Thoracic fetus in fetu.

A rare case of thoracic fetus in fetu is reported. Complete excision was curative.

INTRODUCTION

Thoracic fetus in fetu is rare. Fetus in fetu occurs due to the inclusion of a monozygotic, diamniotic twin.[1] In this condition, a malformed parasitic twin is found within the body of the surviving twin. The incidence is 1:500,000 live births.[23] It is distinguishable from a mature teratoma by the presence of vertebral column-like structures.[4] It commonly presents in infants. Common site is intra-abdominal but it can occur in other sites.[5] We could not identify any thoracic fetus in fetu confined to one hemithorax in a live baby reported earlier in the literature.

CASE REPORT

A 10-month-old female presented with complaints of recurrent episodes of lower respiratory tract infections and cough with fever since two weeks. On examination, there was decreased air entry on the left side of the thorax and it was dull on percussion. Serial chest radiographs showed persistent opacity of the left hemithorax with calcifications filing the whole of the left hemithorax. A computed tomogram (CT) scan of the thorax revealed a well-defined heterogeneous mass measuring 101×75×86 mm with calcifications, fat densities, and cystic densities (24 HU) noted within the lesion occupying the left hemithorax, causing a shift of the trachea and the mediastinum (heart) to the right and causing compression of the left lung and hyperinflation of the left lower lobe [Figure 1].

Figure 1

High-resolution computed tomography (HRCT) of the chest showing a mass in the left hemithorax with coarse calcifications

A left thoracotomy was done and a large mass was found occupying the entire left hemithorax extending on to the right hemithorax. The mass was covered by a membrane. It was lobulated with a rudimentary head, thorax, and three limb buds. Upon opening the cystic cephalic-like structure, CSF-like fluid (CSF: Cerebrospinal fluid) was found with a brain-like tissue, ventricular space, and a spinal cord extending into distal rudimentary vertebrae [Figure 2]. The mass also had multiple bony segments. It was adherent to the posterior chest wall. The mass was excised in toto.

Figure 2

Specimen picture showing the rudimentary brain with ventricles (arrow head) and vertebral column (arrow)

The histopathological examination confirmed the presence of a rudimentary spinal cord and vertebral column, the brain matter showed ventricles, and the body cavity had coelomic spaces. All the three germ cell layers were seen. The structures of pancreas, spinal cord, and brain with meninges were seen. The postoperative course was uneventful and the child was asymptomatic at one year follow up.

DISCUSSION

Fetus in fetu is rare and two theories are postulated: fission and fusion. In fission, the primary hypothesis is failure of a single zygote to divide completely.[6] Fusion is the union of two initially distinct embryos that took place during early embryonic period. It has been reported to occur in the brain, mouth, skull, neck, mediastinum, liver, retroperitoneum (most common), scrotum, and even sacrococcygeal region.[7] The location of fetus in fetu in the thorax is extremely rare. Only four cases have been previously reported.[8-11] In three patients, the mass was in the mediastinum, and in one, it was in the lung parenchyma.

The fetus in fetu is usually anencephalic[5] and acardiac.[2] It has rudimentary limbs, the lower more developed than the upper. It has a varying amount of organogenesis[1] and has an axial skeleton.[4] The presence of the vertebral column is the differentiating factor between a fetus in fetu and a teratoma.[4] Teratoma is a true neoplasm arising from totipotent cells. It lacks a vertebral column as it has not passed through the primitive streak phase.[12] A fetus in fetu is a benign condition. Symptoms occur due to the mass compressing the surrounding organs. Our patient presented with recurrent respiratory tract infections due to compression of the lung and shift of the mediastinum. She improved after excision of the mass. In rare cases, fetus in fetu is known to become malignant if any of the membranes or tissues are not completely excised.[13]

In conclusion, fetus in fetu is a rare occurrence and thoracic fetus in fetu is extremely rare. CT or magnetic resonance imaging (MRI) gives an accurate preoperative diagnosis. Complete excision is curative.