Abstract
A six‐year‐old male, neutered, flat‐coated retriever presented with a suspicion of localised histiocytic sarcoma (HS) in the right axillary region, fine needle aspirate was suggestive of HS. The dog was prescribed four doses of radiotherapy (4×850 cGy). At the first treatment tru‐cut biopsies of the mass were taken, and histopathology was consistent with the cytological diagnosis. At the third treatment, the dog developed five cutaneous nodules, and microscopy confirmed HS. Seven weeks after radiotherapy, the patient presented with severe progressive hind limb ataxia and a flaccid tail. Due to the severity of clinical signs, euthanasia was performed. Postmortem examination revealed neoplastic cells within the right axillary mass, liver, kidney, adrenal gland and diffusely infiltrating the brain, lumbar spinal cord, cauda equina and spinal nerves, consistent with metastatic HS. This case is unusual in the diffuse and infiltrative dissemination of the tumour throughout the CNS and nerve roots.