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

• MR findings of pCR-achieved rectal cancer after CCRT are highly variable.
• The residual lesion of pCR-achieved rectal cancer often maintains large volume.
• pCR lesions mostly show decreased T2 SI, but sometimes intermediate SI remain.

AimTo 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 methodsA 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.ResultsTumour 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.ConclusionMRI 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.