Pathology and molecular biology of teratomas in childhood and adolescence

The biologic behaviour of teratomas depends on various interdependent clinical and epidemiologic variables such as the age at diagnosis, sex, tumor site, histology which all correlate to different cytogenetic and molecular biologic aberrations. Thus, testicular teratomas of infancy are generally benign. Accordingly, prepubertal teratomas show no cytogenetic or molecular genetic aberrations. In contrast, postpubertal testicular teratomas can present as clinically malignant tumors and may show complex cytogenetic aberrations such as the isochromosome 12p, which is pathognomonic of malignant germ cell tumors. Notably, teratomas of both age groups show an at least partial erasure of the genomic imprinting, correlating with their origin from primordial germ cells. The Kiel Pediatric Tumor Registry includes 541 teratoma specimens, and among these, the most frequent tumor sites (in descending order) are: the sacrococcygeal region (33.8 %), the ovaries (31.2 %) and the testes (10.5 %). Rare localizations include the mediastinum, the retroperitoneum, the head and neck region as well as the central nervous system. The WHO classification of germ cell tumors distinguishes mature and immature teratomas as well as teratomas with malignant transformation. In immature teratomas, primitive neuroectodermal structures predominate. According to the grading system (Gonzalez-Crussi, 1982), mature teratomas (G0) are more frequent (54.5 %) than immature teratomas (G1-G3, 45.5 %). Only 7.8 % of all teratomas show the highest grade of immaturity (G3). The frequency of additional microscopic foci of malignant yolk sac tumor correlates with the grade of immaturity. In sacrococcygeal teratomas, the yolk sac tumor microfoci may give rise to a malignant relapse after incomplete resection. The rare teratomas with malignant transformation contain components with "conventional" somatic type malignancy such as leukaemia, carcinoma or sarcoma. Here, molecular genetic analysis has demonstrated the origin of the somatic malignancy from a malignant transformation within the germ cell tumor with retention of the cytogenetic changes characteristic of malignant germ cell tumors.