Demystifying Fetus-in-fetu: A Systematic Review of Its Clinical and Pathological Attributes

Fetus-in-fetu (FIF) is an exceedingly rare condition, characterized by a fetal-like or fetiform mass with a calcified vertebral axis surrounded by other organs or limbs. This systematic review was conducted to comprehensively analyze the clinicopathological attributes, management strategies, and prognosis of FIF to consolidate existing knowledge on FIF.

Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, a comprehensive search was performed across various electronic databases, using the keywords "fetus-in-fetu," "fetus-ex-fetus," "homunculus," or "fetiform teratoma" to look for published articles until December 2023. Data extraction and analysis were carried out for cases meeting the defined criteria for FIF diagnosis, that is "presence of both/either the vertebral column and/or long bones of extremities" and "absence of immature elements."

A total of 249 case reports/case series comprising 241 single FIF cases and 33 multiple FIF cases were included for analysis. The majority of single FIF cases presented within the 1^st year of life, with a slight male predominance. Retroperitoneal location was most common, and imaging modalities played a crucial role in preoperative diagnosis. Complete surgical removal of the mass with detailed histopathological examination is the cornerstone of treatment for FIF, with favorable outcomes in the majority of cases. Both mature and immature teratoma can rarely be found in association with FIF.

This comprehensive systematic review enhances understanding of FIF, emphasizing the importance of accurate diagnosis by diligent histopathological examination, appropriate management, and vigilant postoperative monitoring for favorable outcomes.