Primary “amyloid tumor” of the mammary gland in a dog

The present report describes an unusual mammary lesion in a dog, with gross and histological features similar to the lesion known as “amyloid tumor of the breast” in the woman. A fourteen year-old female spayed Dalmatian dog was presented to the practitioner with a large, bilobated lump involving left and right IV mammary gland. The mass was surgically excised and submitted for histopathological examination. Grossly, two large subcutaneous nodular, firm masses, merging on the median line, expanded left and right IV mammary glands. Histologically, masses were encapsulated and mainly composed of large, coalescing lakes of pale eosinophilic, homogenous hyaline amorphous material, that stained positive with Congo Red (amyloid). Numerous plasma cells with moderate anysocytosis and anysokariosis and rare mitoses were multifocally recognizable. Multinucleated giant cells (MNGCs) with nuclear features similar to plasma cells were occasionally present. Moreover, MNGCs with scant eosinophilic cytoplasm and up to 25 dense nuclei were visible closely associated with amyloid deposits. Moderately atrophic mammary parenchyma was present at periphery. Immunohistochemistry (ABC method) was performed to characterize MNGCs and amyloid, applying anti-Lysozyme; Lambda-light-immunoglobulin-chains and HLA antibodies. Plasma cells and MNGCs were Lysozyme and HLA negative. Lambda-chains stained plasma cells, the majority of MNGCs and amyloid (AL amyloid). Bilateral mammary extramedullary plasmacytoma with abundant AL amyloid deposition was diagnosed. Clinical workup did not reveal systemic amyloidosis or multiple myeloma. Sixteen months later the dog was humanely killed because of cardiac failure. No signs of multiple myeloma or recurrence of the tumor were reported. Primary amyloidosis of the breast not associated with mammary carcinoma (amyloidoma), is a rare entity that has been documented in women since 1973, affecting mostly elderly patients, with bilateral involvement and related neither to systemic amyloidosis nor to multiple myeloma. Despite similar gross and histological appearance, some cases have been described as primary amyloidosis, while others as plasmacytoma with massive amyloid deposition In the canine species, mammary gland amyloid deposition was described in association with mammary carcinoma whereas mammary primary amyloidosis or plasmacytoma with massive amyloid deposition have never been reported so far. To the best of author’s knowledge this is the first report in the dog of a bilateral mammary extramedullary plasmacytoma with features consistent with primary amyloidoma of the breast in the woman.