# Spine SBRT: Indications, Dose Constraints, and Spinal Cord Tolerance

## Introduction

Stereotactic body radiation therapy (SBRT) to the spine delivers ablative doses directly to vertebral tumors while carefully respecting the stringent dose constraints of the adjacent spinal cord. This approach has revolutionized the management of spinal metastases by providing superior local control compared to conventional palliative radiation therapy. In this lecture, we will explore the indications for spine SBRT, appropriate dose-fractionation schemes, spinal cord tolerance parameters, and the technical considerations necessary to ensure safe and effective treatment.

## Indications

The primary indications for spine SBRT include oligometastatic spinal disease, defined as one to five metastatic lesions, and tumors with radioresistant histologies such as renal cell carcinoma, melanoma, and sarcoma, where conventional radiation therapy often results in poor local control. Spine SBRT is also indicated for reirradiation of previously treated spinal metastases, post-operative treatment following surgical stabilization or decompression, and solitary spinal metastases in patients with controlled primary tumors and no other distant disease.

Relative contraindications to spine SBRT include epidural disease causing spinal cord compression, which typically requires surgical decompression or conventional radiation therapy prior to SBRT. An unstable spine, as assessed by the Spinal Instability Neoplastic Score (SINS) greater than 12, is another contraindication. Additionally, involvement of more than three contiguous vertebral levels and frank spinal cord compression accompanied by myelopathy are considered relative contraindications.

The SINS score evaluates spinal stability based on factors such as tumor location, pain, bone quality, spinal alignment, vertebral body collapse, and posterolateral involvement. Scores from 0 to 6 indicate a stable spine where SBRT is safe. Scores between 7 and 12 suggest potential instability, warranting surgical consultation. Scores from 13 to 18 denote an unstable spine, necessitating surgical stabilization before proceeding with SBRT.

## Dose and Fractionation

Common dose-fractionation regimens for spine SBRT include single-fraction treatments delivering 16 to 24 Gy, with 16 Gy being most common for retreatment and 24 Gy for de novo treatment. Multi-fraction regimens typically involve three fractions of 24 to 27 Gy (8 to 9 Gy per fraction) or five fractions of 30 to 40 Gy (6 to 8 Gy per fraction). The choice of regimen depends on factors such as tumor volume, proximity to the spinal cord, and prior radiation history.

| Clinical Scenario | Dose / Fractions | Spinal Cord Dmax (Thecal Sac) | Expected 1-yr LC |
|---|---|---|---|
| De novo (single fraction) | 16–24 Gy / 1 fx | < 14 Gy | 85–95% |
| De novo (3 fractions) | 24–27 Gy / 3 fx | < 21.9 Gy (7.3 Gy/fx) | 85–95% |
| De novo (5 fractions) | 30–40 Gy / 5 fx | < 28–30 Gy (5.6–6 Gy/fx) | 85–95% |
| Postoperative | 24–30 Gy / 3 fx | < 21.9 Gy | 80–90% |
| Reirradiation (single fx) | 16 Gy / 1 fx | ~70% of de novo limit | 70–85% |
| Reirradiation (5 fractions) | 20–25 Gy / 5 fx | Cumulative guided | 70–85% |

For de novo treatments, higher doses such as 24 Gy in a single fraction or 27 Gy over three fractions provide optimal local control. In contrast, reirradiation protocols favor lower doses per fraction, often using five-fraction regimens to respect the cumulative spinal cord tolerance. Post-operative treatments typically involve 24 to 30 Gy delivered in three fractions. The RTOG 0631 trial demonstrated the feasibility and safety of single-fraction spine SBRT doses ranging from 16 to 18 Gy.

![Table summarizing common spine SBRT dose-fractionation regimens by clinical scenario with expected local control rates](images/spine-sbrt-dose-table.jpg)

## Spinal Cord Dose Constraints

In de novo treatments, the thecal sac serves as the primary organ at risk and is used as a surrogate for the spinal cord. For single-fraction treatments, the maximum dose (Dmax) to the thecal sac should be less than 14 Gy, with the dose to 0.03 cc of the thecal sac also kept below 14 Gy. For three-fraction regimens, the Dmax should be under 21.9 Gy, equivalent to 7.3 Gy per fraction, while five-fraction treatments should limit the Dmax to 28 to 30 Gy, corresponding to 5.6 to 6 Gy per fraction. The cauda equina, located below the conus medullaris at approximately L1-L2, is more tolerant than the spinal cord, allowing for modestly relaxed dose constraints in this region.

When considering reirradiation, cumulative spinal cord dose must account for prior radiation exposure. It is generally assumed that the spinal cord recovers approximately 25% of its tolerance after more than six months from the previous treatment. Consequently, reirradiation Dmax to the thecal sac is typically limited to about 70% of the de novo constraints. Pre-treatment MRI is recommended to assess for any signs of myelopathy before retreatment. A conservative approach is essential because radiation-induced myelopathy is irreversible and devastating.

