New entities, concepts, and questions in childhood tumor pathology.

There are major differences between tumors in children and adults, viz. the incidence of tumor types, the predisposition of certain organs and tissues (e.g. sympathetic nervous tissue, kidney, and soft tissues) to develop tumors, problems related to tumor classification, and the biologic behavior of childhood malignancies, which are usually characterized by high rates of proliferation activity. A large number of new entities, especially in soft tissue tumors, have been published over the past years, including nodular mesothelial hyperplasia, which is a tumor-like lesion derived from peritoneal macrophages; infantile myofibromatosis, which can mimic leiomyosarcoma; intermediate grade fibrohistiocytic tumors, like dermatofibrosarcoma protuberans-related giant-cell fibroblastoma, plexiform fibrohistiocytic tumor and angiomatoid malignant fibrous histiocytoma displaying evidence of myogeneous differentiation; finally, the high-grade intraabdominal desmoplastic small cell tumor. With modern methods we can gain better insights into the biology of tumors. For example, tumors of the Ewing's sarcoma family have in common a characteristic t(11; 22) chromosomal translocation, the Ewing's sarcoma (EWS) (22q12) gene rearrangement, and the MIC2 gene. The EWS gene rearrangement is not restricted to tumors of the Ewing's sarcoma family (classic Ewing's sarcoma and malignant peripheral neuroectodermal tumor), however, but occurs in malignant melanoma of the soft tissue and in intraabdominal desmoplastic small cell tumor. Rhabdomyosarcomas (RMS) can be divided into two basic types with different prognoses: embryonal RMS, including botryoid and spindle-cell variants, and alveolar RMS, including the solid variant. The prognosis of alveolar RMS is poorer than that of classic embryonal RMS, mainly due to early tumor dissemination in alveolar RMS. The prognosis of neuroblastoma is mainly based on chromosomal and molecular biologic findings. Structural chromosome 1 abnormalities, double minute chromosomes, homogeneously staining regions, N-myc amplifications, and DNA diploidy are indications for an unfavorable outcome. Despite progress in childhood solid tumor pathology, many questions remain open, including those relating to basic chromosomal defects in germ cell tumors and the obscure nature of tumor heterogeneity.