Fetal Tumours

Introduction

Neck masses are most commonly noted in the second trimester at the time of the detailed anatomy scan, although they may present as an incidental finding on later scans or be referred due to polyhydramnios and measuring large for gestational age. The majority of neck masses detected at this gestational age are mixed cystic and solid in nature and arise in the anterior triangle. Posterior cystic neck lesions and those evident in the first trimester commonly have a different aetiology as outlined later and in Chapter 19 .

The two most common neck masses are lymphangiomas and teratomas. A lymphangioma will be a well-circumscribed, predominantly avascular mass consisting of thin walled cystic areas. Lymphangiomas arise laterally but often crosses the midline, and although not invasive, they can extend into the mediastinum and thoracic cavity and can obstruct the airway ( Fig. 37.1 ).

A teratoma has a heterogeneous appearance with varying degrees of cystic and solid components ( Fig. 37.2 ). They are often well vascularised. There is no pathognomonic ultrasound appearance, and teratomas can be difficult to differentiate from lymphangiomas. However, areas of calcification on ultrasound (or fat signal on magnetic resonance imaging (MRI)) make the diagnosis of teratoma more likely. As with lymphangiomas, fetal teratomas are rarely invasive but can cause oropharyngeal obstruction and deviation. Compressive effects can also cause significant disruption to the bony skull and the skull base; ultrasound (US) and MRI should be used to detect signs of this. Care should be taken to differentiate a cervical teratoma from an epignathus, a rare teratoma arising from the palate-pharyngeal region around the basisphenoid (Rathke pouch), which fills the buccal cavity. This is associated with poor prognosis because of the location and potential for invasion into the skull base and brain tissue.

Differential Diagnoses

Before confirmation of a cystic neck mass, other differentials must be considered. Posteriorly, this would primarily be gross nuchal oedema. Careful US examination of the skull and brain structure should be performed to exclude encephalocele or cervical myelomeningocele. Anteriorly, cervical teratoma is a primary differential. Anomalies of thyroid development, including tumours and goitres can present similarly. Masses with a more cystic appearance may represent thyroid or branchial cleft cysts or thyroglossal duct cysts.

Cystic Lymphangioma

Embryologically, the lymphatic system develops after the formation of blood vessels. The most widely accepted model of lymphatic development to date was developed more than a century ago. It proposes that the endothelial cells bud from the veins to form primary lymphatic sacs. These then sprout towards the periphery. The two main lymphatic sacs develop near the junction of the subclavian and anterior cardinal veins, and the lymphatic capillaries spread from these towards the head, neck, arm and thorax. The failure of these jugular lymph sacs to drain into the developing lymphatic system is thought to cause abnormal lymphatic sprouting, lymph accumulation and the development of lymphangiomas.

The terms cystic hygroma and lymphangioma are synonymous and have been used to describe congenital dilated cystic malformations, predominantly in the region of the neck. However, it should be recognised that these masses may arise from or extend into the thorax and mediastinum or axilla. Less commonly, lymphangiomas have been reported elsewhere in fetuses, including the extremities and abdominal wall.

In the first trimester, the term cystic hygroma is used to describe a significantly increased nuchal translucency, often with internal septations (see Chapter 19 ). Such cases are more frequently associated with underlying chromosomal aneuploidies, particularly monosomy X (Turner syndrome), trisomy 18 and trisomy 21. In the absence of aneuploidy, there is a higher incidence of underlying genetic disorders that may not be detected antenatally. However, in the era of microarray and improved targeted testing for Noonan syndrome (in which >80% can be diagnosed), the antenatal detection rate is improving. A detailed first trimester anomaly scan, including cardiac anatomy, should be performed in such cases and further investigations offered. In the event of a normal karyotype or microarray, if the findings do not resolve, and particularly if there is a further anomaly found on ultrasound, genetic counselling should be considered. If antenatal testing is declined, careful assessment of the infant should be performed in the neonatal period.

Increased nuchal translucency is also associated with cardiac abnormalities, and in some cases, ultrasound signs of fluid in other fetal compartments such as the thorax, abdomen and generalised skin oedema may be noted; this is known as fetal hydrops or hydrops fetalis. In cases of hydrops, investigations as described earlier should be performed, as well as consideration of other causes such as red cell alloimmunisation or nonimmune causes, including viral infection and metabolic disease (see Chapter 36 ). In cases of persistent hydrops presenting in the first trimester, the outlook is very poor with few fetuses surviving to delivery.

In cases of first trimester cystic hygromas that have been appropriately investigated and no genetic abnormality found, particularly in those cases that resolve, the prognosis is very good.

The development of a cystic neck mass in the second trimester is likely to have a different aetiology ( Table 37.1 ) from those seen in the first trimester and may carry a better prognosis unless associated with hydrops or generalised oedema, in which cases the prognosis remains poor.

Teratomas

Fetal teratomas are rare tumours with an incidence of 1 in 40,000 live births. Although most commonly found in the region of the lower spine and pelvis (sacrococcygeal teratomas (SCTs); discussed in detail later), 6% are related to the neck. Teratomas are (almost always) formed of tissues derived from the three germ cell layers: endoderm, mesoderm and ectoderm. Although teratomas are predominantly (>80%) benign tumours with well-differentiated tissues, the rapid growth and the mass effect can cause significant complications; primarily by obstruction or deviation of the trachea and oesophagus. In cases of immature teratoma, with more poorly differentiated tissues, there in increased risk for invasion and metastases; however, the prognosis remains good with a greater than 80% 5-year survival rate.

The origin of fetal teratomas remains poorly understood. The most widely accepted hypothesis is that aberrant pluripotent cells are sequestered during embryogenesis, in the fourth to fifth week of gestation, that are able to proliferate to form disorganised structures comprising tissue types derived from the three embryonic germ layers.