Patterns Of Failure After Single Fraction Stereotactic Ablative Radiotherapy (SAbR) With A Simultaneous Integrated Boost (SIB) In The Treatment Of De Novo Spinal Metastases

Abstract

In spine SAbR, the International Spine Radiosurgery Consortium Consensus Guidelines recommend targeting both the gross tumor volume (GTV) and clinical target volume (CTV) to a single total dose of 16-24 Gy (Cox et al. IJROBP, 2012). However, it is unclear if such a high dose to the CTV is necessary for local control. We hypothesize that a lower dose is sufficient for control of microscopic disease and may decrease risk of toxicity, such as vertebral compression fracture (VCF). This study reports outcomes and analyzes patterns of failure after spine SAbR using a novel dose painting SIB technique. An IRB-approved, retrospective review was performed of 156 de novo spine metastases from 138 patients treated with single-fraction SAbR using a SIB approach from 2014-2019. The most commonly prescribed GTV and CTV doses were 20 Gy (range, 16-25 Gy) and 14 Gy (range, 12-20 Gy), respectively. Pre-existing VCF was present in 19% of cases, and prophylactic vertebroplasty after SAbR was performed in 22% of cases. Competing risk analysis was used to calculate cumulative incidences of local failure (LF) and VCF, with death as a competing risk. LF and VCF were assessed on follow up imaging with VCF defined as new or progression of pre-existing fracture after SAbR. Overall survival (OS) was estimated with Kaplan Meier analysis. Patterns of failure were examined in detail, and all ‘in-field’ failures were further subclassified as failure in the high-dose GTV area alone, low-dose CTV area alone, or in both the low- and high-dose areas. Median follow-up was 11 months. The 1- and 2-year cumulative incidence of LF was 10.7% and 22.6%, respectively, with a median time to LF of 4 months. OS was 68.3% at 1 year and 46.6% at 2 years, with median survival of 23 months. The 1- and 2-year cumulative incidence of VCF was 13.8% and 19.8%, respectively. There were 24 local failures in total. Of these, 18 (75%) were in-field, 5 (21%) were marginal/epidural, and 2 (4%) were outside the treated area. Of the in-field failures, 3 recurred only in the high-dose GTV area, 5 recurred only in the low-dose CTV area, and 12 were equivocal, with recurrence in both the low- and high-dose areas. For those who failed only in the low-dose area, the median isodose line at which failure occurred was 73% or 14.6 Gy (range, 70-95%). Single fraction spine SAbR using a dose painting SIB technique results in good overall local control and low rates of VCF. The patterns of local failure suggest that although rare overall, in-field failures are most common, with the majority of these occurring in the lower dose CTV. It is plausible that some of these failures may have been prevented with higher doses, at the risk of higher toxicity. Further study is needed to compare outcomes with the SIB technique described above to those with standard single-dose spine SAbR.