What Is Histopathology?

The National Cancer Institute defines histopathology as "the study of diseased cells and tissues using a microscope." Histology is the study of tissues, and pathology is the study of disease. So taken together, histopathology literally means the study of tissues as relates to disease. A histopathology report describes the tissue that has been sent for examination and the features of what the cancer looks like under the microscope. A histopathology report is sometimes called a biopsy report or a pathology report.

The specialist doctor who does the examination under the microscope is called a pathologist. The tissue that is studied comes from a biopsy or surgical procedure whereby a sample of the suspect tissue is selected and sent to the laboratory. It is then processed and cut into very thin layers (called sections), stained, and examined under microscopes to characterize the details of the cells in the tissue.

For some diseases, the surgeon can get a sample of the tissue interpreted very quickly through the use of frozen sections. Frozen sections or slices are used sparingly in lymphoma, however, due to problems in interpretation and sampling. In lymphomas, lymph nodes are the tissue most commonly examined in histopathology. For many types of blood cancers, a bone marrow biopsy may also be required for a definitive diagnosis.​

Histopathology reports on surgical cancer specimens are getting more and more complex. They may include:

• The microscopic appearance of the involved tissue
• Special stains
• Molecular techniques
• Other tests

Many of the findings from such examination of the tissues are linked to prognosis. Prognostic indicators may include tumor grade and extent of spread, and whether or not the cancer was removed with a margin of healthy cells surrounding it, or if there is evidence the cancer has spread beyond what was removed.

Grading systems differ depending on the kind of cancer being graded, but generally, the cells are scored based on how abnormal they appear under the microscope, with Grade 1 tumors being more normal looking and Grade 4 tumors reflecting more abnormalities. A high-grade tumor, then, is generally one in which the cells have more abnormalities. Grading is not the same as staging. Staging has more to do with where the cancer is found in the body and how far it has spread.

In addition to the histopathology, other techniques may be used to assess the presence of cancer in the tissues, including fine needle aspiration cytology, and some of these techniques may be used more extensively in health care settings around the world.

Leukemias and lymphomas are diagnosed using a combination of their appearance—how the cells look (morphology), their markers or surface proteins that can be detected using antibody tests (immunophenotype), their enzymes that can enable certain chemical reactions to occur (cytochemistry), and their chromosomal changes (karyotype).

Often times in lymphomas and other cancers, a technique called immunohistochemistry is used to help assess the tumor type, prognosis, and treatment.

For example, CD23 and CD5 are microscopic tags that, if present in the cancer cells, might support the notion that chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) is a possible diagnosis. These same markers are also present in other malignancies, however, so doctors use a sort of process of elimination based on the available information and what is known about the various malignancies and their "typical" CD markers.

Another example of a CD marker is CD20, which is present in some lymphomas but absent in others. Diffuse large B cell lymphoma, or DLBCL, is a very common lymphoma that is associated with the CD20 marker.

For a given sample of leukemia or lymphoma cells, markers can be tested using an entire panel of antibodies that stick to different markers, with positive and negative controls, built in.

Molecular and chromosomal studies may be done to look at gene rearrangements and specific changes to the chromosomes. Sometimes inserted or deleted genes are linked to information about prognosis. For instance, in chronic lymphocytic leukemia, or CLL, a specific piece of a chromosome is lost, and often times lost along with it is a gene that helps suppress cancer. The 17p deletion is found in about three to 10 percent of people with CLL, overall. The 17p deletion CLL is a form of CLL that is harder to treat with conventional chemotherapy.