Retained Foetus-in-Fetu or Fetiform Teratoma - A Diagnostic Dilemma and Operative Challenge

Foetus-in-fetu (FIF) is a condition where one of the twin foetuses is found developing inside the other. Differentiating this condition from fetiform teratoma (FT), in which the teratoma becomes highly differentiated to take a foetus-like appearance, often becomes arduous. Unfamiliarity with the situation and the presence of bone and hair within such lesions can potentially mislead the surgeon to diagnose it as a dermoid cyst erroneously. Suboptimal surgery can complicate things further when it comes to redoing operations and the final histopathological reporting. We report this case of a partially removed cystic retroperitoneal mass in a 14-year-old male, posing a significant surgical challenge due to dense adhesions to the vital retroperitoneal structures, such as common iliac vessels and ureter. We also faced similar diagnostic dilemmas during histopathology due to the tissues removed during the previous surgery. Based on gross and microscopic features, the possibility of a FIF or FT was rendered.