Final Diagnosis -- Intratesticular fetus in fetu


Intratesticular fetus in fetu.

The presence of compressed testicular parenchyma at the periphery of the lesion associated with rete testis and epididymis elements, in the context of a cryptorchid testis, indicate that the fetus in fetu developed within the abdominal testis. However, no vas deferens elements were identified.

Cytogenetic studies subsequently showed an apparently normal male chromosome analysis.


Fetus in fetu is a very rare entity (estimated at 1 in 500,000 live births1) involving incorporation and development of a malformed diamniotic-monochorionic-monozygotic twin into the host. The etiology is unknown. The most common site is retroperitoneal, but cases have been reported in the cranial vault, mediastinum, scrotal sac, and other locations2,3. Diagnosis is typically made in infancy or childhood; however, cases have also been identified on prenatal ultrasound. Prenatal three-dimensional sonographic imaging has even demonstrated spontaneous limb movement in a highly developed case of fetus in fetu4.

The main diagnostic challenge is whether the tumor represents a very well-differentiated mature teratoma or fetus in fetu. The most widely accepted criteria of fetus in fetu are the presence of a vertebral structure with surrounding organotypic tissue development5. Some require more stringent criteria such as the presence of an axial skeleton or a fetus with metameric segmentation, skin, encapsulation, and a two vessel cord6. Others suggest that all cases of fetus in fetu are actually teratomas.

Fetus in fetu is a benign condition. Surgical treatment is recommended as there are very rare reported cases of malignant recurrence7. Patients are also typically monitored for the tumor markers β-HCG and AFP.


  1. Grant P, Pearn JH: Foetus-in-foetu. Med J Aust 1969, 1(20):1016-1019.
  2. Marynczak L, Adamek D, Drabik G, Kwiatkowski S, Herman-Sucharska I, Lankosz-Lauterbach J: Fetus in fetu: a medical curiosity-considerations based upon an intracranially located case. Childs Nerv Syst 2014, 30(2):357-360.
  3. Aoki K, Matsumoto Y, Hamazaki M, Sano M, Fukumoto K, Fukaya T, Kuroda K, Tsutsumi R: MRI reveals fetus in fetu in the mediastinum. Pediatr Radiol 2004, 34(12):1017-1019.
  4. Jones CD, Reyes-M˙gica M, Gallagher PG, Fricks P, Touloukian RJ, Copel JA: Three-dimensional sonographic imaging of a highly developed fetus in fetu with spontaneous movement of the extremities. J Ultrasound Med 2001,20: 1357-63.
  5. Gonzalez-Crussi. Extragonadal teratomas. Washington, DC: Armed Forces Institute of Pathology; 1982.
  6. Willis RA: The structure of teratoma. J Pathol Bacteriol 1935; 40:1-36.
  7. Hopkins KL, Dickson PK, Ball TI, Ricketts RR, O'Shea PA, Abramowsky CR: Fetus-in-fetu with malignant recurrence. J Pediatr Surg 1997, 32(10):1476-1479.

Contributed by Diana Thomas, MD, PhD and Miguel Reyes-M˙gica, MD

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