Clinical features and imaging diagnostic challenges of fetus-in-fetu associated with abdominal cryptorchidism: a case report

Fetus-in-fetu (FIF) is a rare congenital anomaly in which a malformed monozygotic twin is incorporated within its host twin. FIF is usually retroperitoneal, whereas association with an intra-abdominal undescended testis is exceptionally uncommon. Because imaging may show a heterogeneous mass containing soft tissue, fat and calcified elements, FIF can be difficult to distinguish preoperatively from mature teratoma, especially when cryptorchidism is also present. A 3-day-old male neonate was admitted because of jaundice and a palpable right abdominal mass. Physical examination revealed an underdeveloped right hemiscrotum without a palpable right testis; the left testis was palpable and a left hydrocele was suspected. Prenatal ultrasound demonstrated a mixed echogenic mass in the right lower abdomen and raised the possibility of FIF. Fetal magnetic resonance imaging (MRI) and postnatal contrast-enhanced computed tomography (CT) showed a well-circumscribed cystic-solid abdominal mass with fat-like signal/density and multiple calcified stripe-like components, leading to diagnostic uncertainty between FIF and teratoma. Serum alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG) showed no abnormal elevation after interpretation according to neonatal age. Exploratory laparotomy revealed a 5.0 cm x 4.0 cm x 4.0 cm encapsulated cystic-solid mass arising from the right intra-abdominal undescended testis, without identifiable viable right testicular parenchyma. The mass and the atrophic right testicular/epididymal tissue were excised. Gross and histopathological examination demonstrated an organized axial skeleton/spinal structure and multiple differentiated tissues, including brain, intestine, liver, bone, skeletal muscle, skin, and epididymal duct tissue, fulfilling the Willis criterion for FIF. This case highlights a very rare presentation of FIF associated with abdominal cryptorchidism. Recognition of calcified axial skeletal elements, fat components, organoid structures and the relationship to the spermatic cord/testicular structures may help pediatric surgeons and radiologists anticipate FIF rather than misdiagnosing the lesion as a testicular or abdominal teratoma. Complete surgical excision and long-term follow-up with ultrasonography and tumor markers are recommended.