Histiocytosis and Neoplasms of Macrophage-Dendritic Cell Lineages: Multimodality Imaging with Emphasis on PET/CT

Histiocytosis is a rare inflammatory process characterized by pathologic infiltration and accumulation of cells derived from the monocytic lineage in normal tissue. It encompasses more than 100 different subtypes of disorders that were recently classified into five main groups: (a) Langerhans-related histiocytosis, (b) Rosai-Dorfman histiocytosis, (c) cutaneous and mucocutaneous histiocytosis, (d) malignant histiocytosis, and (e) hemophagocytic lymphohistiocytosis and macrophage activation syndrome. Langerhans cell histiocytosis is the most common histiocytic disorder. Less common types include Erdheim-Chester disease, Rosai-Dorfman disease, adult and juvenile xanthogranuloma, necrobiotic xanthogranuloma, histiocytic sarcoma, interdigitating dendritic cell sarcoma, Langerhans cell sarcoma, and hemophagocytic lymphohistiocytosis. Although the pathogenesis of these disorders may be attributable to mutations in the oncogenic driver, recent discoveries have shown that inflammation and fibrosis secondary to mutated histiocytes, rather than a proliferative cell mechanism, result in manifestation of the disease. Diagnosis, which relies on a multidisciplinary approach, is challenging and often delayed because clinical findings are nonspecific and may mimic malignant processes at radiologic evaluation. Compared with conventional imaging, PET/CT allows detection of the increased metabolic activity in histiocytes. Diagnostic algorithms for histiocytic disorders should include functional imaging with fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG) PET/CT, which provides a comprehensive whole-body evaluation of their potential involvement with multiple organ systems and allows monitoring of therapeutic response. The most recent revised classification, pathophysiologic and clinical manifestations, sites of involvement, and imaging features of histiocytosis are described in this review and a multimodality approach is used, with emphasis on ¹⁸F-FDG PET/CT evaluation. ^©RSNA, 2021.