The fibroblastic and myofibroblastic lesions of childhood with variable biologic potential covered in this chapter include neurothekeoma, plexiform fibrohistiocytic tumor, angiomatoid fibrous histiocytoma, inflammatory myofibroblastic tumor, and infantile fibrosarcoma.

Neurothekeoma is a benign myofibroblastic tumor separate from true nerve sheath myxoma. It is included here because of rare occurrence of atypical variants and its resemblance to plexiform fibrohistiocytic tumor. All other lesions have potential mainly for local recurrence; however, they also have a variable but usually low risk for metastasis.

Understanding of the molecular genetics of all of these tumors has improved because of the discovery of tumor-specific fusion translocations in angiomatoid fibrous histiocytoma, inflammatory myofibroblastic tumor, and infantile fibrosarcoma. These gene rearrangements are diagnostic markers, and the corresponding gene products probably play a pathogenetic role.

Other borderline to malignant fibroblastic lesions that are more typical of adults can also occur in children, for example, low-grade fibromyxoid sarcoma and dermatofibrosarcoma protuberans. These tumors, including giant cell fibroblastoma, the juvenile variant of DFSP, are discussed in Chapter 13.

NEUROTHEKEOMA

Originally described by Gallager and Helwig in1980 and then thought to be a nerve sheath tumor, neurothekeoma has recently been verified conclusively as a fibroblastic-myofibroblastic neoplasm that is unrelated to nerve sheath myxoma and therefore should be separated from it. The original description of neurothekeoma contained a minor component of nerve sheath myxomas (because these tumors are far less common than neurothekeomas), and similarly, the early reports on nerve sheath myxomas probably contained examples of myxoid neurothekeomas, because at that time immunohistochemical studies were not available for conclusive separation of these entities.