How Skin Cancer Is Diagnosed

If you or your physician find an abnormality on your skin that might be skin cancer, a biopsy is needed to determine both the type and extent of the cancer. A simple shave or punch biopsy is often done if a basal cell carcinoma or squamous cell carcinoma is suspected, but an excisional biopsy is usually the better choice if it could be a melanoma. For melanomas and squamous cell cancers that have spread, further tests such as CT, MRI, PET, and/or a sentinel node biopsy may be needed to determine the stage of the disease.

skin cancer diagnosis
Illustration by Verywell

Physical Exam

If you develop an abnormal skin lesion, you may see your primary care doctor or a dermatologist, a physician who specializes in skin diseases. For those who have a skin lesion that could be a melanoma, however, referral to a dermatologist is often recommended before any testing is done (such as a biopsy). 

Your doctor will first do a careful skin examination of your suspicious finding, as well as a general skin exam. This is important, as other skin findings, such as the presence of many moles, may increase the chance that your skin lesion is a skin cancer.

In addition to studying your skin lesion with a naked eye, your doctor may also use a dermascope, a special instrument that magnifies the skin, to get a closer look. What he or she sees may prompt further evaluation.

Procedures

Unfortunately, the only way to definitively diagnose a skin cancer is to do a biopsy. Your doctor may suggest this step if he doesn't like what he sees during your physical examination.

Skin biopsy options recommended to you will vary depending on the expertise of your doctor and the type of skin cancer suspected. Some primary care physicians are comfortable performing biopsies if a basal cell carcinoma or squamous cell carcinoma is suspected, whereas others will refer you to a dermatologist. With either type of physician, a biopsy may be done at the time one has been suggested or in a follow-up visit. If a melanoma is suspected, it's likely that an appointment will be set up for you to have this done at a later time, since a wide excisional biopsy (and sometimes a sentinel node biopsy) may be needed, and these procedures are more involved than others.

After a biopsy is done, the tissue is sent to a pathologist for evaluation. Your results will include information on the type of skin cancer, and if a melanoma is found, will include information on the "mitotic rate" of the tumor or how aggressive it appears. Genetic testing of the tumor cells may also be done (see below).

Shave Biopsy

A shave biopsy is the most common type of biopsy used when a basal cell or squamous cell carcinoma is suspected. In a shave biopsy, the area beneath the skin lesion is numbed with lidocaine and a doctor uses a thin, sharp blade to shave off either part or all of an abnormal growth. Sometimes the area is cauterized (burned) after the shave biopsy is performed.

A shave biopsy and/or cautery should not be done if a melanoma is suspected, as this could create problems with staging and ultimately making the best choices for treatment.

Punch Biopsy

A punch biopsy may also be done if a non-melanoma skin cancer is suspected (and on rare occasions, a melanoma). In a punch biopsy, the skin is numbed with lidocaine and a doctor uses a sharp, hollow tool to remove a piece of tissue. The punch tool is inserted to a particular depth by the physician and then twisted to remove a circle-shape sample of tissue.

Excisional Biopsy

In an excisional biopsy, the entire area under an abnormal growth and surrounding tissue is numbed. An incision is then made that includes the growth plus some surrounding tissue (a fairly large margin of tissue if a melanoma is suspected).

This is the best method of obtaining a biopsy if a melanoma is suspected, as it preserves the original cancer and tissue surrounding it so an accurate measurement of the depth of the tumor can be made. Depending on the location of cancer and its size, however, an excisional biopsy may not always be possible.

Incisional Biopsy

An incisional biopsy is similar to an excisional biopsy, but only a portion of the growth is removed. 

Sentinel Lymph Node Biopsy (Lymphatic Mapping)

If you have a melanoma that is thicker than 0.75 millimeters or is thinner but is ulcerated, has a high mitotic rate (looks more aggressive under the microscope), or lymphovascular invasion (has extended into lymph vessels or blood vessels near the tumor), your dermatologist may recommend a sentinel node biopsy. This may be done at the same time as a wide local excisional biopsy (ideally), or as a separate procedure following an excision. 

The theory behind a sentinel lymph node biopsy is that cancers drain in a specific fashion, beginning with the sentinel node and then to other nodes. Since the sentinel node or nodes are cancer's first stop as it's spreading, the absence of tumor cells in these nodes indicates that it's unlikely cancer has made its way to any lymph nodes. If cancer is found in the sentinel node(s), there is a possibility that it has spread to other nodes (or distant tissues).

In this procedure, the melanoma (or the area where the melanoma was found) is anesthetized and injected with a blue dye (isosulfan blue) and a radioactive dye (technetium-labeled sulfur colloid). The dyes are then given time to be absorbed and filtered through the lymphatics into the nearest lymph nodes.

