Cutaneous plasmacytomas with amyloid in six dogs.

Cutaneous plasmacytomas associated with local deposition of amyloid were diagnosed by light microscopy in a series of six older dogs (mean age 10.7 years) consisting of two Cocker Spaniels, a Poodle, a Weimeraner, and two mixed-breed dogs. The neoplasms occurred on the digits (2 dogs), forelimb (2 dogs), lip (1 dog), and ear (1 dog). In most cases, groups of neoplastic plasma cells were widely separated by large homogeneous islands of amyloid. The neoplastic cells had characteristic plasmacytoid features, but the degree of pleomorphism varied greatly between different neoplasms. In four of the six tumors, the diagnosis of plasmacytoma was confirmed by the demonstration of a monoclonal plasma cell population using immunofluorescent staining for anti-canine immunoglobulins. In these tumors, the neoplastic cells reacted with only one class of immunoglobulins (IgG). The amyloid did not react with any of the reagents used. The suspicion that the amyloid was of immunoglobulin origin (primary amyloid) was supported by its retention of birefringence under polarized light after treatment with potassium permanganate and staining with Congo red.