Risk of Vertebral Fracture and Local Recurrence after Palliative Spine Stereotactic Body Radiotherapy (SBRT)

Abstract

Bony metastases to the spinal column are a common manifestation of cancer that can lead to significant morbidity. Although stereotactic body radiotherapy (SBRT) has been increasingly employed as an effective treatment for spinal column metastases, the optimal dosimetric parameters to maximize disease control and minimize toxicity remain unclear. Thus, we sought to understand the dosimetric factors associated with pain palliation, local control, and toxicity with spinal column SBRT for bone metastases. Patients from a single institution treated with spine SBRT between 2010 and 2017 for metastatic disease had electronic records retrospectively reviewed. Baseline patient characteristics, treatment parameters including spinal cord dose, and follow-up data were collected. Treatment-related outcomes including freedom from fracture rate (FFFR), myelitis, and freedom from local recurrence (FFLR) were assessed. Statistical analysis of categorical variables was done with Fischer’s exact test and continuous variables with Wilcoxon rank sum, with the threshold for significance of all tests at a p-value <0.05. During the study period 96 patients were treated with SBRT to 137 sites in the cervical, thoracic, and lumbar spine, with a median clinical follow up of 15 months per lesion (IQR: 4-33). There were 79, 19, and 39 treatments with 1, 3, and 5 fractions, with dose ranges of 14-20 Gy, 24-27 Gy, and 30-40 Gy, respectively. Eight lesions were treated post-operatively, 19 lesions were treated with SBRT as re-irradiation, with the rest of the lesions undergoing SBRT as the first treatment of choice. Pain was a presenting symptom at 104 of treated sites, and the median pain score was 3/10 (IQR: 1-7). On post treatment clinical follow up approximately 2-4 weeks after treatment, 89% of patients noted improved pain, with a median decrease in score of 3 points. FFFR at 12 and 24 months was 91% (CI: 86-97) and 86% (CI: 78-94) respectively without significant association with fractionation, dose, SINS score, or pre-existing compression fracture. The median D0.035cc of the true spinal cord for 1, 3, and 5 fraction treatments were 12.9 Gy (IQR:10.4-14.4), 18.5 Gy (IQR: 17.2-20.8), and 25.2 Gy (IQR: 21.8-28.4), with only one case of asymptomatic radiographic myelitis noted. FFLR at 12 and 24 months was 84% (CI: 77-92) and 78% (CI: 69-87) respectively, with no significant association between histology, dose, or fractionation and local recurrence. Spine SBRT demonstrated high rates of pain relief with acceptable toxicity profile over a range of relatively conservative dose-fractionation schema. Treatments typically exceeded TG101 guideline spinal cord “threshold doses,” while respecting maximum point doses, for the spinal cord without evidence of symptomatic myelitis. These results show superior FFFR and FFLR than prior reported studies and supports increasing use of SBRT in a wide range of patients.