Teratomas represent neoplasms in which there are histological tissue components recapitulating all three embryonic germ cell layers, which form recognisable structures but a histologically disorganised mass lesion. There may be a wide variation in the proportion showing differentiation towards each of the different germ cell layers and in very rare cases in childhood, monodermal teratomas may exist in which only one element appears present, the characteristic example being struma ovarii of the ovary composed of thyroid-type tissue [16].

In general, the most common elements identified in pediatric teratomas regardless of site are neural components followed by cutaneous tissue, fat, fibrous tissue, cartilage and gastrointestinal type epithelium, but with a wide range of other tissue types being described [2, 3]. Due to the large variation in elements present there is usually a mixture of cystic and solid components, corresponding to the general imaging appearances described above. Depending on the site, teratomas may locally infiltrate widely and markedly distort the normal anatomy, this being particularly the case in congenital tumours in which the teratoma is presumed to have grown in utero; in such cases surgery may be technically very difficult due to the unusual anatomy, including atypical vascular anatomy supplying, or running through, the tumour [17, 18].

In the majority of cases, all elements within a teratoma are histologically mature. Most cases of mature teratoma, provided excision is complete, behave in a benign fashion. There are some exceptions to this, however, specifically that the presence of an apparently mature teratoma within the testis of young adult may be associated with malignant behaviour [19] (Figs. 11.4, 11.5, 11.6, and 11.7).

Immature teratomas represent cases in which, in addition to any other tissues present, there is a component of immature embryonal tissue. In most cases, the majority of the tumour represents mature teratoma but with presence of immature neuroepithelial structures, the frequency and extent of which is related to the histological grading of such cases (Figs. 11.8, 11.9, and 11.10). The clinical significance of immature tissue within a teratoma appears to be related to the age of the patient and anatomical site. For example, immature elements in a congenital sacrococcygeal teratoma appear to have little significance for prognosis whereas immature tissue in the ovary of an adolescent girl is of adverse prognostic significance. The diagnosis of immature teratoma should be reserved for cases in which definite immature neuroepithelial tissue can be identified. The presence of apparently immature mesenchymal components, in isolation, is of uncertain significance, and is not usually a clinically relevant issue in immature teratoma of the ovary, in which immature neuroepithelial elements are almost always additionally present. The main relationship in the literature between histological grading of immature teratomas and clinical behaviour and outcome is provided for ovarian immature teratomas in young adults but such data is not available for teratomas at extragonadal sites in younger children [2, 20].

For the diagnostic pathologist, the most important aspect in the histological diagnosis of a teratomatous GCT is adequate sampling and histological assessment in order to detect any possible areas of malignant elements, almost always represented by yolk sac tumour. Therefore, it is important that the entire tumour is examined and sectioned and any atypical areas are sampled. The clinical management of GCTs in which a small focus of malignant yolk sac tumour is identified depends on the age and stage in terms of completeness of excision. Whilst the serum alpha-fetoprotein is often raised in the presence of the malignant yolk sac elements it should also be recognised that there may be cases in which serum alpha-fetoprotein is raised but despite an extensive examination of the specimen, no yolk sac tumour can be identified. In such cases, it has been suggested that the alphafetoprotein may be derived from other sources such as immature hepatic tissue [2, 3, 21].

Teratomas manifest as typical GCTs with cystic and solid areas, with lesions in individual areas of the body having a propensity to invade local structures. Some important specific radiological features of intracranial, mediastinal and sacro-coccygeal tumours are described elsewhere in this chapter.

Benign mature teratoma accounts for between 23 and 48 % of prepubertal testicular GCTs. These are usually found in boys under 4 years of age and appear radiologically as complex solid and cystic masses on ultrasound evaluation, often being large relative to the size of the testis. Echogenic foci may be evident as a result of calcification or fat within the lesion [22].

Most ovarian teratomas are diagnosed late in the first decade of life, with a peak age incidence at diagnosis of 10 years, or in early adolescence. The majority are benign. Most patients present as an abdominal mass (Fig. 11.11) or with pain due to torsion of the lesion (Fig. 11.12).

