Skin Tumor Consisting of Intermingled Lipoma and Extramedullary Plasmacytoma in an Old Dog

* Correspondence to: Kinji Shirota (Laboratory of Veterinary Pathology, School of Veterinary Medicine, Azabu University), Fuchinobe 1-17-71, Chuo-ku, Sagamihara, Kanagawa 252-5201, Japan TEL +81-42-850-2468 FAX +81-42-769-1628 E-mail: shirota@azabu-u.ac.jp A cutaneous solitary soft neoplasm of approximately 20 mm in diameter developed on the back of a 14-yearold male mongrel dog (Fig. 1). Cytologic examination of a fine-needle biopsy of the neoplasm revealed round tumor cells with eccentric nuclei and occasional perinuclear halos. No significant clinical symptoms were found in the dog. The neoplasm was resected under local anesthesia. The mass lesion was easily separated from the adjacent subcutaneous tissue, and the gross appearance of the cut surface was shiny and similar to lipoma (Fig. 2). The resected neoplasm was fixed in 10% neutral buffered formalin, in which it floated, and was then submitted to our laboratory for histologic examination. Histologically, a major portion of the neoplasm consisted of a lipomatous structure composed of mature adipocytes with a small distinct proliferating focus of closely packed neoplastic plasmacytoid cells (Fig. 3). The well-demarcated lipomatous structure was surrounded by thin collagen fibers. In addition, the neoplastic plasmacytoid cells had diffusely infiltrated within and around the lipomatous structure (Fig. 4). The neoplastic plasmacytoid cells showed mild anisokaryosis and frequent mitosis, especially in the proliferating focus. Many of the neoplastic plasmacytoid cells infiltrating between the adipocytes had a fine vacuolar cytoplasm. Immunostaining revealed that the cytoplasm of most of the neoplastic plasmacytoid cells was positive for the lambda light chain of immunoglobulin, but negative for the kappa light chain of immunoglobulin (Fig. 4), suggesting monoclonal proliferation of the plasma cells. From these histologic findings, the tumor was diagnosed as a benign collision tumor of the skin that consisted of lipoma and extramedullary plasmacytoma. A collision tumor is defined as a tumor in which the neoplastic proliferation of cells from two originally different cell lineages occurs in close proximity in the same tumor tissue. Collision tumors are rare in dogs, although malignant collision tumors in the skin and