An imaging study called lymphoscintigraphy (a test that detects radioactive activity) is then done so that the surgeon knows where to look for the sentinel nodes and which should be removed (typically, one to five are biopsied).

The lymph nodes are then sent to a pathologist to look for evidence of "macrometastases" (obvious tumor in the lymph nodes) or micrometastases (tumor cells in the lymph node that can only be seen under the microscope). 

In the past, all of the lymph nodes in a region were usually removed, a procedure that may result in lymphedema, a collection of fluid in the region of the nodes due to disruption of the flow of lymph. If cancer is not found in the sentinel nodes, surgery to remove other lymph nodes is not usually needed. On the other hand, if cancer is found in the sentinel nodes, a surgeon may recommend removing more lymph nodes (a full lymph node dissection), and cancer will probably require more aggressive treatment than if cancer wasn't in the nodes.

There are a number of pros and cons to lymph node dissection with melanoma that your doctor can discuss with you if your sentinel node biopsy is positive.

Complications of a sentinel node biopsy may include infection, bleeding, a buildup of fluid in the area where the nodes were removed (a seroma), or sometimes, lymphedema. The risk of lymphedema, however, is less common than when a full lymph node dissection is performed.

Labs and Tests

Most of the time tests, other than a biopsy, are not needed with non-melanoma skin cancers or early melanoma. With other cases of melanoma, lab tests will include a complete blood count (CBC) and a chemistry profile including a test for LDH (lactate dehydrogenase). LDH, in particular, may give helpful information regarding the prognosis of cancer.

Gene Mutation Testing

Molecular differences among melanomas define them and can provide a direction for treatment. Testing for gene mutations (done on a sample of the tissue removed via biopsy or excision) has been a major advancement, allowing physicians to address these cancers with "targeted therapies," drugs that target specific pathways in the growth of a cancer cell.

A few of the gene mutations that may be present in a melanoma, and that can be detected in a blood sample, include:

  • BRAF
  • NRAS
  • NF-1
  • KIT

It's important to note that these are "acquired" gene mutations (somatic mutations) that develop in the process of a cell becoming a cancer cell, in contrast to mutations that are present from birth (inherited or germ-cell mutations).

Imaging

A sentinel node biopsy used for evaluation of melanomas has an imaging component, but tests dedicated to imaging alone are not usually needed for basal cell cancers or early squamous cell carcinomas. For more advanced squamous cell cancers and melanomas, however, imaging can be very helpful in determining the stage of the disease. Tests may include:

CT Scan

CT scan uses a series of X-rays to create a 3-D picture of the inside of the body. It can be used to look for the spread of cancer to lymph nodes or distant regions of the body.

The most common site of spread is the lungs (lung metastases) and can be detected on a chest CT. An abdominal and/or pelvis CT may be done as well, depending on the location of the tumor. After the lungs, the most common sites of distant metastases are the bones, liver, and brain, but a melanoma may spread to nearly any region of the body.

MRI

Magnetic resonance imaging (MRI) uses magnetic fields to create a picture of the inside of the body. While an MRI may be used to look for metastases in any region, it is particularly helpful in detecting metastases to the brain and spinal cord.

PET Scan

Positron emission tomography (PET scan) is different than many imaging tests in that it looks at the function of the body rather than structure, though it's usually combined with CT.

A small amount of radioactive glucose is injected into a vein and allowed to travel through the body. Actively growing areas of the body (such a cancer cells) take up more of the glucose and can be seen in the images generated.

A PET scan can be helpful as a staging test and to help detect recurrences of previous cancer. Unlike structural tests, a PET scan can discriminate between an area that appears abnormal due to scar tissue and an area that looks abnormal due to active tumor growth.

Differential Diagnoses

There are a number of conditions that can look similar to skin cancer, even to a trained eye. In fact, without a biopsy, it is sometimes impossible to tell the difference between a skin cancer and another condition. Some conditions that can cause signs and similar to skin cancer include:

  • Dysplastic nevi (atypical moles that are more likely to develop into melanomas)
  • Benign melanocytic nevi (moles that can look very much like melanomas but are usually smaller)
  • Actinic keratosis (benign skin lesions that are considered precancerous for squamous cell carcinoma)
  • Metastatic cancer to the skin (for example, breast cancer metastases to the skin)
  • Keratoacanthoma
  • Dermatofibroma
  • Blue nevi
  • Junctional or compound nevi
  • Subungual hematoma (these "black and blue" marks under nails are due to bleeding in the area and can usually be traced back to trauma, like someone stepping on your foot; the dark color does not usually extend into the cuticle)
  • Pyogenic granuloma
  • Cherry hemangioma
  • Keloid scars
  • Vitiligo

Staging

Most of the time, staging is not needed with a basal cell carcinoma or an early squamous cell carcinoma. If the biopsy shows that you have melanoma, however, your doctor needs to know the extent (stage) of the disease to effectively plan treatment.

