FETUS IN FETU (RETROPERITONEAL TERATOMA)

Fetus in fetu is a rare condition in which a malformed parasitic twin is found within the body of its sibling. It presents as a congenital, usually retroperitoneal tumor. It occurs in monozygotic diamniotic twins and usually presents within the first year of life.

The diagnosis is usually made at surgery or autopsy.

It is thought to be caused by embedding of a twin due to vitelline circulation anastomoses. Vascular anastomoses between twins have a variable presentation depending on the vessels involved and the location of the anastomoses. Anastomoses between vitelline vessels (only occur in monochorionic twins), are assumed to cause fetus-in-fetu in a mechanism similar to the formation of an acardiac twin (reversal of flow to the heart stunts growth of the affected twin which progressively embeds in the host twin while it grows).

• Definition is controversial.
• Usually found in the upper retroperitoneum (1-3), but may be found in the mesentery, adrenal gland, intracranially, pelvis, sacrum, coccyx, inguinal region, testis and scrotum.
• Well formed fetal bones within the mass in 62% of cases (2)
• Slow growing with symptoms due to compression of genitourinary and gastrointestinal organs (1).
• Usually single but multiple masses have been reported (2,3).
• Most are connected to the host by vessels originating from or near the superior mesenteric artery. Direct branches from the aorta, renal or adrenal vessels may be involved.
• Surgical excision appears to be curative with no recurrence demonstrated even when small remnants were left (3).
• Associated anomalies result from hypoxic damage. Anencephaly, abdominal wall defects and limb body wall complex has been described.

DIFFERENTIAL DIAGNOSIS

• Teratomas. Differentiation from teratoma is important as teratomas do have a malignant potential. Differentiation may however be difficult as teratomas may show a high degree of differentiation, and two patients with both fetus in fetu and teratoma have been reported (3).

Double fetus in fetu has been described (4).

REFERENCES

1. Eng HL, Chuang JH, Lee TY et.al. Fetus in fetu: a case report and review of the literature. J Pediatr Surg 1989;24:296-299.
2. Sutherland SF, Singh S. Fetus in fetu. Pediatr Surg Int 1988;2:368-372.
3. Federici S, Ceccarelli PL, Ferrari M et.al. Fetus in fetu: report of three cases and review of the literature. Pediatr Surg Int 1991;6:60-65.
4. Luzzatto C, Talenti E, Tregnaghi A et.al. Double fetus in fetu: diagnostic imaging. Pediatr Radiol 1994;24:602-603.