Heterogeneous Water Diffusion on DWI/ADC in Moderately Differentiated Rectal Adenocarcinoma: A Case Report and Pathologic Correlation

Background: Colorectal tumors are common, and magnetic resonance imaging (MRI) with diffusion-weighted imaging (DWI), apparent diffusion coefficient (ADC), and contrast-enhanced sequences can assist in distinguishing benign from malignant lesions. However, certain atypical cases exist in which malignancies do not exhibit restricted water molecule diffusion. Case Presentation: We present the case of a 66-year-old male with moderately differentiated rectal adenocarcinoma showing heterogeneous diffusion restriction on MRI. The tumor surface demonstrated diffusion restriction (hyperintense on DWI, hypointense on ADC), whereas the base did not. Quantitative ADC analysis showed mean values of 0.82 × 10-3 mm2/s at the surface and 1.36 × 10-3 mm2/s at the base. Conclusion: This case highlights that regions with lower cellularity in moderately differentiated adenocarcinoma may lack diffusion restriction. Correlating imaging findings with histopathological results is essential to prevent diagnostic misinterpretation.