Fetiform teratoma: a systematic review with insights into concepts and controversies in differentiating it from fetus-in-fetu.

Fetiform teratomas are rare tumors mimicking malformed fetuses, posing diagnostic challenges especially when differentiating them from fetus-in-fetu. Fetiform teratomas result from neoplastic processes of germ cells, while fetus-in-fetu arises from aberrant embryogenesis, specifically the inclusion of a parasitic monozygotic twin. This review systematically examined 17 cases of fetiform teratoma identified through strict criteria, excluding cases with vertebral columns or long bones. A thorough search of multiple electronic databases, using the keywords 'fetus-in-fetu,' 'fetus-ex-fetus,' 'homunculus,' or 'fetiform teratoma' was conducted to identify relevant studies published till December 2024. Common locations of fetiform teratoma included the ovaries, retroperitoneum, and sacrococcygeal region, with clinical presentations varying from neonatal masses to pelvic pain in adults. Imaging typically revealed complex masses with solid-cystic components and calcifications, while histopathology demonstrated tissues from all three germ layers, including rare elements like thymic or retinal tissue. Accurate diagnosis relies on integrating imaging, histopathology, and ideally, cytogenetic studies, considered the gold standard for differentiating these entities based on genetic makeup. Surgical excision remains the primary treatment. Post-operative follow-up is recommended due to a small risk of recurrence or malignant transformation. This review underscores the need for standardized diagnostic criteria, primarily focusing on the absence of a vertebral column and long bones for fetiform teratomas. The study's limitations include the small sample size and lack of molecular or genetic data, highlighting the need for further research into the pathogenesis and genetic characteristics of these rare entities.