Mediastinal teratoma is a germ cell tumor that is composed of mature and/or immature somatic tissues derived from two or three germinal layers (ectoderm, endoderm, and mesoderm). The WHO classification of germ cell tumors distinguishes mature and immature teratomas as well as teratomas with somatic-type malignancy.¹

Mature teratoma is a tumor composed exclusively of mature, adulttype tissues. Dermoid cyst is a variant that is common in the ovary and rare in the mediastinum and testis, consisting of one or more large cysts lined predominantly by keratinizing squamous epithelium with skin appendages and usually hair.² Monodermal teratomas analogous to struma ovarii have not been described in the mediastinum.

Teratomatous component is the term used to describe differentiated somatic tissues associated with a seminoma, embryonal carcinoma, yolk sac tumor, or choriocarcinoma. The teratomatous component of mixed germ cell tumors is very often immature.¹

The terms “malignant teratoma” and “teratocarcinoma” are obsolete and were used to indicate teratoma with mixed germ cell elements, namely, those that result in elevations of serum tumors markers (α-fetoprotein or β-human chorionic gonadotropin) (so-called secreting tumors).

Among teratomas of all sites, up to 27% occur in the mediastinum in adults, and 4% to 13% in children.^3,4,5 Mediastinal teratomas account for 7% to 9.3% of mediastinal tumors.⁴ Approximately 43% to 70% of all mediastinal germ cell tumors⁶ contain teratoma and include mature teratoma (63%), immature teratoma (4%), and teratoma with other malignant components (33%).^7,8 In a recent series of mediastinal germ cell tumors, 73% were mature teratomas, 1% immature teratoma, 1% teratoma with somatic malignancy, and the remainder mixed germ cell tumors (22%) and seminomas (3%).⁹

More than 80% of mature teratomas occur in the anterior mediastinum, 3% to 8% in the posterior mediastinum, and 2% in the middle mediastinum, while 13% to 15% involve multiple mediastinal compartments.^10,11,12

Pure teratomas are found in both sexes, most frequently during the neonatal period and infancy¹³ (Table 121.1). Overall, there is an equal sex distribution or a slight female preponderance (M:F = 1:1.4), but there may be a slight male predominance in infants and young children.^4,12 A strong female predominance has been reported in China in adult teratomas.⁹

The mean age of adults is 28 years (range 18 to 60).¹ In children, teratoma is the predominant mediastinal tumor during the first year and has also been detected in fetuses.⁴ The proportion of immature teratomas (up to 40%) is much higher in the first year of life than at older age (˜4% to 6%).⁴

Mediastinal teratomas, particularly those in adults, are asymptomatic in up to 53% of cases and are frequently discovered incidentally on chest radiography performed for other reasons.^4,14 Most common initial complaints are dyspnea, cough, anorexia, fatigue, fever, and chest pain.^4,12 Patients with mediastinal teratomas that contain pancreatic tissue have more presenting symptoms (pleural effusions, cardiac tamponade) due to the occurrence of rupture than do those whose tumors were without pancreatic tissue.^9,15,16

In newborns, these tumors are immature and present with respiratory distress and may reach large sizes.^17,18 Perforation of a mediastinal teratoma is rare but is always symptomatic.¹⁹