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13 April 2016 - Case of the Week #385

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Dr. Dhiraj B. Nikumbh, ACPM Medical College Dhule (India) and Dr. Sudhir Singhavi, Singhavi Hospital Dhule (India) for contributing this case and the discussion. This case was reviewed in May 2020 by Dr. Jennifer Bennett, University of Chicago and Dr. Carlos Parra-Herran, University of Toronto. To contribute a Case of the Week, follow the guidelines on our main Case of the Week page.

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Case of the Week #385

Clinical history:
An 18 month old girl presented with abdominal distension and pain. Ultrasound and CT scan were suggestive of a retroperitoneal tumor. Laparotomy was posted, and the specimen was excised.

Gross images:

Intraoperative retroperitoneal mass

Micro images:

What is your diagnosis?

Fetus in fetu (FIF) - excised retroperitineal mass


Radiograph showing non-calcified vertebral column

The mass measured 18 x 10 x 6 cm and weighed 910 g. The external surface was sac-like and smooth, with partial skin covering the upper region. Vertical cut section showed solid and large cystic areas containing mucoid material. The upper segment of the mass contained hair, pultaceous (pulpy) material, scalp and brain substance. The middle segment showed vertebral column, cartilage, intestine and grey-white solid areas. The lower segment contained a cyst with a flattened wall and limb bud-like structures attached to the external aspect.

Histologic examination of the upper segment showed primitive brain parenchyma, cartilage and skin with adnexa and hair. The middle segment contained stomach, duodenum, small intestine, vertebral column, bone and bone marrow, pancreas, soft tissue, lymph nodes and adipose tissue. The lower segment contained a cyst wall with flattened lining and no adnexal structures.

Fetus-in-fetu (FIF) is an extremely rare pathological condition with an estimated incidence of 1 in 500,000 births. Less than 200 cases have been reported worldwide. To our knowledge, FIF was originally described by Meckel in the 18th century. Subsequently, Willis described it as rare condition in which a malformed parasitic twin was found inside the body of its partner, usually in the abdominal cavity. Willis defined FIF in 1935 as a "mass containing a vertebral axis often associated with other organs or limbs around this axis" (Willis: The Borderland of Embryology and Pathology, 1962). The presence of a vertebral axis in a fetiform mass has since been considered pathognomonic of FIF.

Although several theories as to the pathogenesis of FIF had been proposed, the "twinning" theory has become accepted, as the mass is always found enclosed within a sac resembling the amniotic sac - in fact, the lining of the sac histologically resembles the amniotic membrane. Of late, Spencer has proposed that one or more of the following criteria needs to be present to label any fetiform mass as FIF: (1) the mass must be enclosed within a distinct sac, (2) it should be partially or completely covered by normal skin, (3) it should have grossly recognizable anatomic parts, (4) it should be attached to its host by only few relatively large blood vessels, (5) it should either be located immediately adjacent to one of the sites of attachments of conjoined twins or be associated with neural tube or the gastrointestinal tract (Clin Anat 2001;14:428). In our case, both the criteria of Willis and Spencer were fulfilled, and thus the diagnosis of FIF was made.

The clinical differential diagnosis of FIF includes other retroperitoneal masses such as neuroblastoma, Wilms tumor, teratoma and hydronephrosis. The main diagnostic challenge lies in differentiating FIF from retroperitoneal teratoma, which contains pluripotent cells without organogenesis or vertebral segmentation. This distinction is important as the latter can be associated with risk of malignancy. The most important feature that has been used to distinguish between FIF and teratoma is the presence of the vertebral column.

Surgical excision is curative, although the presence of immature elements indicates the need for close clinical, radiological and serological (AFP) follow-up.

References: Med J Aust 1969;1:1016, J Pediatr Surg 1989;24:296, J Med Case Rep 2010;4:96, Pediatr Surg Int 2009;25:289, J Indian Assoc Pediatr Surg 2008;13:30

Discussion edited by Dr. Jennifer R. Kaley, University of Arkansas for Medical Sciences (USA).

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