoperative SBRT is the modern paradigm; reirradiation is bounded by cumulative cord dose.

See also Recognition and management of vertebral compression fracture, pain flare, and radiation myelopathy — with the imaging workup and the steroid / bevacizumab / LITT / surgery ladder — are consolidated on the Adverse Radiation Effects page.

References

Sahgal A, Myrehaug SD, Siva S, et al. Stereotactic body radiotherapy versus conventional external beam radiotherapy for painful spinal metastases (CCTG SC.24/TROG 17.06): a randomised, controlled, phase 2/3 trial. Lancet Oncol. 2021;22(7):1023–1033. PubMed
Ryu S, Deshmukh S, Timmerman RD, et al. Stereotactic radiosurgery vs conventional radiotherapy for localized vertebral metastases (NRG Oncology/RTOG 0631): a randomized phase 3 trial. JAMA Oncol. 2023;9(6):800–807. PubMed
Sahgal A, Weinberg V, Ma L, et al. Probabilities of radiation myelopathy specific to stereotactic body radiation therapy to guide safe practice. Int J Radiat Oncol Biol Phys. 2013;85(2):341–347.
Sahgal A, Atenafu EG, Chao S, et al. Vertebral compression fracture after spine stereotactic body radiotherapy: a multi-institutional analysis. J Clin Oncol. 2013;31(27):3426–3431.
Laufer I, Iorgulescu JB, Chapman T, et al. Local disease control after decompressive surgery and adjuvant high-dose single-fraction radiosurgery: outcomes in 186 patients (separation surgery). J Neurosurg Spine. 2013;18(3):207–214.

Educational synthesis for neurosurgery and radiation-oncology trainees; not a treatment directive. Cord-tolerance values are protocol-specific. Randomized spine-trial references verified during review.