OBJECTIVETo investigate the association of mesorectal invasion depth before neoadjuvant radiochemotherapy with pathological outcome and to provide evidence for individualized treatment in T3 rectal cancer.

METHODSRetrospective analysis was performed on the clinical records of 73 consecutive rectal cancer patients treated with neoadjuvant radiochemotherapy and radical surgery in the Shanghai Cancer Center from January 2010 to December 2012. All the patients underwent high-resolution MRI and the depth of mesorectal invasion, lymph node status, tumor length, and mesorectal fascia status were evaluated. The category T3 was subdivided according to the measurement of the maximal tumor invasion beyond the outer border of the muscularis propria: T3a(<5 mm), T3b(5-10 mm) and T3c(>10 mm). The association of mesorectal invasion depth,other MRI and clinical features with short-term efficacy was analyzed,especially with pathological complete response(pCR).

RESULTST3a, T3b and T3c accounted for 19.2%, 64.4% and 16.4% in 73 rectal cancer patients who underwent high resolution MRI, respectively. There were 42.9% of T3a patients achieved pathological complete response,significantly higher than those of T3b(14.9%)and T3c(0%) (P=0.017).

CONCLUSIONST3a rectal cancer patients are more likely to achieve pCR than those of T3b and T3c after neoadjuvant radiochemotherapy. The maximal tumor invasion beyond the outer border of the muscularis propria less than 5 mm may act as a predictive factor and guide the follow-up treatment of T3 rectal cancer.