Spinal cord tolerance increases when only a partial cross-section of the cord is irradiated. Therefore, point maximum dose constraints may be overly conservative for very small volumes. Volume-based constraints such as D0.03 cc and D0.35 cc are commonly used to better represent partial volume effects. Emerging data support the use of volumetric constraints over strict point-dose limits to optimize safety and efficacy.

## Target Volume Delineation

The International Spine Radiosurgery Consortium (ISRC) guidelines define the clinical target volume (CTV) based on the anatomical compartments of vertebral body involvement. These compartments include six segments: the vertebral body, right and left pedicles, right and left transverse processes/laminae, and the spinous process. The CTV encompasses the involved compartment(s) plus adjacent compartments at risk. In cases of epidural disease, the CTV extends to include the epidural space, and a planning organ at risk volume (PRV) is created for the spinal cord.

The gross tumor volume (GTV) corresponds to the visible tumor on MRI, typically identified on T1 post-contrast and STIR sequences. The CTV includes the entire involved vertebral body segment, even if the GTV occupies only part of it. Expansion to adjacent vertebral bodies is not standard practice unless imaging suggests involvement.

The planning target volume (PTV) generally involves a 2 mm expansion from the CTV, which may be reduced to 0 mm at the interface between the cord and CTV to minimize cord dose. This reduced margin near the spinal cord is compensated by a steep dose gradient. The accuracy of image guidance must support the chosen PTV margin to ensure precise delivery.

## Treatment Planning and Delivery

Imaging for spine SBRT requires MRI sequences including T1 post-contrast, T2, and STIR to accurately delineate the target and spinal cord. CT simulation with thin slices of 1 to 2 mm is fused with MRI for precise anatomical localization. Cone-beam CT (CBCT) is used at the time of treatment for verification.

Treatment planning typically employs volumetric modulated arc therapy (VMAT) or intensity-modulated radiation therapy (IMRT) using multiple non-coplanar or coplanar arcs. Dose painting techniques are utilized to deliver a high dose to the vertebral body while creating a steep dose gradient at the spinal cord interface. The typical prescription isodose line ranges from 80 to 90%, allowing a hotspot within the vertebral body. Plans are evaluated using conformity index, gradient index, and dose-volume histogram (DVH) constraints.

Image guidance is critical, with CBCT combined with a six-degree-of-freedom (6DOF) couch correction being mandatory to achieve the submillimeter accuracy required for spine SBRT. Additional verification is provided by systems such as ExacTrac or other kilovoltage stereoscopic imaging. Intrafraction monitoring is recommended for treatments lasting longer than 15 to 20 minutes. Patient immobilization is achieved using custom body cradles or vacuum cushions combined with head and body masks.

![Spine SBRT treatment plan showing steep dose gradient between vertebral body target and adjacent spinal cord with DVH](images/spine-sbrt-plan-dvh.jpg)

## Outcomes and Complications

De novo spine SBRT achieves excellent local control rates, with one-year local control ranging from 85 to 95%. For radioresistant histologies such as renal cell carcinoma and melanoma, local control rates are 80 to 90%, significantly higher than the 40 to 60% achieved with conventional radiation therapy. Reirradiation yields one-year local control rates between 70 and 85%.

The most common complication following spine SBRT is vertebral compression fracture (VCF), with an incidence ranging from 11 to 39% depending on dose and lesion location. Risk factors for VCF include lytic lesions, involvement of more than 40% of the vertebral body, and baseline fractures. Prophylactic vertebroplasty or kyphoplasty may be considered in high-risk patients. The SINS score is useful in identifying patients at increased risk for VCF.

Radiation myelopathy is a rare but devastating complication, occurring in less than 1% of cases when appropriate dose constraints are adhered to. It is irreversible, underscoring the importance of strict compliance with spinal cord constraints and accurate image guidance. MRI follow-up is essential to differentiate between tumor progression and radiation-induced changes.

![Algorithm for clinical decision-making in spine SBRT including assessment of stability, cord compression, and dose selection](images/spine-sbrt-decision-algorithm.jpg)

## Key Clinical Pearls

Spine SBRT offers superior local control compared to conventional palliative radiation therapy, particularly for radioresistant tumors such as renal cell carcinoma, melanoma, and sarcoma. The thecal sac is used as a surrogate for the spinal cord in dose constraints, with a maximum dose of less than 14 Gy for single-fraction treatments being standard. Vertebral compression fracture remains the most common complication, occurring in 11 to 39% of patients, and risk assessment using the SINS score and evaluation of lytic tumor burden is essential. Achieving submillimeter accuracy requires mandatory six-degree-of-freedom couch correction combined with cone-beam CT image guidance. In reirradiation scenarios, it is prudent to assume only 25% recovery of spinal cord tolerance and to apply conservative cumulative dose constraints to minimize the risk of myelopathy.

## References

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