Spatially Fractionated Radiation Therapy in the Modern Era: The Mayo Clinic Experience

Abstract

<h3>Purpose/Objective(s)</h3> Spatially fractionated radiation therapy (SFRT) is a specialized technique that may enhance response to conventionally fractionated radiation in bulky radioresistant tumors. We hypothesized that SFRT using modern radiotherapy techniques would result in acceptable local control and low toxicity. <h3>Materials/Methods</h3> Medical records were retrospectively reviewed for clinical characteristics, dose prescription, toxicity, and outcomes under an IRB approved protocol. Descriptive statistics were used to collate patient characteristics, treatment outcomes and toxicities. Local control, progression free survival, and overall survival were estimated using Kaplan Meier method. Univariate Cox analysis was used to find predictors of outcomes and grade 3+ toxicity (CTCAE v5.0). <h3>Results</h3> A total of 126 patients with 129 treatment courses were treated from October 2019 through November 2021 with a median follow-up until death of 313 days. Of the 126 patients, 102 had follow up imaging and visits post treatment. Median age was 62 years (interquartile range [IQR] 22-96 years). Lesions in the pelvis (21%), abdomen (26%), and chest (38%) were the most common SFRT targets. The most common histologies treated were sarcoma (33%) and non-small cell lung cancer (16%). Most patients received an SFRT dose of 20 Gy (85%) with VMAT placed spheres (84%). Twenty-five patients (20%) received SFRT alone without subsequent EBRT, 50 patients (40%) received follow up palliative EBRT (EQD2 <= 40 Gy10), and 51 patients (40%) received follow up definitive EBRT (EQD2 > 40 Gy10). Median time between SFRT and follow up EBRT delivery was 4 days (IQR 3-7 days). Local control was excellent at 86% at 6 months and 83% at 12 months. There were 14 grade 3+ toxicity events potentially attributable to any radiation therapy (7.5% cumulative incidence at 6 months, 13% at 12 months). Grade 3+ toxicities included: 2 non healing wound/skin ulcers, 7 fistulae (bowel/esophageal/bronchopleural), 1 esophagitis requiring hospitalization, 1 grade 3 enteritis and GI bleed, 2 radiation pneumonitis, 1 radionecrosis of the cervix. No patient or treatment characteristics were predictive of local control, survival or toxicity. <h3>Conclusion</h3> The current study is one of the largest modern series in which most patients received VMAT SFRT. While local control was excellent, there was grade 3 toxicity in the abdomen/pelvis with fistulae post radiation, some from complete clinical response and others from disease progression following SFRT + EBRT. SFRT remains a valuable tool for treating patients who otherwise have limited local control options.