Primary malignant melanoma of the rectum: CT findings in eight patients. 
To retrospectively evaluate computed tomographic (CT) findings in patients with pathologically proved primary malignant melanoma of the rectum.  CT scans of eight patients (three men and five women; age range, 38-74 years; mean age, 62 years) with histologically proved primary rectal malignant melanomas were retrospectively evaluated by two radiologists in consensus.  Scans were evaluated for the involved site, approximate size and morphologic appearance of the primary mass, degree of perirectal infiltration, and presence or absence of lymphadenopathy, bowel obstruction, and distant metastases.  All of the tumors were located in the distal rectum just above the anal verge.  The approximate average sizes of the tumors were as follows: length, 4.8 cm (range, 3.8-6.9 cm); width, 3.8 cm (range, 2.8-5.2 cm); and mean diameter, 4.3 cm (range, 3.3-5.8 cm).  Tumors usually appeared as polypoid or fungating intraluminal masses (n = 7).  Perirectal infiltration commonly extended to the pelvic side wall or the presacral space (n = 5).  All eight patients had lymphadenopathy, frequently larger than 3 cm in diameter (n = 3), which most commonly involved the perirectal lymph node station (n = 7).  There was no evidence of bowel obstruction in any of the patients.  Distant metastasis involving the liver was noted in one patient.  On CT scans, primary rectal malignant melanomas appeared as bulky intraluminal fungating masses in the distal rectum, focally expanding and obscuring the lumen without causing obstruction, with perirectal infiltration and frequently enlarged lymph nodes.