Introduction The optimal time interval between chemoradiotherapy (CRT) and surgery for rectal cancer has not been clearly defined. A longer interval may facilitate greater tumour down-staging. There is, however a paucity of high-quality evidence, with the majority of published data being from observational studies.

Method This study explores down-staging of rectal cancer assessed by Magnetic Resonance Imaging (MRI) within a pilot Randomised Controlled Trial (RCT) comparing an interval of six weeks versus an interval of 12 weeks.

Patients with rectal cancer requiring long-course CRT from seven sites were randomised to have surgery after an interval of either 6 weeks or 12 weeks following completion of long-course CRT. MRI scans were performed at baseline and at approximately 1 week prior to surgery. Additionally in the 12-week arm an additional “interval” MRI scan was performed approximately 5–6 weeks after completion of CRT. All scans were de-identified and analysed by a single blinded consultant gastrointestinal radiologist. Tumours were staged using the TNM system and the response to CRT by the tumour regression grade (TRG) system.

Results Fifteen patients were randomised to surgery at 6-weeks and 16 to surgery at 12-weeks. Six patients (40%) from the 6-week arm and seven (50%) from the 12-week arm showed down-staging of the primary tumour by 1 or more stage between baseline and pre-operative MRI scans. Lymph node down-staging was observed for 10 patients (67%) in the 6-week arm and nine patients (64%) in the 12-week arm. No patients had an increase in T stage, although two patients in the 6-week arm had an increase in N stage between baseline and 6 weeks. Three patients (21%) in the twelve week arm showed ongoing down-staging of T stage on MRI between 6 and 12-weeks, and four patients had a reduction of their TRG between scans, indicating ongoing down-staging.

Conclusion Although limited by small sample size, this analysis of data from a randomised controlled trial demonstrates that ongoing down-staging may continue between six and twelve weeks following completion of CRT. A longer interval might be considered where resection margins are still threatened on MRI at six weeks.

Disclosure of interest None Declared.