TheFetus.net

Fetus-in-fetu

Philippe Jeanty, MD, PhD Luís Flåvio de Andrade Gonçalves, MD Sandra Rejane Silva, MD

Article Published: May 31, 2002

[1] Florianopolis, SC Brazil, [2] Sao Paulo — SP [3] Nashville, TN jeanty@pov.net

Adapted and reproduced with permission from TKI Medcon Inc. http:/www.tki.com/

Definition

A fetus-in-fetu is an encapsulated, pedunculated vertebrate tumor. It represents a malformed monozygotic, monochorionic diamniotic parasitic twin included in a host (or autosite) twin. Characteristically the fetus-in-fetu complex will be composed of a fibrous membrane (equivalent to the chorioamniotic complex) that contains some fluid (equivalent to the amniotic fluid) and a fetus suspended by a cord or pedicle. The presence of a rudimentary spinal architecture is used to differentiate a fetus-in-fetu from a teratoma, since teratomas are not supposed to develop through the primitive streak stage (12-15 days). This last criterion has been considered too stringent by many authors who regard a rudimentary body architecture (metameric segmentation, craniocaudal and lateral differentiation, body coelom, "gestational sac"), or the presence of an associated fetus-in-fetu as equivalent criteria. Although teratomas can achieve striking degrees of differentiation by the inductive effect of adjacent tissues on one another, they do not present the criteria mentioned above.

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A fetus-in-fetu recovered from a 16-year old boy (From Highmore, 1815)

Sonographic features

The few cases detected prenatally all presented as a complex mass. The general appearance is a well-delineated capsule, with an echogenic mass suspended in fluid or partially surrounded by fluid. Occasionally, the diagnosis can be suggested by the recognition of a rudimentary spine.

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Longitudinal and axial section of a fetus-in-fetu. Notice the faintly visible spinal organization on the longitudinal view (Courtesy Dr. Patricia Dix).

Differential diagnosis

When discovered in a newborn child during physical examination, the differential diagnosis includes all the common masses such as Wilms" tumor, hydronephrosis, and neuroblastomas. Prenatally, the main differential diagnosis is with teratoma. Teratomas are disorganized congregations of pluripotential cells from all three primitive tissue layers. By differentiation and induction, they can achieve striking organization, with examples of several organs being well formed. However, teratomas do not have vertebral segmentation, craniocaudal and lateral differentiation, body coelom or systemic organogenesis. Thus the presence of a mass with a spinal organization and surrounded by fluid suggests the correct diagnosis.

When spinal structures are not present, most authors have considered that the diagnosis of fetus-in-fetu can still be made when the alternate criteria described under Definition are found. These criteria are sufficiently restrictive that even well-organized teratomas cannot fulfill all of them.

Teratomas have a definite malignant potential, a feature that has not been reported in fetus-in-fetu. Teratomas occur predominantly in the lower abdomen, not the upper retroperitoneum. Yet, the coexistence of a fetus-in-fetu and a teratoma as well as the occurrence of a teratoma 14 years after removal of a twin fetus-in-fetu have been reported, supporting the older hypothesis of a continuum between twin and teratoma. Cases of sacrococcygeal fetus-in-fetu should probably be regarded and treated as teratoma, because of the high incidence of teratoma in this region.

Ectopic testicles have a higher incidence of germ cell tumors, and the differentiation between fetus-in-fetu and teratoma is particularly important.

Although the characteristics of intracranial teratoma differ from those of intracranial fetus in fetu, Wakai found, in a large review of 245 intracranial teratomas, that there are some transitions between certain teratoma and fetus-in-fetu.

In the older literature, several descriptions of fetus-in-fetu were too vague to be acceptable by current criteria. For example, the case reported by Phillips does not unequivocally suggest the criteria described above and therefore should probably be considered a teratoma.

Some have argued that fetus-in-fetu should be considered as teratomas since they do not evolve into lithopedion like fetuses of abdominocyesis. That argument is probably not valid since in abdominocyesis the antigen complements of the host and fetus are different, which contrasts with fetus-in-fetu.

Associated syndromes

None

References

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