Intra-tumoral adipocytes in canine extramedullary plasma cell tumors

Canine extramedullary plasma cell tumors (cePCTs) are common integumentary round-cell tumors. The presence of adipocytes among round cells, which is thought to be the result of trapping of subcutaneous adipocytes, is reported as a helpful diagnostic feature to distinguish cePCTs from other round-cell tumors in the skin. Here, we report our retrospective analysis of the prevalence, extent, and distribution of adipocytes in cePCTs from a single institution. We found adipocytes in 73 of 265 (27.5%) cePCT cases diagnosed from 2020-2024, suggesting that intra-tumoral adipocytes are a frequent histologic feature in cutaneous cePCTs. We found that 1-96% of the mass was composed of adipocytes, as quantified on whole-slide images. Of the cePCTs with adipocytes, 27 (37%) cases did not have significant involvement of the subcutis. Adipocyte-containing cePCTs were more frequent in our case series than were amyloid-producing cePCTs (19; 7%). A subset of 9 cases had limited involvement of the subcutis. The proportion of intra-tumoral adipocytes was far greater (>10%) than would be expected for simple entrapment of subcutaneous adipocytes, suggesting the promotion of proliferation of mature adipocytes by neoplastic cells.