Fetus in fetu is a rare condition in which a fetiform calcified mass often is present in the abdomen of its host, a newborn or an infant. We report on a case of a 19-month-old girl whose plain abdominal radiograph, ultrasonography, and computed tomography scan revealed a mass in which the contents favor a fetus in fetu rather than a teratoma. The noncalcified vertebral column invisible on the radiographs was identified by the pathologist; therefore, the nonvisualization of the vertebral axis on radiography or on computed tomography scan does not exclude the diagnosis of fetus in fetu.
Key words: fetus in fetu, spine of fetus, teratoma, twin, intraabdominal mass, parasitic twin.
The term fetus in fetu is used to point out an unequal division of totipotential cells of a blastocyst where the result is the inclusion of a small cellular mass in the more mature embryo. This is a form of monozygotic diamniotic twin pregnancy where the parasitic twin installs and grows in the body of its partner.1 This is a rare malformation that has some similarities with the retroperitoneal teratoma, but it is different from the latter by its fetiform aspect and the metameric segmentation of its spinal axis. The presence of a capsule covering this formation and a vascular pedicle is frequently encountered.2
In reality, one may find cases whose vertebral column is insufficiently calcified and therefore invisible on the plain radiograph. It may also be nonexistent but, because of the organogenetic differentiation, one may link this fetiform mass to the diagnosis of fetus in fetu.
A 19-month-old girl was hospitalized because of an abdominal mass of some months’ duration. She was the only child in her family where there was neither twin pregnancy nor fetal malformation. Her body weight was 10 kg. Except for the presence of a mass in the upper right abdomen, the clinical examination and laboratory findings were normal. The plain film of the abdomen showed left-sided calcifications adjacent to the spine at the fourth and fifth lumbar vertebrae and the first sacral vertebra (Fig 1). These calcifications had the appearance of limb bones rather than vertebral bodies. Abdominal ultrasonography showed a hypoechoic mass with hyperechoic structures inside that extended from the lower border of the liver to the hypogastric region. An abdominal computed tomography (CT) scan (Figs 2, 3, and 4) showed a thick-walled mass that measured approximately 90 × 65 mm including solid, cystic, and calcified components. The calcifications resembled cranial bones and long bones, but a vertebral axis was not visualized. The mass was displacing the transverse colon downward and the small bowel downward and posteriorly.
Based on these imaging findings, a diagnosis of fetus in fetu was thus made preoperatively.
The surgeon discovered a well-encapsulated retroperitoneal mass behind the transverse mesocolon. This mass had a pedicle that was connected to the superior mesenteric artery. Excision of the capsule revealed a yellowish fluid and an incompletely developed fetus covered by vernix caseosa.
The postoperative course was uneventful and the patient was discharged on the eighth postoperative day.
A radiograph of the specimen showed cranial bones and long bones but no vertebral column. On the macroscopic pathologic examination, the mass measured 20 × 8 × 5 cm and was composed of a head with hair, a trunk, and rudimentary limbs corresponding to an incompletely developed fetus. A soft vertebral axis was found behind the intestinal loops, colon, and liver. (Figs 5 and 6). Microscopically, there were hepatic cells, biliary tract cells, splenic tissue, and pulmonary tissue. Most of these cells were in blastic stage and not well-differentiated. The bony tissue was essentially composed of osteoblasts with little bone marrow. The nervous tissue was mostly formed by glial cells and not well-differentiated neurons
The living children with fetus in fetu were 15 years (3 cases). Sex ratio was 47 boys to 35 girls (the sex of 5 other cases was undetermined). In 70% of cases, the chief complaint was an abdominal mass.9 As far as location was concerned, it was predominantly retroperitoneal in 80% of cases,10 but could be atypical including the skull as in 6 cases,11-14 the sacrum as in 6 cases,15-19 the scrotum as in 1 case,20 and the mouth as in 1 case.21
In almost all cases (88%), there was a single parasitic fetus apart from 5 reports in which the number of the fetus ranged from 2 to 5.1,1322-24 The size and weight of the fetus varied, from 4 cm24 to 24.5 cm, respectively,25 and from 1.2 g23 to 1.8 kg, respectively.26
The organs present in the fetus in fetu were as follows: vertebral column, 91%; limbs, 82.5% (number varied from 1 to 4); central nervous system, 55.8%; gastrointestinal tract, 45%; vessels, 40%; and genitourinary tract, 26.5%.
The fetus was always anencephalic, the vertebral column and the limbs were present in the fetus in fetu in almost all cases (91% and 82.5%, respectively). The lower limbs were more developed than the upper limbs. Fetus in fetu was rarely found in the central nervous system, gastrointestinal tract, vessels, or the genitourinary tract; however, it was found in 55.8%, 45%, 40% and 26.5% of cases, respectively. It was rarer still to find fetus in fetu in the lungs, adrenal glands, pancreas, spleen, and lymph nodes. The heart was very rarely found in fetu.15,27
The absence of cardiovascular system almost led to misdiagnosis of acardiac fetus in 1 case as the morphology is otherwise similar with findings of anencephaly, absent or rudimentary limbs, absent lungs, short intestine, and single umbilical artery. However, in the case of acardiac twin fetus, the karyotype is abnormal in at least 50% of cases including both trisomy or triploidy whereas the karyotype of fetus in fetu is normal and similar to his host’s.28
Eighty-nine per cent of fetus in fetu lesions were noted before 18 months of age.28
In reviewing literature most case reports up to 1980 showed the preoperative diagnosis of fetus in fetu was made only in 16.7% of cases because CT scan was not performed. Nowadays, CT scan has proven very helpful in suggesting the preoperative diagnosis.2,14,2429-31 Magnetic resonance imaging was also used in 4 cases.32-35
The differential radiologic diagnoses were teratoma and meconium pseudocyst.28 Indeed, these masses often had calcified components, so they were sometimes difficult to differentiate with fetus in fetu.
Treatment was complete resection of the mass except when it was adherent to the host’s organs.36 Relapse was observed in 1 case (out of 87 cases) with recurrent right abdominal mass 4 months after surgery. This was a teratoma, which contained cystic, solid, and calcified components. It measured 13 cm in diameter and 5% of the tumor was yolk sac carcinoma. After surgical excision, the patient was treated with chemotherapy and recovered at 2 years of age.34
Most of the fetus in fetu were cases of pedunculated masses within a capsule containing fluid37 and with an umbilical cord composed of only 2 vessels.28
In our case, like some of the cases described in the literature, the vertebral column was detected by the pathologist. It was radiolucent on radiography because it was insufficiently calcified.29 So, our case truly had a vertebral column like 91% of cases in the literature. It was therefore in accordance with Willis’ theory38 that fetus in fetu is a mass containing a vertebral axis often associated with other organs or limbs around this axis.
However, review of the literature showed that in about 9% of cases of fetus in fetu, there was no vertebral column, even on pathologic examination.39 This has led to another definition of fetus in fetu by Gonzalez-Crussi39: “Fetus in fetu is applied to any structure in which the fetal form is in a very high development of organogenesis and to the presence of a vertebral axis.”39
On the contrary, teratoma is an accumulation of pluripotential cells in which there is neither organogenesis nor vertebral segmentation.27
Although the hypothesis regarding the origin of the fetus in fetu was widely accepted as the included twin theory, which was postulated as a diamniotic monochorionic monozygotic twin, some supporters of the teratoma theory have suggested that the fetus in fetu mass represents a well-differentiated, highly organized teratoma.40