Histiocytic Sarcoma: A Review and Update.

Histiocytic sarcoma (HS) is an ultra-rare hematopoietic neoplasm that frequently occurs in extranodal sites of adults. Clinically, HS demonstrates aggressive behavior and can arise de novo or in association with other hematological neoplasms. The median overall survival from the time of diagnosis is approximately six months. Histologically, HS is composed of sheets of large, round to oval cells with abundant eosinophilic cytoplasm and can be confused with a variety of benign and malignant conditions. Immunohistochemistry plays a crucial role in the diagnosis of HS, showing expression of CD163, CD68, lysozyme, and PU.1 and negative staining with follicular dendritic cell markers and myeloid cell markers. Recent studies have demonstrated a high rate of PD-L1 expression, suggesting a potential therapeutic target. Several genomic alterations have been identified in HS, including mutations involving the RAS/MAPK and PI3K/AKT/mTOR signaling pathways, mutations/deletions, and mutations. There is no standard protocol for the management of HS. Surgical resection with or without radiotherapy is the most common first-line treatment for unifocal/localized disease. The systemic treatment options for multifocal/disseminated disease are very limited. This review provides an overview of the current knowledge on the clinicoradiological features, histopathology, pathogenesis, and management of HS.