Immunohistochemistry (IHC) is used to diagnose the type of cancer and to help determine the patient's prognosis. In cases such as metastases or carcinoma of unknown primary origin, where it may be difficult to determine the type of cell from which the tumor originated, immunohistochemistry can identify cells by the characteristic markers on the cell surface. IHC can also help distinguish between benign and malignant tumors.
Immunohistochemistry requires a sample of tissue from a biopsy; usually the tissue sample is examined fresh, but frozen or chemically preserved material can be used. A blood sample or bone marrow may also be examined. The tissue sample is sliced extremely thinly, so that it is approximately one cell thick, then the sample is fixed onto a glass slide. The tumor cells in the sample have characteristic markers, or antigens, on their cell surfaces which can be used to help identify the specific type of tumor cell. Antibodies against these characteristic antigens are added to the sample on the slide, and the antibodies bind wherever the antigens are present. Excess antibody is then washed away. The antibodies that remain bound to the cell have labels on them that either fluoresce (glow) or undergo a chemical reaction that makes them visible by microscope. The pathologist is able to see the specially labeled tumor antigens as they appear in the patient's tissue.
The pathologist will try to assess the level of maturity of the tumor cells, which will help him to determine their origin. He will be checking for cell types that are found in an inappropriate part of the body, for example prostate cells in a lymph node. He will also look for cell characteristics that will indicate if the tumor is benign or malignant. Proteins involved in the replication of genetic material and cell growth may be present in greater amounts; for example, antibodies against the antigen Ki-67 are used to evaluate malignant melanomas, breast carcinomas, and non-Hodgkin's lymphomas. Hormone receptors may also be examined. The presence of receptors to estrogen and progesterone indicate a good prognosis for breast cancer patients. Pathologists may also look for an increase in tumor suppressor proteins. A wide variety of antibodies are available to help determine the origin of the tumor, whether it is growing rapidly, and whether it is a type of tumor that responds well to particular treatments.
The physician will choose the type of sample to be taken based on the type of tumor. If the patient has a solid tumor, a tissue sample may be biopsied; if the entire tumor is being removed a biopsy may be taken during surgery. In this case the patient should prepare for the surgery or the biopsy as the physician suggests. A routine blood sample may also be required; in most cases, no additional preparation is required.
The only aftercare that might be required is from the sample collection process.
The risks associated with IHC are the risks associated with the sample collection, either the biopsy of the tumor or the drawing of blood. The only other concern is the possibility that the test could yield unclear results.
Normal results will simply look like normal cells. The cells will have a high level of maturity and be located only in sites appropriate to their cell type. For example, analysis of lymph nodes will show only the cells that belong there, not cells that would normally be present in the breast. No specific tumor antigens will be present in increased numbers.