Introduction

Fetal tumors are rare, with an incidence of around 1 in 12 000 to 1 in 30 000 births [1]. Whilst almost all childhood tumors have also been reported to occasionally occur in the perinatal period, most tumors diagnosed in utero are biologically distinct from, and behave differently to, tumors more common in older children. Congenital solid fetal tumors are, as in older children, generally of mesenchymal rather than epithelial origin, and often recapitulate aspects of normal embryonic development; it is postulated that they arise from oncogenic mutations in immature cells or fetal stem cells, and are generally referred to as blastomas or embryonal sarcomas. In view of the unique fetal circulation, peculiar metastatic patterns are observed in this age group which are not seen in older children, and the size and site of the tumor are often more important in determining outcome than the specific histological type. Furthermore, from a histological perspective, features such as a high mitotic index, tumor necrosis, and even vascular invasion do not necessarily indicate malignant behavior in this setting [1]. Complications specific to this age group include development of fetal hydrops, polyhydramnios, and obstructed labor, and some tumors may be associated with specific malformation syndromes [1].

As in older children, fetal tumors can be broadly categorized as either benign or malignant, although this distinction is not always clear. Benign tumors are relatively common and include entities such as hemangiomas which may not necessarily be adequately reported in the literature [1], although many “hemangiomas” are now considered to be vascular malformations rather than true neoplasms [2]. Indeed, many fetal and neonatal tumors are hamartomas rather than true neoplasms, although the distinction between neoplasms and hamartomas is again not always obvious. Moreover, many of the tumors that are regarded as malignant in older children may behave in a more benign fashion in the fetus and neonate, including neuroblastoma, congenital myeloproliferative disorder in Trisomy 21, congenital fibrosarcoma, and hereditary retinoblastoma [1].