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

## TL;DR

This study finds that MRI scans done 24 weeks after treatment are most reliable for evaluating response in anal cancer patients.

## Contribution

The study introduces a new MRI-based tumor regression grading system (comrTRG) and identifies optimal timing for MRI evaluation in anal cancer.

## Key findings

- MRI positive predictive value increases significantly from 33% at 6 weeks to 88% at 24 weeks post-treatment.
- Combined T2+diffusion-weighted MRI tumour regression grade (comrTRG) is a practical tool for response evaluation in squamous cell carcinoma of the anus.
- High-risk patients may benefit from frequent MRI scans to detect treatment failure early.

## Abstract

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).

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.

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 tumour (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%.

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.

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.

ClinicalTrials.gov: NCT01937780.