A dosimetric study of single-fraction stereotactic body radiotherapy using proton versus photon for spinal metastases

Bone metastases are a common indication for palliative radiotherapy. Stereotactic body radiotherapy (SBRT) enables the delivery of ablative doses in 5 or fewer fractions, potentially improving local control and symptom relief. Proton-based SBRT (SBPT) may enhance sparing of organs at risk (OARs) due to its favorable dose distribution. This study compared the dosimetric profiles of single-fraction photon versus proton SBRT for spine metastases. Twelve patients with thoracic or lumbar spine metastases were included in this IRB-approved dosimetric analysis. Each patient had SBRT (photon) and SBPT (proton) plans generated to deliver 20 Gy in one fraction. Target volumes and OARs were contoured following international guidelines. Plans were evaluated using dose-volume histograms, and comparisons were performed using Wilcoxon signed-rank test with significance defined as p < 0.05. Target coverage was similar between modalities, with no significant differences in clinical target volume Dmax, D90, or D95. SBPT resulted in significantly lower spinal cord dose (Dmax 0.5 cc: 7.1 Gy vs 11.9 Gy, p = 0.008) and lower lung mean dose (0.2 Gy vs 0.7 Gy, p = 0.040). Bowel doses were also significantly reduced with SBPT (Dmax 0.5 cc: 3.2 Gy vs 6.1 Gy, p = 0.016). The esophageal D5cc was significantly lower in SBPT plans (8.0 Gy vs 1.9 Gy for proton, p < 0.001), although all values were within safe clinical thresholds. SBPT offers equivalent target coverage with improved sparing of critical organs, including the spinal cord, lungs, bowel, and esophagus, supporting its potential clinical advantages in spinal metastases treatment.