Canine meningeal histiocytic sarcoma

Siobhan O’Sullivan 

Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2025;29(1):27.

A 6-year-old Belgian Malinois dog presented to the referring veterinarian with neurologic clinical signs that were localized to the spinal cord. MRI imaging revealed nodules in and around the spinal cord at multiple levels of the lower thoracic and lumbar spine. Large mononuclear cells were observed on cytologic examination of the cerebrospinal fluid. Postmortem histopathology confirmed an extensive infiltrate of neoplastic round cells that markedly expanded the meninges and subdural space, surrounded nerve roots, and infiltrated the parenchyma of the spinal cord (Fig. 1A). These cells were highly atypical, with round to reniform to bizarre nuclear outlines, frequent binucleation or multinucleation, 5-fold anisokaryosis, and >50 mitotic figures in ten high-power fields (40x/2.37mm2) (Fig. 2). Histiocytic sarcoma was the foremost consideration given the marked cellular pleomorphism, and this was confirmed by immunohistochemistry (IHC) for the marker Iba-1, with neoplastic cells demonstrating strong cytoplasmic reactivity (Fig. 1B). No information concerning metastasis or lymph node involvement was provided in the history.

Histiocytic sarcoma is a malignant neoplasm of histiocytic cell/dendritic cell/macrophage origin, which tends to be highly invasive and highly metastatic. As in this case, it can appear sporadically in any breed, however, Bernese mountain dogs are genetically predisposed. Commonly-reported primary locations for this neoplasm include the skin, joints of the limbs, and spleen. Histiocytic sarcoma of the central nervous system is less common, and most often presents as a focal mass rather than a diffuse meningeal infiltrate. Only two other infiltrative meningeal cases such as this have been seen at the AHL since 2015. Highly pleomorphic neoplastic cells are a typical feature of histiocytic sarcoma, however, confirmation of the diagnosis by IHC is still recommended, because marked atypia may also be observed in anaplastic round cell neoplasms.

Figure 1. Canine, spinal cord. 1A. The meninges are markedly expanded by an infiltrate of neoplastic round cells which surround and invade the spinal cord and adjacent nerve roots. H&E stain.1B. Neoplastic cells demonstrate strong reactivity (brown colour) for the histiocytic immunohistochemical marker Iba-1.

Figure 2. Canine, spinal meninges. Neoplastic histiocytic cells with atypia (variation in nuclear shape, size, and number). H&E stain.

Reference

1. Moore, PF. Canine and feline histiocytic diseases. In: Tumors in Domestic Animals, 5th ed. Meuten, DJ, ed. Wiley Blackwell, 2017:327-336.