Comparing the microscopic features of tumor cells with their normal cellular counterparts is the key to diagnosing a neoplastic
disease. Thus, cellular identification is frequently achieved with well-differentiated tumors; however, assigning a cell of
origin in a poorly differentiated tumor is difficult. Identifying the tissue of origin in an undifferentiated tumor is important
in determining the patient's prognosis and planning the treatment.
Laboratories have access to hundreds of antibodies for immunohistochemical testing. (Veterinary laboratories are preferred
because human immunohistochemical tests may not be useful for animal tissue.) Some antibodies only react with one type of
cancer, but other antibodies react with several types, so testing with several different antibodies helps determine the cancer's
origin. Most procedures may be performed on formalin-fixed tissues; however, fresh or frozen tissue samples prepared on unstained
slides may be necessary for others.
Epithelial tissue and tumors of epithelial origin (carcinomas) stain positive for various cytokeratin filaments. Neural filaments
are diagnostic of neural cells and some neuroendocrine tumors. Vimentin, an intermediate filament, is typically present in
mesenchymal cells and their derived tumors, such as sarcomas and other nonepithelial neoplasms. Desmin, an intermediate filament,
is characteristically present in muscle cells and most tumors derived from muscle cells. Kappa and lambda light chains are
excellent markers for canine B cell lymphomas and plasmacytomas. Neuron-specific enolase and S-100 protein are often present
within amelanotic melanomas. The combined use of vimentin, S-100 protein, and neuron-specific enolase helps distinguish amelanotic
melanoma from other undifferentiated round cell tumors in dogs. CD18 can detect all bone-marrow-derived cells, while CD3 will
stain T cells, and CD79a will mark B cells. Other connective tissue sarcomas can be identified by the presence of smooth muscle
actin, desmin, factor VIII-related antigen, and glial fibrillary acidic protein. Carcinomas can be further characterized by
staining for specific proteins, such as various hormones and hormone receptors (e.g. an estrogen receptor). Consultation with a veterinary pathologist regarding the selection of an appropriate stain or with
a veterinary oncologist before tissue submission can be helpful.
By considering the immunohistochemical test results in the context of the tumor's appearance after a routine histologic examination,
the location of the tumor's metastasis, and other information such as the patient's age and sex, it is often possible to find
the source of the cancer or to classify the cancer in a way that helps guide treatment.
Kevin A. Hahn, DVM, PhD, DACVIM (oncology)
Gulf Coast Veterinary Specialists
1111 West Loop South, Suite 150
Houston, TX 77027