• Jabehdar MaralaniPejmanP0000-0001-9975-4379Department of Medical Imaging, University of Toronto, Toronto, Canada., Chia-Lin Tseng, Hamidreza Baharjoo, Erin Wong, Anish Kapadia, Archya Dasgupta, Peter Howard, Aimee K M Chan, Eshetu G Atenafu, Hua Lu, Pascal Tyrrell, Sunit Das, Seyedmehdi Payabvash, Jay Detsky, Zain Husain, Sten Myrehaug, Hany Soliman, Hanbo Chen, Chris Heyn, Sean Symons, and Arjun Sahgal.
• Department of Medical Imaging, University of Toronto, Toronto, Canada.
• Neurosurgery. 2021 Oct 13; 89 (5): 884891884-891.

BackgroundThere are no established threshold values regarding the degree of growth on imaging when assessing response of spinal metastases treated with stereotactic body radiation therapy (SBRT).ObjectiveTo determine a magnetic resonance imaging-based minimum detectable difference (MDD) in gross tumor volume (GTV) and its association with 1-yr radiation site-specific (RSS) progression-free survival (PFS).MethodsGTVs at baseline and first 2 post-SBRT scans (Post1 and Post2, respectively) for 142 spinal segments were contoured, and percentage volume change between scans calculated. One-year RSS PFS was acquired from medical records. The MDD was determined. The MDD was compared against optimal thresholds of GTV changes associated with 1-yr RSS PFS using Youden's J index, and receiver operating characteristic curves between timepoints compared to determine which timeframe had the best association.ResultsA total of 17 of the 142 segments demonstrated progression. The MDD was 10.9%. Baseline-Post2 demonstrated the best performance (area under the curve [AUC] 0.90). Only Baseline-Post2 had an optimal threshold > MDD at 14.7%. Due to large distribution of GTVs, volumes were split into tertiles. Small tumors (GTV < 2 cc) had optimal thresholds of 42.0%, 71.3%, and 37.2% at Baseline-Post1 (AUC 0.81), Baseline-Post2 (AUC 0.89), and Post1-Post2 (AUC 0.77), respectively. Medium tumors (2 ≤ GTV ≤ 8.3 cc) all demonstrated optimal thresholds < MDD, with AUCs ranging from 0.65 to 0.84. Large tumors (GTV > 8.3 cc) had 2 timepoints where optimal thresholds > MDD: Baseline-Post2 (13.3%; AUC 0.97) and Post1-Post2 (11.8%; AUC 0.66). Baseline-Post2 had the best association with RSS PFS for all tertiles.ConclusionGiven a MDD of 10.9%, for small GTVs, larger (>37%) changes were required before local failure could be determined, compared to 11% to 13% for medium/large tumors.© Congress of Neurological Surgeons 2021.

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