Histiocytic sarcoma arises from dendritic cells, which are specialised immune cells that normally help the body recognise and respond to threats. In the localised form, the cancer develops as a single mass, most commonly in or around joints (periarticular), the spleen, lungs, or under the skin. The Bernese Mountain Dog is dramatically predisposed, with roughly one in four developing this disease during their lifetime — a hereditary susceptibility that remains an active focus of breed health research. Flat-Coated Retrievers, Rottweilers, and Golden Retrievers are also at increased risk. While the localised form has a better outlook than the disseminated form, it remains an aggressive cancer that frequently spreads if not treated promptly, and surgery combined with chemotherapy offers the best chance of meaningful disease control.