General practitioners are increasingly faced with a histology report which suggests they use immunohistochemistry to further classify their tumour. With the development of immunohistochemical markers, retrospective studies have shown that some tumours were previously misclassified based on histological examination alone. Although there are often clear histological indicators of a tumour origin, some masses exhibit multiple conflicting features, or they are so poorly differentiated that no typical features are present and so further diagnosis can only be made with the use of immunohistochemistry. Also, some excessively reactive lesions, particularly those of lymphocytic origin, can mimic neoplasia. Here, immunophenotyping may help to differentiate between reactive and neoplastic change.
Many of you will remember a time when immunohistochemistry was rarely recommended as a further diagnostic test. This was in part due to the limited availability of antibodies for accurate tumour identification but also due to the limitations and costs of veterinary oncology. For many owners, hearing their animal has cancer is very distressing and for some the thought (and expense) of chemotherapy or radiotherapy is more than they wish to consider. However, with an increasing number of insured pets, and rapid advances in veterinary oncology, survival times for even some of the most aggressive cancers have been greatly improved. The increasing range of chemotherapeutics available for our patients makes an accurate diagnosis imperative, if the most up to date advice regarding the prognosis and possibilities of further treatment are to be provided. So how does it work?
Immunohistochemistry is the process of detecting tissue antigens in histologic sections using target antibodies. It is mainly performed on formalin-fixed paraffin embedded tissue sections. In the case of tumours, the antigens are typically structural proteins inherent to the specific cell of origin (for example, cytokeratins in carcinomas). The antibodies bind specifically to these antigens and the subsequent antibody-antigen complex is then visualised, typically by means of a conjugated enzyme, such as peroxidase, that can catalyse a colour-producing reaction. Alternatively, a fluorophore, such as fluorescein or rhodamine, can be used to label the antibody. The sections are then examined microscopically, and the neoplastic cells can be identified by means of positive or negative staining.
Although there are a large number of commercially available antibodies, the time and costs of processing an individual antibody are huge and many are prohibitively expensive. As a result, many published antibodies are, as of yet, only available in research establishments. However, the approach we use offers a diverse range of antibodies, which covers the majority of the neoplastic lesions encountered in practice. We are here to offer help and advice on the most appropriate test or tests for your particular case. Please contact us to discuss your options.