Ectopic Splenic Adenocarcinoma in a Dog.

A 10-year-old spayed female Border Collie developed a ductal adenocarcinoma in the spleen. Clinically, the spleen was enlarged and a small liver nodule was present but there were no other abnormalities. Most of the splenic parenchyma was diffusely infiltrated by variably shaped atypical neoplastic cells that formed small clusters or larger nests, arranged as duct or duct-like structures within a fibrous matrix. There was acinar differentiation in a few portions of the tumour with a sheet-like solid growth pattern and occasional squamous metaplasia or exocrine acinus-like structures. Mitotic figures were frequent. Neoplastic cells with ductal differentiation were diffusely immunoreactive for AE1/AE3, CAM5.2 and CK7 cytokeratins but negative for CK20, while cells with acinar differentiation were immunolabelled only for AE1/AE3 cytokeratins and were also immunopositive for mucin-1 and trypsin. A few regions of tumour with ductal or acinar differentiation were immunopositive for pancreatic lipase. All neoplastic cells were negative for mucin-2, vimentin, smooth muscle actin, chromogranin A, CD31, hepatocyte paraffin 1 and thyroglobulin antigens. Because of the formation of exocrine acinus-like structures and an immunolabelling pattern consistent with exocrine pancreas tissue, an adenocarcinoma of ectopic exocrine pancreas within the spleen was diagnosed.