The information gained from the pathologist can be invaluable and should be maximized at every opportunity. The pathologist's job is to determine 1) tumor vs. no tumor 2) malignant vs. benign 3) tissue of origin 4) margins in excisional specimens and 5) a histologic grade when available. The history provided to the pathologist is invaluable and should be as complete as possible. It should always include basic information including the signalment, clinical signs and their duration. Significant laboratory and diagnostic imaging information is also important. Be very specific in the location of the lesion. It's somewhat surprising the number of samples that come in labeled stomach, only to be haired skin from the ventral abdomen. Remember you cannot change what the pathologist sees, so you don't need to be concerned about biasing them.

Cytology can be an extremely useful tool when performed properly. Before submitting a slide, take a minute to stain one of the slides to be reasonably certain you have the cells of interest on the slide. Neoplastic cells are often very friable so be gentle when spreading the material on the slide. The goal of cytology is not necessarily to obtain a definitive diagnosis, but rather to tell you whether to worry or not worry i.e. is this a mass that needs to be biopsied or removed, and if it is to be removed what kind of margins would be appropriate. Benign masses such as lipomas and sebaceous adenomas can usually be identified cytologically and may only need to be identified and measured for future reference. Lymphomas, mast cell tumors, plasma cell tumors, transmissible venereal tumors and histiocytomas can routinely be definitively identified cytologically and treated accordingly. At the very least the broad categories of mesenchymal or epithelial cell neoplasms can be made with the exception of the highly undifferentiated neoplasm, which in itself is valuable information.

Properly performed biopsies will not negatively influence survival times and the benefits far outweigh the risks. When performing incisional biopsies it is important not to contaminate tissue planes that may need to be excised later. For that reason, drains are inappropriate and dead space should be obliterated as much as possible. Incisional biopsies on the extremities or tail should be longitudinal rather than transverse to facilitate removal of masses later if indicated. As a general rule, the larger the sample, the more likely it is to be diagnostic. When procuring small endoscopic or needle biopsies, try to take multiple samples to increase the likelihood of a definitive diagnosis. Before performing the procedure, be certain that owners don't have unrealistic expectations. Remember that if the diagnosis is not what you expected, don't hesitate to get a second opinion. They are a way of life in every pathology laboratory.

Immunohistochemistry is the identification and localization of antigens in tissue using antibodies bound to a marker. It may provide a more definitive diagnosis for the pet owner which will better define the prognosis, and in some cases it will dictate the treatment regimen. Tumors may be so undifferentiated that their cell of origin can be difficult to determine. Numerous immunohistochemical stains are available to help identify them further starting with whether the tumor is mesenchymal or epithelial in origin. Vimentin is present in all mesenchymal cells. Cytokeratin is found in predominantly in epithelial cells, although also found in synovial cell sarcomas, rhabdoid tumors and mesotheliomas.

Numerous stains are available to further delineate mesenchymal tissues. Three vascular markers are available that can help in the identification of hemangiosarcoma tissues: Factor VIII-related antigen (von willebrands factor), CD-31, and CD 34. CD 31 (PECAM-1, platelet endothelial cell adhesion molecule) is expressed by 80-100% of angiosarcomas and hemangiosarcomas in human patients and is easier to interpret than Factor VIII and more specific than CD 34.

Muscle markers include desmin, actin and myoglobin. Desmin is expressed by the majority of muscle tumors.

Melanoma markers include vimentin, S-100 protein, neuron-specific enolase, Melan-A and HMB-45. Melan-A is the most sensitive and specific for canine oral melanomas. Sensitivity is lower for feline tumors.

The only other epithelial stain used with any frequency is uroplakin III which is specific for transitional epithelium.

Leukocyte markers should be used routinely in lymphoma samples as it impacts prognosis and the treatment regimen. B-cell markers include CD 79a, CD 45, CD 19 and CD 20. T-cell markers include CD3, CD 4, CD 8 and TdT. CD 34 is a stem cell marker. CD 18 is more reactive for histiocytes than lymphocytes and is often used to differentiate histiocytic sarcoma from synovial cell tumors and other sarcomas. At times it is impossible on histology or cytology alone to differentiate myeloid leukemias from lymphomas. Myeloid antigens include CD 13, CD 33 and MPO. Acute lymphoblastic leukemias should react with antibodies to CD 7, CD 10 and/or TdT. NK cells generally are positive for CD 16.

Another tumor that almost always needs immunohistochemical stains is synovial cell sarcomas. Roughly 50% of these will be histiocytic sarcomas, 15% synovial cell sarcomas and the remainder a combination of synovial myxomas and other soft tissue sarcomas. The median survival time for synovial cell sarcomas is 31.8 months with an amputation vs. only 5.3 months with surgery alone.A variety of case examples will be used to illustrate the indications for special stains.