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 tumour regression grade (comrTRG). Methods: We assessed the positive and negative predictive values (PPV, NPV) of post-treatment MRI in a retrospective mono-centre diagnostic accuracy study that prospectively included consecutive patients treated between 2013 and 2017. Index tests were MRI at 6-, 12-, and 24-weeks post-treatment (6w, 12w, and 24w) to detect locoregional treatment failures (LRTF). Clinical outcome served as reference standard. Tumour 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: Conclusions: 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.