Infiltrative lipoma: An entity to keep in mind
Rebecca Egan
Animal Health Laboratory, University of Guelph, Guelph, ON
AHL Newsletter 2025;29(4):32
A 7-year-old male neutered Great Pyrenees dog presented to a referral veterinary center for recurrent growth of a mass associated with the left body wall following removal by his primary care veterinarian several months prior. CT imaging was performed and revealed a fatty mass with evidence of invasion between muscle layers of the body wall. A second surgery to remove this large mass was performed, and the specimen was sent to the Animal Health Laboratory for gross examination, sampling and fixation, and histopathology. The mass was 36 cm x 21 cm x 12 cm and was comprised of fatty tissue that infiltrated between a few slender pieces of muscle at the periphery and extended widely across surgical margins (Fig. 1).
Figure 1. Photograph of the 36 x 21 x 12 cm lipoma excised from the left body wall of a dog during gross examination and sample collection at the Animal Health Laboratory. Note the presence of a few slender pieces of striated muscle at the periphery (*).
Microscopic examination of several formalin-fixed paraffin-embedded samples from the mass confirmed that it was composed of adipose tissue formed by sheets of well-differentiated neoplastic adipocytes, among which there were a few thin fibrous septa and occasional small groups of blood vessels. In one piece, the adipose tissue was intimately associated with a few bundles of striated muscle at the periphery. There was also adipose tissue bordering both sides of a 10 mm long by 500 um thick band of fibrous tissue that contained multiple bundles of collagenous stroma harbouring widely-spaced wavy fibrocyte nuclei (reminiscent of ligament or epimysium).
In this case, the clinical history and histopathology results confirmed a diagnosis of infiltrative lipoma. Lipomas are typically slow‐growing and expansile tumors that are cured by excision, but infiltrative lipomas can be more difficult to completely excise and may require repeated surgical resections. Over the last 10 years at the Animal Health Laboratory, 8-9% of lipomas in both dogs and cats have been diagnosed as infiltrative (Table 1). The diagnosis of infiltrative lipoma is made based on invasive nature and tumor recurrence, not by the bland appearance of the adipocytes histologically; thus, the diagnosis hinges upon a clinical description of these features from the submitting veterinarian.
Table 1. Lipoma diagnoses in dogs and cats at the Animal Health Laboratory from 2015-2025
Reference
1. Hendrick, MJ. Mesenchymal Tumors of the Skin and Soft Tissues. In: Domestic Animals, 5th ed. Meuten DJ, ed. Wiley-Black Sons, 2017:158-59.