Optimal time for MRI response evaluation in squamous cell carcinoma of the anus

Objectives To identify the optimal time for MRI response evaluation after chemoradiotherapy (CRT) in squamous cell carcinoma of the anus (SCCA) and to employ combined T2+diffusion-weighted MRI tumor regression grade (comrTRG). Methods We assessed the positive and negative predictive values (PPV, NPV) of post-treatment MRI in a retrospective mono-center diagnostic accuracy study that prospectively included consecutive patients treated between 2013-2017. Index tests were MRI at 6-, 12-, and 24-weeks post-treatment (6w, 12w, 24w) to detect locoregional treatment failures (LRTF). Clinical outcome served as reference standard. Tumor regression was assessed using comrTRG based on radiological reports. Mixed-effects logistic regression was used to compare the comrTRG score across time points. The analyses were stratified by patients` T/N stage and human papillomavirus (HPV) status. Results For 127 included patients (62 years ± 11 [mean±SD]; 92 women), 261 post-treatment MRI reports (6w: n = 45, 12w: n = 125, 24w: n = 91) were scored using comrTRG. LRTF occurred in 13 patients; 12/13 were high-risk patients (T3/T4, N+, or HPV-negative); 1/13 progressed early (<24 weeks). The rate of radiologic complete response (comrTRG1) increased over time (6w: 27%, 12w: 66%, 24w: 75%), while the rate of indeterminate (comrTRG2) and minor definite tumor (comrTRG3) decreased. PPV of MRI increased over time: 6w: 33% (95%CI: 9.9-65.1%), 12w: 46% (16.7-76.6%), and 24w: 88% (47.3-99.7%). NPV was stable high >90%. Conclusion MRI performed more reliably after 24 weeks. Timely assessment may aid early LRTF detection. Tailoring follow-up with frequent MRI scans may be sufficient for high-risk patients. Combined mrTRG is practical for describing response in SCCA. Advances in knowledge Post-treatment MRI assessment at later time points is preferable in SCCA to avoid inconclusive imaging and unnecessary salvage surgery. The introduced comrTRG is a practical tool for response evaluation. Registration ClinicalTrials.gov: NCT01937780