Purpose To determine the diffusion-weighted imaging (DWI) characteristics of ano-rectal mucinous carcinoma. Apparent diffusion coefficient (ADC) and DWI findings for mucinous and tubular adenocarcinomas were retrospectively compared. Materials and methods Eight-one consecutive surgically resected ano-rectal adenocarcinomas were evaluated. The patient group consisted of 15 mucinous and 66 tubular adenocarcinomas. The DWI signal intensity (SI) pattern of each tumor was visually classified into one of three groups. Differences in the proportions in each group were statistically compared for the two types of tumor. The ADCs of each tumor, calculated from two different b-factors (0 and 1500 s/mm 2) were compared statistically. Results Visual evaluation revealed ten mucinous carcinomas as predominantly hypointense lesions, and the remaining five as mixed SI lesions. Sixty-one of 66 tubular adenocarcinomas had hyperintensities
the remaining five had mixed SI. The different proportions in each group were statistically significant. The mean ADC for mucinous carcinomas was 1.49 ± 0.34 9 10-3 mm 2/s whereas that for tubular adenocarcinomas was 0.80 ± 0.15 9 10-3 mm 2/s. The difference between those figures was statistically significant. Conclusion In DWI, mucinous carcinomas had higher ADCs and lower SI than tubular adenocarcinomas. DWI and ADC measurements were useful for differentiating these two tumors. © 2011 Japan Radiological Society.