A 47-year-old woman who presented with
abdominal pain was found to have a 20 cm cystic retroperitoneal mass. Pathology indicated a colonic-type
adenocarcinoma arising in a primary retroperitoneal mature cystic
teratoma. The
adenocarcinoma was predominantly intracystic with focal superficial invasion into the
cyst wall but not beyond the
teratoma capsule. Immunohistochemistry showed that the
adenocarcinoma cells were diffusely positive for
cytokeratin 20 (CK20) and caudal-type homeobox transcription factor-2 (CDX2) but negative for CK7, confirming the colonic phenotype. In addition, the
adenocarcinoma was seen adjacent to teratomatous colonic-type mucosa with adenomatous change (i.e.
adenoma), suggesting that it was probably arising from a colonic-type
adenoma within the
teratoma. The
carcinoma had a higher Ki-67 proliferation index and had a higher percentage of cells stained for p53 than the adjacent adenomatous lesion. To the authors' knowledge this is the first documented case in which a colonic-type
adenocarcinoma was seen arising from a precursor lesion (i.e. a colonic-type
adenoma in a primary retroperitoneal mature cystic
teratoma) and is the second case of intestinal-type
adenocarcinoma arising in a primary retroperitoneal mature cystic
teratoma.