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Objective: To correlate imaging parameters from baseline MRI diffusion-weighted imaging (DWI) and fludeoxyglucose (FDG) positron emission tomography (PET)-CT with synchronous and metachronous metastases in mucinous carcinoma (MC) and non-mucinous carcinoma (NMC) rectal cancer.Methods: 111 patients with extraperitoneal locally advanced rectal cancer, who underwent pelvic MRI, DWI and FDG PET-CT, were stratified into MC (n = 23) and NMC (n = 88). We correlated adverse morphologic features on MRI [mT4, mesorectal fascia involvement, extramural venous invasion (mEMVI), mN2] and quantitative imaging parameters [ minimum apparent diffusion coefficient (ADC(min)), maximum standardized uptake value, total lesion glycolysis, metabolic tumour volume, T-2 weighted and DWI tumour volumes] with the presence of metastatic disease. All patients underwent preoperative chemoradiation therapy (CRT); 100/111 patients underwent surgery after CRT and were classified as pathological complete response (PCR) and no PCR [tumour regression grade (TRG) vs TRG2-5] and as ypN0 and ypN1-2. Median follow-up time was 48 months. Metastases were confirmed on FDG PET-CT and contrast enhanced multidetector CT.Results: The percentage of mucin measured by MRI correlates with that quantified by histology. On multivariate analysis, the synchronous metastases were correlated with mEMVI [odds ratio(OR) = 21.48, p < 0.01] and low ADC(min) (OR = 0.04, p = 0.038) in NMC. The difference of metachronous recurrence between the MC group (10-90% mucin) and NMC group was significant (p < 0.01) (OR = 21.67, 95% confidence interval 3.8-120.5). Metachronous metastases were correlated with ypN2 (OR = 8.24, p = 0.01) in MC and in NMC. In NMC, mEMVI correlated with no PCR ( p = 0.018) and ypN2 (p < 0.01).Conclusion: mEMVI could identify patients with NMC, who are at high risk of synchronous metastases. The MC group is at a high risk of developing metachronous metastases.Advances in knowledge: Patients at high ris