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Objectives: Stereotactic body radiation therapy (SBRT) for the spine is challenging due to highdose gradients sparing the cord in the treatment plans. We present our findings of initial setup error and intrafraction motion from Cone-beam computed tomography (CBCT) imaging. Materials and methods: A total of 47 patients treated with spine SBRT with a total of 154 fractions following a fractionation schedule of 16 Gy in 1, 24 Gy in 2, and 30 Gy in 5 fractions were part of this study. Pre-treatment CBCT was used for localization of the target and couch shifts were applied based on target volume matching to the planning CT image set. Post-treatment CBCT was acquired for all fractions. Intrafraction motion (IFM) was calculated by matching post-treatment CBCT to planning CT for the target volume. Results: The average Intrafraction motion was 1.6 ± 0.9 mm for the study cohort. The average and standard deviation of intrafraction motion were 0.4 ± 1.1 (AP), 0.3 ± 0.9 (SI) and 0.2 ± 1.2 (RL) respectively. The average Initial setup error tabulated from the offline review showed a mean value of 7.8 ± 5.3 mm. The average and standard deviation of the initial setup error were 2.5 ± 5.5 (AP), 2.4 ± 5.3(SI), and 0.8 ± 4.5(RL) respectively. The correlation of intrafraction motion with body mass index (BMI) and the number of consecutive vertebrae levels did not show any statistical significance, however, there was a significant association with gender as women showed more IFM Conclusions: Our study on intrafraction motion from CBCT images reinforced the importance of immobilization and imaging for positioning spine SBRT patients. [ABSTRACT FROM AUTHOR]

Purpose: Stereotactic body radiotherapy (SBRT) improves complete pain response for painful spinal metastases compared to conventional external beam radiotherapy (cEBRT). We report mature local control and reirradiation rates in a large cohort of patients treated with SBRT vs. cEBRT enrolled previously in the Canadian Clinical Trials Group Symptom Control (SC).24 phase II/III trial. Methods/materials: 137/229 (60%) patients randomized to 24 Gy in 2 SBRT fractions or 20 Gy in 5 cEBRT fractions were retrospectively reviewed. By including all treated spinal segments, we report on 66 patients (119 spine segments) treated with SBRT, and 71 patients (169 segments) treated with cEBRT. The primary outcomes were MR-based local control and reirradiation rates for each treated spine segment. Results: The median follow-up was 11.3 months (IQR:5.3-27.7 months), and median OS in the SBRT and cEBRT cohorts were 21.6 and 18.9 months (p=0.428), respectively. The cohorts were balanced with respect to radioresistant histology and presence of “Mass” (paraspinal and/or epidural disease extension). Risk of local failure after SBRT vs. cEBRT at 6, 12 and 24 months were 2.8% vs. 11.2%, 6.1% vs. 28.4% and 14.8% vs. 35.6%, respectively (p<0.001). cEBRT (HR:3.48, 95%CI:1.94-6.25, p<0.001) and presence of “Mass” (HR:2.07, 95%CI:1.29-3.31, p=0.002) independently predicted local failure on multivariable analysis. The 1-year reirradiation rates and median times to reirradiation after SBRT vs. cEBRT, were 2.2% vs 15.8% (p=0.002) and 22.9 months vs. 9.5 months respectively. Radioresistant histology (HR:2.66, 95%CI:1.43-4.94, p=0.002) and cEBRT (HR:2.34, 95%CI:1.14-4.78, p=0.002) independently predicted for reirradiation. 8/12 iatrogenic vertebral compression fractures (VCFs) were after SBRT and 4/12 after cEBRT; Grade 3 toxicities were isolated to the SBRT cohort (5/12). Conclusions: Risk of local failure and reirradiation is lower with SBRT compared to cEBRT for spinal metastases. Although the iatrogenic VCF rates were within expectations, Grade 3 VCF were isolated to the SBRT cohort. [ABSTRACT FROM AUTHOR]

Introduction: Stereotactic body radiotherapy (SBRT) planning for spinal metastases is a challenging task that involves complex target shapes and steep dose gradients proximal to the spinal cord. The aim of the present study is to investigate dosimetric variability among delivery systems and institutions doing spine SBRT. Materials and Methods: Three institutions (in Japan, Canada, and the USA) participated in this retrospective treatment planning study. Computed tomography (CT) datasets for three patients including fully delineated targets and organs-at-risk (OAR) were distributed to all three institutions for planning. Delivery systems included the Clinac 21EX, Vero4DRT, Synergy S, and CyberKnife. All treatment plans were generated using a prescribed dose of 24 Gy in 2 fractions and met the following objectives: the evaluated planning target volume (PTVevl, defined as the PTV minus spinal cord) should receive greater than 16.8 Gy in at least 95% of the volume (D95 > 16.8 Gy) and a maximum dose to the less than 140% of the prescribed dose (Dmax < 33.6 Gy). The maximum dose of planning risk volume (PRV) cord or thecal sac was limited to 0.035 cm3 receiving less than 17 Gy. Aside from minimum and maximum dose objectives for the PTVevl, there were no criteria regarding the shape of the PTVevl dose-volume histogram (DVH). For each completed treatment plan, the following DVH parameters were evaluated for the PTVevl: D95, D80, D50, D2 and sigma-index (S-index, standard deviation of the differential DVH). Results: The PTVevl and OAR dose volume constraints were satisfied in all treatment plans. For Case 1, the mean PTVevl D50 was 25.4 ± 1.5 Gy (range: 23.7 - 27.8 Gy), for Case 2 it was 26.7 ± 2.0 Gy (23.6 - 28.6 Gy), and for Case 3 it was 26.0 ± 1.3 Gy (24.1 - 27.3 Gy). The mean PTVevl D2 was 27.3 ± 2.2 Gy (24.4 - 30.2 Gy), 28.9 ± 3.0 Gy (24.5 - 31.4 Gy) and 28.7 ± 2.7 Gy (25.2 - 31.6 Gy) for Cases 1, 2, and 3, respectively. However, there were statistically significant variations in the DVH parameters of PTVevl between apparatuses (CyberKnife versus non-CyberKnife) and among institutions (between 2 CyberKnife sites or between 2 conventional accelerator sites). Conclusions: Although all institutions met the minimum prescribed objectives, inter-institutional and interapparatus target dose variations were observed. Further study is necessary to determine target dose constraints that may minimize inter-institutional variations and lead to plan standardization. [ABSTRACT FROM AUTHOR]

Purpose: Determine the risk of vertebral compression fracture (VCF) following stereotactic body radiotherapy (SBRT), specific to osteolytic renal cell carcinoma (RCC) spinal metastases, and associated predictive factors. Methods: 187 RCC osteolytic spinal tumor segments in 116 patients obtained from a multi-institutional pooled database were reviewed. Each segment was evaluated according to the Spinal Instability Neoplastic Score (SINS). Results: The median follow-up was 8.0 months. 34 VCF (34/187, 18%) were observed and median time to VCF was 2.4 months. VCF was observed in 43% (10/23), 24% (4/17) and 14% (20/147) of segments treated with 24Gy/fraction (fx), 20-23Gy/fx and ≤19Gy/fx, respectively. Multivariate analysis identified dose per fx (p=0.005), baseline VCF (p<0.001) and spinal misalignment (p=0.002) as predictors of VCF. Prior conventional radiotherapy (p=0.029) was found to be protective. Conclusions: 18% of osteolytic RCC spinal metastases fractured post-SBRT. The presence of a baseline fracture, spinal mal-alignment and treatment with ≥20Gy/fx predicted for VCF. [ABSTRACT FROM AUTHOR]