MRI T1 relaxation time for evaluating early complete response to neoadjuvant treatment in rectal cancer: measurement at six weeks – a protocol article

Management of rectal cancer requires accurate staging and treatment. Neoadjuvant chemoradiotherapy offers tumour size reduction and mitigation of the risk of local relapse. Patients with complete response to neoadjuvant treatment can be enclosed in watchful waiting (WW). Recent studies have explored magnetic resonance imaging (MRI) T1 relaxation time (T1RT) as a predictive biomarker for treatment response in rectal cancer. Preliminary findings indicate that lower T1RT correlates with pathologic complete response. However, inclusion of patients in WW remains unexplored. This prospective study aims to investigate T1RT 6 weeks after neoadjuvant treatment and the ability to determine complete response. MRI scans are conducted on a 1.5 T MRI-unit. T1RT is measured at time of diagnosis and 6 weeks after neoadjuvant treatment. Experienced radiologists analyse T1RT using specialised software. Treatment decisions are made in multidisciplinary team conferences based on tumour staging. Endpoints include tumour visibility on MRI and endoscopy, along with histopathological analysis of surgical specimens. Statistical methods include t test and receiver operating characteristic curves. Sample size calculations showed we must enrol 76 participants to achieve a statistical power of 80% with an α = 0.05. Data analysis begins in winter 2025. Results are planned to be submitted in spring 2026. The implications of this study extend to the potential refinement of treatment strategies, offering patients the prospect of improved outcomes and the potential avoidance of surgery-associated risks. We expect to find a lower relaxation time in fibrotic tissue compared to non-responsive cancerous tissue after 6 weeks.