Antenatal Diagnosis of Retroperitoneal Cystic Mass: Fetiform Teratoma or Fetus in Fetu? A Case Report.

BACKGROUND Teratoma, a tumor containing a variety of tissues, is a broad diagnosis containing mature teratoma, immature teratoma, and teratomas with malignant transformation. The tumor forms during embryological development secondary to unsuccessful migration of primordial germ cells. A specific type of mature teratoma, containing human-like features, is called a fetiform teratoma. The fetiform teratoma is often compared and confused with fetus in fetu, a reabsorbed twin. While these tumors have commonly been described in the gonads, the retroperitoneal location finding on antenatal imaging is rare. The distinction between the aforementioned subtypes is not well established, proving a challenging diagnosis prior to resection. CASE REPORT We present a case of a newborn male with a prenatal diagnosis of retroperitoneal cystic mass. Although prenatal imaging was obtained, the diagnosis remained unclear. After birth, planned surgical excision on day of life 7 showed the suprarenal mass contained contiguous intestinal elements. Histopathology examination revealed a mature cystic teratoma with multiple tissue types, including colonic, brain, respiratory, lymphatics, and nerves, reminiscent of fetiform teratoma. This case report presents an interesting example of differentiating elements straddling the diagnoses mentioned above. CONCLUSIONS This is the first reported case of fetiform teratoma diagnosed in a newborn and is especially unique for having the element of intestinal duplication within the retroperitoneal mass. The differentiating features of fetus in fetu and fetiform teratoma depend on subjective distinctions. The case provides an opportunity to discuss the differentials and management strategies.