Primary abdominal and retroperitoneal teratomas are extremely rare and represent only 4–6 % of teratomas [23]. Although usually benign, abdominal teratomas in infants are often large and have a particular tendency to infiltrate widely such that describing them as merely retroperitoneal masses is anatomically simplistic. They have a propensity to expand anteriorly and often occupy virtually the entire abdomen such that contact with the anterior abdominal wall is common. Their large size often makes ascribing a definite organ or anatomical compartment of origin difficult. Whilst pancreatic, gastric, mesenteric and gallbladder origins are frequently described for these masses, the location may simply reflect the largest component of a tumour mass originating in the retroperitoneum or root of the mesentery. The typical radiological features of a retroperitoneal teratoma are a large, complex mass, usually with well-circumscribed cystic components, containing fat and areas of calcification [24]. The sonographic appearances of mature retroperitoneal teratoma are usually those of a mixed tumour with cystic and solid components. Calcification and characteristic adipose content or fat-fluid levels are better demonstrated on CT or MRI. Contrast enhancement of an intra-cystic solid component has been suggested as a criterion to distinguish benign from rarer malignant lesions in adults (with a sensitivity and specificity of approximately 80 %) and this may also be applicable to paediatric GCTs [25].

It is noteworthy, that vascular displacement seems to occur in a different pattern in abdominal teratomas to that seen with other infiltrative paediatric tumours such as neuroblastoma. These tumours almost invariably envelop the aorta and vena cava [26]. Mass effect can cause marked distortion of normal anatomical relationships and compression or complete effacement of the IVC. In addition to being compressed or effaced, unusual but marked anterior displacement of the IVC away from the aorta is a surgical risk with these tumours and this should be carefully considered pre-operatively [26] (Fig. 11.13). Furthermore these masses may prove difficult to remove at surgery not only due to vascular encasement but also from being densely adherent to adjacent structures. Current CT angiographic techniques should be adequate to assess the arterial anatomy but the compressed abdominal veins can be very difficult to evaluate even with the best sonographic technique.

Seminoma, whilst the most common testicular tumour in adults, is very rare in childhood but when it does occur, it is the neoplasm most commonly associated with a cryptorchid testis [22]. Seminomas tend to have homogeneous, soft-tissue attenuation while the more malignant lesions frequently have large necrotic components but there is wide variation in the appearance of all these tumours.

YSTs appear on imaging similar to other GCTs, but elevated serum levels of AFP suggest the diagnosis. They tend to be more infiltrative than benign GCTs and may arise from the ovary, testis or elsewhere in the abdomen (Fig. 11.17). Testicular yolk sac tumours, which occur mainly in pre-pubertal boys, are confined to the testis (stage 1 disease) in 85 % at presentation [22] (Fig. 11.18). In the upper abdomen yolk sac tumours may mimic a primary hepatic tumour and it can be very difficult to identify the true site of origin of the lesion (Fig. 11.19). Extragonadal yolk sac tumours may occur virtually anywhere in the body, such as in the brain, mediastinum, common bile duct or thyroid gland as examples. Routine chest CT to detect or exclude pulmonary metastases is recommended at diagnosis.

A hemorrhagic mass or a hemoperitoneum may be seen with an ovarian primary. A large unilateral, hypervascular adnexal mass with multiple cystic cavities and the presence of haemorrhage associated with elevated β–HCG levels and an “empty uterus” are highly suspicious for an ovarian choriocarcinoma [31]. An MRI of the pelvis can confirm the contents of a mass to be hemorrhagic. It may nevertheless be challenging to differentiate choriocarcinoma from ectopic pregnancy in a young woman by imaging findings alone [31]. Distant hematogenous metastases may be present, however, with choriocarcinoma and help point to that diagnosis.

GCTs arising in the gonad have different epidemiological characteristics in boys and girls. In boys there are two distinct peaks of GCT development, corresponding to distinctly different histological entities and underlying pathophysiological basis. GCTs occurring in young children represent either mature or immature teratomas or malignant yolk sac tumour. The majority of these present as asymptomatic mass lesions confined to the testis and do not show an association with intratubular precursor germ cell lesions, almost all of them in young children occurring in otherwise normal testes. Yolk sac tumour in young children does not show the characteristic cytogenetic abnormalities associated with adult testicular GCTs [28] (Figs. 11.20, 11.21, 11.22, 11.23, 11.24, 11.25, and 11.26).