Deviation From Consensus Contouring Guidelines Predicts Inferior Local Control After Spine Stereotactic Body Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Stereotactic body radiotherapy (SBRT) is increasingly utilized for the treatment of spine metastasis. Although consensus guidelines for spine SBRT target delineation have been published, data regarding their impact on patient outcomes are limited. The goal of this study is to analyze predictive factors for long-term local control (LC) in a large, mature series. <h3>Materials/Methods</h3> Patients with de novo metastasis of the spine treated at an academic institution between 2009 and 2020 were retrospectively reviewed. Patients with prostate and hematologic malignancies were excluded. Variables examined included primary tumor type, prior surgery and radiation, location, disease extension into the epidural space and paraspinal soft tissue, adherence of target volume to consensus guidelines (retrospectively reviewed from treatment plans), and prescribed dose and fractionation. Target lesions and epidural disease (ED) were identified on MRIs obtained at the time of treatment planning within 4 weeks of SBRT delivery. ED was graded as minimal (Bilsky grade 0-1a), low grade (LG-ED, Bilsky grade 1b) and high grade (HG-ED, Bilsky grade 1c-3). Univariable and multivariable Cox proportional hazard models were constructed using the Fine and Gray competing risk method with death as a competing risk for local progression. <h3>Results</h3> A total of 360 discrete lesions in 283 patients were treated with SBRT. The majority of lesions received 24-27Gy in 2-3 fractions. At median follow up of 14.6 months (range 1.2-131.3), 68 lesions had local progression, with 1 and 2-year LC rate of 87.6% (95% CI 82.6-91.2%) and 73.8% (95% CI 66.3-80.0%), respectively. In total, 58 deviations (16.1%) from consensus contouring guidelines were identified. Targets treated before 2012 and sacral lesions were more likely to deviate from guidelines. On univariable analysis, deviation from guidelines was associated with inferior LC (hazard ratio [HR] 2.68, 95% confidence interval [CI] 1.65-4.34, <i>P</i> < 0.001). One-year LC was 89.6% (95% CI 84.1-93.3%) for targets that adhered to guidelines, vs 78.2% (95% CI 63.3-87.7%) for those with deviations. Other factors associated with inferior LC included ED (univariable HR 4.10 for LG-ED, 95% CI 1.77-9.53, <i>P</i> = 0.001; HR 4.24 for HG-ED, 95% CI 1.90-9.45, <i>P</i> < 0.001), gastrointestinal (GI) primary (HR 2.45, 95% CI 1.36-4.40, <i>P</i> = 0.003) and paraspinal extension (HR 2.46, 95% CI 1.43-4.22, <i>P</i> = 0.001). On multivariable analysis, deviation from guidelines (HR 2.56, 95% CI 1.55-4.22, <i>P</i> < 0.001), ED (HR 3.38 for LG-ED, 95% CI 1.43-7.96, <i>P</i> = 0.005; HR 3.45 for HG-ED, 95% CI 1.52-7.86, <i>P</i> = 0.003), GI primary (HR 2.19, 95% CI 1.19-4.02, <i>P</i> = 0.01) and paraspinal extension (HR 2.24, 95% 1.29-3.87, <i>P</i> = 0.004) demonstrated significant association with progression. <h3>Conclusion</h3> SBRT provides durable tumor control for spine metastasis. These data support the use of consensus contouring guidelines to maximize the likelihood of LC.