Laboratory Tests Related to Diagnosing Melanoma
If a dermatologist finds a growth that may be cancerous, they will perform a biopsy. They will numb the area and then remove a skin sample. The sample will be sent to a lab, where a pathologist will look at it under a microscope. There are three types of biopsies that are used to diagnose skin cancer.
Punch Biopsy: The doctor uses a tool called a biopsy punch to remove a small circular piece of skin.
Shave Biopsy: The doctor uses a sharp blade to remove the top few layers of skin.
Excisional and Incisional Biopsies: The doctor uses a scalpel to remove a tumor that has grown deeper in the skin. They usually remove the entire growth, which is called an excisional biopsy. In some cases, they may only remove part of the growth, which is an incisional biopsy.
Melanoma can spread quickly throughout the body and reach lymph nodes, the brain, or other organs before the skin lesion is identified. Rarely, the cancer will spread, and the skin lesion will go away without any treatment. If there is concern that the melanoma has spread, doctors will often perform a biopsy on lymph nodes or other areas of concern. These biopsies are often more intensive than the ones used to examine the skin. Types of biopsies that may be used include the following:
Fine Needle Aspiration: The doctor inserts a very thin, hollow needle into the lymph node and collects a small amount of cells. If the lymph node is near the body’s surface, the doctor can aim the needle by feeling with their hands. If the lymph node is deeper in the body, they will use imaging like ultrasound or a CT scan to guide the needle. The samples removed during the biopsy will be sent to the lab and examined under a microscope by a pathologist to confirm if the cancer has spread.
Excisional Lymph Node Biopsy: The doctor will remove the whole lymph node through a small cut in the skin. Depending on the location of the lymph node, the area will be numbed, or the patient will be sedated. These biopsies are performed if the doctor suspects the cancer has spread, but a fine needle aspiration wasn’t done or didn’t find any melanoma cells. The samples removed during the biopsy will be sent to the lab to be examined by a pathologist under the microscope to confirm if the melanoma has spread to the lymph node.
Sentinel Lymph Node Biopsy: These biopsies are done on patients diagnosed with melanoma that is at risk of spreading. Doctors will inject the patient with small amounts of a radioactive substance and/or blue dye near the site of the skin lesion. They observe what lymph nodes collect the material first. These are called sentinel nodes, and they are where cancer would likely spread first. The sentinel nodes are removed and examined by a pathologist under the microscope. If there is no melanoma in these nodes, it is very unlikely that the cancer has spread elsewhere, and there is no additional surgery. If the pathologist sees signs of melanoma, the surgeon may remove the rest of the lymph nodes in the area for the pathologist to examine.
In some cases, a pathologist cannot determine the type of cancer just by looking at the biopsy under a microscope. In these cases, they will work with the lab to perform additional tests on the sample.
Immunohistochemistry (IHC): This process uses tissue from patient samples prepared on glass slides and special antibodies to check for certain markers on the tumor cells called antigens. Different types of cancer have different antigens, so when the antibodies bind to specific antigens, they help the pathologist to tell the difference between melanoma and other types of cancer. The antibodies are attached to a chemical that displays a color (usually brown or red), which allows the pathologist to see if the test is positive under the microscope.
Fluorescence in situ hybridization (FISH): This test identifies genetic changes in DNA, like chromosomal abnormalities that create specific proteins that cause cancer cells to grow and reproduce faster. FISH is used both to identify cancer types and to guide treatment options. During a FISH test, small stretches of DNA called DNA probes attach to matching stretches in the tumor cells. The DNA probes are marked with a dye that responds to florescent light. When the sample is put under a fluorescent light microscope, they highlight gene changes in the tumor DNA. FISH is usually performed on tissue on a glass slide or on tumor cells in a Petri dish that have been removed and allowed to multiply in the laboratory.
Comparative genomic hybridization (CGH): This test compares the DNA in two samples to identify changes in the DNA. Pathologists will use it to compare the DNA of tumor tissue to the DNA of healthy tissue. By examining the differences in the DNA, the pathologist can better determine if the tissue is cancer, and if it is, what type.
Gene expression profiling (GEP): Gene expression is the process where information is taken from genes and used to make proteins within cells. DNA transcription creates a substance called messenger RNA. A process called RNA transcription uses messenger RNA to create proteins. At any given time, only some of a cell’s genes are making RNA. Gene expression profiling identifies what genes are “turned on” and creating messenger RNA. This information can be used to help diagnose cancer and guide cancer treatments.
After examining the biopsies, the pathologist will stage the melanoma. Staging identifies how much cancer is in the body and how much it has spread.