The pathogenesis, diagnosis, and treatment of histiocytosis syndromes.

The histiocytosis syndromes previously known, and often still referred to, as histiocytosis-X were originally categorized by many as malignant neoplasms. They have been treated as such with aggressive chemotherapy and radiation therapy. Although these modalities are still used, there are significant differences between the histiocytosis syndromes and the true malignant disease that suggest conservative management for many patients. The clinical course of malignant neoplasia is relentlessly progressive with virtually no survival in untreated patients. The histiocytosis syndromes are characterized by frequent spontaneous remissions and exacerbations, with varying morbidity and survival in untreated patients, depending on the extent of the disease. Pathologically, the lesions of histiocytosis appear as reactive infiltrates, possessing little of the cellular atypicality and homogeneity characteristic of malignancy. Although the etiology of these phenomena is unknown, histiocytosis syndromes appear to represents a reactive autoimmune disorder triggered by unknown stimuli.