Non-pancreatic retroperitoneal mucinous neoplasms and a discussion of the differential diagnosis.

Retroperitoneal mucinous neoplasms are common and have a wide range of differential diagnoses to consider. It is of utmost importance to maintain a multidisciplinary approach when evaluating these lesions. Clinical history, surgical impression and radiographic studies should be obtained and used in conjunction with morphology and immunohistochemistry to help guide the pathologist to the correct diagnosis. We recently encountered a case of a 51-year-old man who presented with an 11.5 cm left-sided retroperitoneal mucinous neoplasm. This lesion was initially diagnosed as a low-grade mucinous carcinoma of probable pancreatic origin at an outside facility, and he was subsequently treated with chemotherapy. One year later he presented to our institution and underwent retroperitoneal en bloc resection of the 7-cm residual multiloculated mucinous neoplasm. When discussed at tumor board additional medical history was obtained, and review of the patient's chart revealed a remote history of left orchiectomy for a mixed malignant germ cell tumor and metastatic embryonal carcinoma in 2 of 34 retroperitoneal/para-aortic lymph nodes. With no clinical evidence of tumor in the pancreas or extension from it, the predominately para-aortic location of the tumor favors a mucinous carcinoma arising from a rest of mature metastatic/proliferating teratoma that persisted after chemotherapy. This case illustrates the importance of a thorough history, however remote, and correlation with imaging in the development of differential diagnoses, as well as, the need to consider non-pancreaticobiliary sources of retroperitoneal mucinous tumors.