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A 62-year-old man with a 40-pack-year history of cigarette smoking, who had a malignant melanoma on his neck 5 years ago, presents to the emergency room with acute intestinal obstruction. Laparoscopic examination of his abdomen demonstrates multiple, large, white masses on the serosa of his small bowel. This patient most likely has which of the following?
A. An inflammatory lesion
B. Metastatic lung cancer
C. Metastatic melanoma
D. Primary intestinal lymphoma
E. Pseudomyxoma peritonei
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The correct answer is C. The obvious answer is correct. The serosal surface of the intestines is a common metastatic site for melanoma, and is an unusual site for other tumors not located in the abdomen or pelvis (tumors of the abdominal and pelvic organs can seed the peritoneal fluid by direct extension, producing tumor throughout the abdominal cavity). An important feature of melanoma is that the metastatic lesions are often not obviously pigmented, and may even be amelanotic when observed microscopically. The production of visible melanin pigment requires that the tumor cells retain a sophisticated level of differentiation, and many of the more aggressive (which are more likely to metastasize) components of the tumor may have lost this function. The immunohistochemical tumor markers S-100 and HMB-45 can be very helpful in these cases in establishing that the masses are indeed melanoma.
An inflammatory etiology (choice A) is unlikely, given the presence of the serosal masses.
Metastatic lung cancer (choice B) would be unlikely to metastasize to the bowel serosa.
Primary intestinal lymphomas (choice D) are quite rare compared with metastatic melanoma in a patient with previous melanoma.
Pseudomyxoma peritonei (choice E) is associated with seeding of the abdomen with gelatinous implants rather than discrete masses, often from a mucinous cystadenoma or cystadenocarcinoma of the ovary or appendix.
