Disseminated histiocytic sarcoma is a rapidly progressive cancer in which malignant dendritic cells spread to multiple organs simultaneously, most commonly the lungs, liver, spleen, bone marrow, and lymph nodes. It may develop as the spread of a previously localised tumour or arise in multiple sites from the outset. This form carries an extremely poor prognosis, with survival often measured in weeks to a few months even with treatment. The same breeds predisposed to the localised form — Bernese Mountain Dogs, Flat-Coated Retrievers, and Rottweilers — are at highest risk. Treatment options are limited, and the primary goal is often to maintain comfort and quality of life for as long as possible.