TNM staging is used to determine the stage of the tumor. Two other measures, the Breslow thickness and Clark level, can give important information about the prognosis. 

The stage of a tumor is determined by four factors:

  • The depth (thickness) of the tumor, using the Breslow scale
  • If the tumor is ulcerated
  • Whether the tumor has spread to nearby lymph nodes (and the degree)
  • Whether the tumor has spread to distant regions of the body

Learning a bit more about this can help you put comments from your doctor in perspective, should he mention these terms.

Melanoma Stages (TNM Staging)

Staging of a melanoma is done using the TNM staging system. "T" stands for tumor, and basically describes the size and depth of the tumor. "N" stands for lymph nodes, and has an associated number that describes whether cancer has spread to any nodes and how many. Sub categories also describe whether the metastases to lymph nodes are macroscopic (able to be detected during an examination) or microscopic (only seen under a microscope). "M" stands for metastasis and is associated with a number only if cancer has spread to distant regions of the body.

How your tumor can be described using the TNM system dictates what stage of melanoma is indicated.

melanoma: stage at diagnosis
Illustration by Verywell

Stage 0: The Cancer involves only the top layer of skin. It is referred to as melanoma in situ or carcinoma in situ. At this stage, the cancer is considered non-invasive and should theoretically be 100 percent curable with surgery.

Stage I: These tumors are broken down into two substages:

  • Stage IA: This staging includes tumors that are less than or equal to 1 millimeter thick and are not ulcerated. (The newest staging guidelines, which are out but still being widely adopted, change this from 1 millimeter to 0.8 millimeters.)
  • Stage IB: These tumors may either be less than or equal to 1 millimeter thick and ulcerated, or between 1 millimeter and 2 millimeters in thickness but not ulcerated.

Stage II: Stage II tumors are broken down into 3 substages, but none of these indicate that the cancer has spread to lymph nodes or other regions of the body:

  • Stage IIA: These tumors are either between 1 millimeter and 2 millimeters thick and ulcerated, or 2 millimeters to 4 millimeters thick and not ulcerated.
  • Stage IIB: This includes tumors that are 2 millimeters to 4 millimeters thick and ulcerated, or more than 4 millimeters in thickness but not ulcerated.
  • Stage IIIC: These tumors are more than 4 millimeters thick and are ulcerated.

Stage III: Stage III tumors may be of any thickness and may or may not be ulcerated, but include one of the following:

  • One or more positive lymph nodes
  • Matted lymph nodes
  • Cancer is found in lymph vessels between the tumor and a lymph node and is 2 cm or further from the primary tumor
  • Small areas of cancer on or in the skin apart from the primary tumor, but not more than 2 cm away from the tumor

Stage IV: The cancer has spread to other regions of the body, such as the lungs, liver, bones, brain, soft tissues, or digestive tract.

Breslow Thickness and Clark Level

While melanomas are now divided into the TNM stages above, and these stages encompass what is known as Breslow thickness and Clark level, you may hear these terms from an oncologist or in your reading if you or a loved one are diagnosed with melanoma.

With melanoma, the single most important finding that determines the prognosis is the depth of the tumor and the number that describes this is the Breslow number. The Breslow number represents the total vertical height of the tumor.

Breslow numbers are divided as follows:

  • Less than 1 millimeter
  • Between 1.01 millimeter and 2 millimeters
  • Between 2.01 millimeters and 4 millimeters
  • Over 4.01 millimeters

Clark levels used to be used more often, but have been found to be less predictive of outcomes than Breslow numbers. These levels may still be helpful, however, in predicting outcomes for thin tumors (less than 1 millimeter thick). Clark levels describe how deeply the tumor has penetrated through the layers of the skin:

  • Level I: These tumors are confined to the topmost layer of skin (the epidermis) and included tumors classified as carcinoma in situ.
  • Level II: The tumor has invaded the upper part of the dermis, the second layer of skin (the papillary dermis).
  • Level III: The tumor is present throughout the papillary dermis, but has not invaded the lower dermis (the reticular dermis).
  • Level IV: The tumor has invaded the reticular dermis.
  • Level V: The tumor has penetrated through the epidermis and dermis and into the deep subcutaneous tissue.
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