A 64-year-old man complains of bloody diarrhea.

Fig. 95.1 Axial T2-weighted MRI image of the pelvis shows ill-defined circumferential thickening of the rectum due to a low signal intensity tumor (arrow). There is extension of the tumor into the perirectal fat (arrowhead), making it a T3 disease, as well as an adjacent metastatic lymph node (curved arrow).

Rectal adenocarcinoma with perirectal extension of tumor

• Rectal lymphoma
  
• Rectal metastases (rare)

Colorectal carcinoma is the second most common cancer in the United States. Approximately one third of these cancers occur in the rectum. Ninety-eight percent of rectal cancers are adenocarcinomas, and they usually arise from adenomatous polyps.