Dosimetric evaluation of target coverage as a predictor of local failure following stereotactic body radiation therapy for spinal tumors

Purpose: To perform a dosimetric analysis of target coverage and determine parameters predictive for local failure (LF) in patients undergoing spinal stereotactic body radiation therapy (sSBRT).Materials and Methods: Sixty-seven spinal tumors in 59 patients were treated with image-guided linac-based sSBRT from 2008-2012. Median prescription dose was 18Gy (8-35) delivered in 1-5 fractions (87% single-fraction). Prescription dose was targeted to cover ≥ 80% of PTV within spinal cord (SC) dose constraints (9/11Gy to 0.1cc SC/SC+2mm). Twelve tumors had local failure (LF, median time-to-failure 3.7 months) and were compared to 14 tumors with >1-year follow-up and local control (LC). Univariate and multivariate analyses were performed to determine parameters predictive of LF.

Results: Median follow-up was 7.4 months and 24.7 months for LF and LC, respectively. Post-SBRT, 42% of LF patients had neurological symptoms due to tumor progression. No patients developed post-SBRT myelopathy. Pre-treatment PTV volumes were not statistically different (median/mean/range 61.8/74.5/19.9-206.4cc for LF vs 39.4/47.1/10.3-119.7cc for LC; p=0.13). LF tumors had larger volumes receiving <80% of prescription dose (5.2cc vs 1.9cc, p=0.02) and larger overlap volume between GTV/SC within 2 and 3mm (p=0.01/p=0.007). LF tumors had lower GTV minimum dose (5.6 vs 8.5Gy, p=0.001) and smaller GTV to SC distance (0.06 vs 0.19mm, p=0.049). Maximum SC doses were not statistically different (6.4Gy LC vs 9.2Gy LF, p=0.33). GTV minimum dose was predictive of LF, with a trend for overlapping GTV/SC volume within 2mm.

Conclusions: Minimum GTV dose, PTV volume receiving <80% prescription dose, smaller GTV-SC distance, and large overlapping volume of PTV/SC are predictive of LF after SBRT. Given the absence of SC toxicity but neurological progression upon LF, less conservative SC constraints should be considered.