Immunohistochemical and histomorphologic characterization of canine neoplasms of the disseminated neuroendocrine system.

Canine neoplasms of the diffuse neuroendocrine system are an enigmatic and heterogeneous group of neoplasms with a wide spectrum of immunohistochemical properties and morphologic features. Through the utilization of tissue microarrays, 82 canine neoplasms of the disseminated neuroendocrine system from 16 different anatomic locations were evaluated. The prototypical canine neoplasm of the disseminated neuroendocrine system was composed of rounded polygonal neoplastic cells arranged in packets supported by delicate fibrovascular stroma. Neoplastic cells typically had moderate quantities of pale eosinophilic cytoplasm stippled by numerous fine argyrophilic granules, round nuclei with finely stippled chromatin, and inconspicuous nucleoli. Immunohistochemical assays utilized in this study included chromogranin A, neuron-specific enolase (NSE), microtubule-associated protein 2 (MAP2), pan-cytokeratin, oligodendrocyte transcription factor 2 (OLIG2), protein gene product 9.5 (PGP9.5), vimentin, synaptophysin, neuronal nuclei (NeuN), S100, SRY-related HMG-box 10 (SOX10), glial fibrillary acidic protein (GFAP), insulinoma-associated protein 1 (INSM1), CD56, and antigen Kiel 67 (Ki67). The 4 immunohistochemical assays that were positive in over 50% of cases included PGP9.5 (77/82, 94%), NSE (68/82, 83%), synaptophysin (59/82, 72%), and chromogranin A (56/82, 69%). In 81/82 (99%) cases, neoplastic cells immunolabeled with at least 1 of these 4 assays, and thus, these 4 immunohistochemical assays are deemed most useful when attempting to substantiate that a neoplasm is of neuroendocrine origin.