Use of short tandem repeat analysis in unusual presentations of trophoblastic tumors and their mimics.

Gestational trophoblastic tumors can be difficult to distinguish from nongestational neoplasms. Somatic and germ cell tumors can mimic gestational choriocarcinoma, and epithelioid trophoblastic tumor (ETT) is known for its histologic, and sometimes clinical, resemblance to squamous cell carcinoma. Short tandem repeat (STR) analysis can separate gestational from nongestational neoplasms and can provide useful information about the type of causative conceptus. We present a series of cases which demonstrate the utility of STR analysis in the evaluation of gestational choriocarcinoma, epithelioid trophoblastic tumor, and their mimics. Samples from normal tissue and tumor were microdissected. DNA was extracted, and STR analysis was performed. Five cases were identified in which there was clinical and/or histologic concern for a gestational trophoblastic neoplasm. Case 1 is a choriocarcinoma presenting concurrently with a 16-week gestation. STR testing on the tumor, mother, and fetus showed that the tumor arose from a previous occult complete hydatidiform mole. Case 2 is an ETT presenting as multiple masses in bilateral kidneys, initially diagnosed as urothelial carcinoma. However, because of an elevated human chorionic gonadotropin, additional workup was performed which showed that the tumor was most likely an ETT. STR analysis showed that the tumor arose from a nonmolar pregnancy. Cases 3-5 illustrate somatic carcinomas mimicking gestational neoplasia. In those cases, STR confirmed a somatic origin. STR can be useful in distinguishing gestational from nongestational neoplasms, particularly in unusual settings. Also, STR analysis can add clinically useful information that is not available from clinical or histologic evaluation.