Identification of lambda light chain amyloid in eight canine and two feline extramedullary plasmacytomas.

Amyloid deposition in varying amounts and with variable patterns of distribution (focal or diffuse) was demonstrated in eight canine and two feline extramedullary plasmacytomas expressing lambda light chains. Frequently, the neoplastic plasma cells had been displaced by the amyloid deposits. Foreign-body giant cells were regularly detected in the vicinity of the amyloid. In all 10 cases, Congo-red staining of the amyloid was resistant to potassium permanganate oxidation. Immunohistochemically, the amyloid reacted positively with cross-reacting antibodies against human and equine A lambda amyloids. Extramedullary plasmacytomas accompanied by localized AL amyloidosis have so far been described in human beings, dogs, cats and horses.