BackgroundIt is widely believed that small rectal tumors are more likely to have a good response to neoadjuvant treatment, which may influence the selection of patients for a ‘watch and wait’ strategy.ObjectiveThe aim of this study was to investigate whether there is a relationship between baseline tumor length on magnetic resonance imaging (MRI) and response to chemoradiotherapy.MethodThe 96 patients with locally advanced rectal cancer randomised (2:1–intervention:control) in the TRIGGER feasibility study where eligible. Baseline tumor length was defined as the maximal cranio-caudal length on baseline MRI (mm) and was recorded prospectively at study registration. Magnetic resonance tumor regression grade (mrTRG) assessment was performed on the post-chemoradiotherapy (CRT) MRI 4–6 weeks (no later than 10 weeks) post completion of CRT. This was routinely reported for patients in the intervention (mrTRG-directed management) arm and reported for the purposes of this study by the central radiologist in the control arm patients. Those with an mrTRG I/II response were defined as ‘good responders’ and those with an mrTRG III–V response were defined as ‘poor responders’.ResultsOverall, 94 patients had a post-CRT MRI performed and were included. Forty-three (46%) patients had a good response (mrTRG I/II) and 51 (54%) patients had a poor response (mrTRG III/IV). The median tumor length of good responders was 43 mm versus 50 mm (p< 0.001), with considerable overlap in tumor lengths between groups.ConclusionBaseline tumor length on MRI is not a clinically useful biomarker to predict mrTRG tumor response to CRT and therefore patient suitability for a deferral of surgery trial.