Initial Post-Therapy Imaging as a Predictor of Local Relapse after Radiotherapy in Pediatric Sarcomas

Abstract

<h3>Purpose/Objective(s)</h3> To assess the value of initial post-therapy scans for predicting local relapse in patients with Ewing sarcoma (EWS) and rhabdomyosarcoma (RMS) who received radiotherapy (RT) for local control. <h3>Materials/Methods</h3> This single-center study included all patients who were diagnosed with EWS or RMS at <22 years of age, completed RT for local control and underwent post-treatment diagnostic imaging evaluations from 2007-2020. Radiologists' impressions of the primary tumor site were categorized as "positive" (residual disease within RT field), "negative" (no residual disease within RT field), or "equivocal." Each patient was assigned to one of these categories, based on the radiologists' overall impression of all off-therapy scans. The primary endpoint was local progression (LP), defined as disease progression within the RT field. Positive predictive value (PPV) and negative predictive value (NPV) quantified the ability of off-therapy imaging studies to predict LP. Actuarial rates of LP were calculated using Kaplan-Meier methods. <h3>Results</h3> Sixty-eight patients were eligible (29 EWS, 39 RMS). The median age at diagnosis was 8 years (range 0-21). All patients received chemotherapy and RT based on disease histology and risk group, per Children's Oncology Group protocols. RT was to a median dose of 50.4 Gy (range 36-55.8); 19 patients (28%) underwent surgery and RT for local control, and 49 (72%) received RT alone. Post-treatment imaging was performed 3 weeks (range <1- 13) after completion of chemotherapy and 17 weeks (range 4-52) after completion of RT. Post-treatment imaging of the primary site included PET in 51 patients (75%), MRI in 54 (79%), and CT in 5 (7%). Radiologists' overall impressions were that 1 patient's (1%) scans were positive, 50 (74%) were negative, and 17 (25%) were equivocal. 4-year LP was 100% for patients with positive scans, 12% for patients with negative scans, and 5.9% for patients with equivocal scans. The difference in the percentages of LP for patients with negative vs. equivocal scans did not reach significance (p= 0.5). Positive scans had a PPV of 100%, and negative scans an NPV of 88%. When equivocal results were considered positive (positive/equivocal vs. negative), the PPV was 11%. When equivocal results were considered negative (positive vs. equivocal/negative), the NPV was 90%. Five patients (29%) with equivocal results underwent short-term surveillance imaging at a median of 7 weeks (range 6-9) after the equivocal studies. No short-term follow-up scan demonstrated local progression. <h3>Conclusion</h3> Equivocal findings on initial post-treatment diagnostic imaging studies did not indicate a higher risk of local recurrence after RT than negative findings. These results may influence the interpretation of initial post-therapy scans and contribute to accurate counseling of patients and families.