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Dosimetric evaluation of target coverage as a predictor of local failure following stereotactic body radiation therapy for spinal tumors
Maha Saada Jawad, Jun Zhou, Joe G. Harb, J. Ben Wilkinson, Shannon K. Prausa, Jennifer Wloch, Daniel J. Kraus, Daniel Fahim, Di Yan and Inga S. Grills

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.

Keywords: Radiosurgery, spinal tumors, SRS, spinal SBRT, Stereotactic Body Radiotherapy, SBRT

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