Profiling of rectal cancers MRI in pathological complete remission states after neoadjuvant concurrent chemoradiation therapy

Aim To fully characterise the magnetic resonance imaging (MRI) traits of rectal cancers in a large sample of patients, each experiencing pathological complete remission (PCR) after neoadjuvant concurrent chemoradiation therapy (CCRT). Materials and methods A total of 120 patients (77 male, 43 female; median age, 59.5 years; range, 32-81 years) with rectal cancers in CCRT-induced PCR states who underwent pre- and post-CCRT MRI and eventual surgery between July, 2005 and September, 2014 were retrospectively reviewed. In most (n=100), diffusion-weighted imaging was also performed. Tumour volume, tumour regression grade (TRG), T-stage, mesorectal fascia (MRF) status, and T2 signal intensity (T2-SI) were analysed. Paired t-test and McNemar's test were applied for statistical comparisons. Results Tumour volume declined sharply after CCRT (pre-CCRT, 21.5±22.4cm³; post-CCRT, 6.6±8.4cm³; p<0.001). TRG distribution was as follows: G1 (clinical CR), 3; G2, 38; G3, 78; G4, 1; and G5 (marked progression), 0. Downstaging of T-stage (34%,16/47) and MRF status (19.7%,13/66) did occur; but on post-CCRT MRI, 25.8% (31/120) remained at T3_{≥5 mm} or T4 stage, and 44.2% (53/120) were MRF-positive. A majority (88.3%, 106/120) of patients displayed intermediate T2-SI prior to CCRT. Most converted to dark T2-SI after CCRT, with 12.5% (15/120) unchanged. On post-CCRT MRI, 11% (11/100) of patients showed diffusion restriction. Conclusion MRI findings in CCRT-induced PCR-status rectal cancers were highly variable. Tumour volume and T2-SI mostly decreased; however, such lesions occasionally presented with unexpected atypical features, such as large residual volume and/or intermediate T2-SI.