Progressive Infiltrative Lipoma of the Masseter Muscle Associated with Unilateral Orofacial Discomfort in a Dog.

Intramuscular lipomas are uncommon benign adipocytic neoplasms characterized by infiltrative growth within skeletal muscle. This report describes the clinical, imaging, surgical, and histopathologic findings of an infiltrative lipoma arising from the masseter muscle in a dog presenting with orofacial discomfort suggestive of neurologic or painful orofacial disorders. Ultrasonography demonstrated heterogeneously hyperechoic adipose tissue within the right masseter muscle. Computed tomography identified a progressively enlarging fat-attenuating intramuscular lesion containing infiltrative extensions and linear soft-tissue strands consistent with entrapped muscle fibers. Although imaging findings strongly favored an infiltrative intramuscular lipoma, well-differentiated liposarcoma could not be definitively excluded. Histopathology confirmed a non-encapsulated infiltrative intramuscular lipoma composed of mature adipocytes infiltrating between skeletal muscle fibers. Surgical debulking resulted in complete resolution of clinical signs during six months of follow-up. This case highlights that infiltrative fat-containing lesions of the masticatory musculature may mimic neurologic or painful orofacial disorders and emphasizes the value of cross-sectional imaging and histopathology for diagnosis and surgical planning.