The skin is the largest organ in the body and is comprised of three main layers; the epidermis, dermis and fat. The dermis and adjacent fatty tissue layers are not visible to the naked eye. Skin is rich in cell types that have the potential to grow cancer if exposed to repeated ultraviolet trauma, such as excessive sun exposure.
Melanocytes clustered in groups form moles.
The level of melanoma relates to how deep the cancer has penetrated the skin layers. In situ melanoma and Level 1 melanoma are found within the epidermis. Levels II, III and IV melanoma occur within the dermis. Level V melanoma occurs in the deepest layer of the skin, the hypodermis.
One of the most common areas of confusion is the difference between the levels of melanoma and the staging of melanoma. The level of melanoma relates to the depth of the melanoma in the skin and the staging of melanoma refers to how limited or advanced the melanoma is at the time of diagnosis.
The stages of melanoma are determined by reviewing different factors including:
Tumour depth or thickness is measured in millimetres by a pathologist using a microscope. This is referred to as the ‘Breslow Depth’. The thicker the tumour, the greater the chance it might have metastasised (spread) to regional lymph nodes or distant sites. Thinner melanomas have a better prognosis.
The tumour depth was traditionally described using a ‘Clark level’ to indicate the number of layers of skin penetrated by the melanoma. The Clark level is a number from I-V with V being the deepest penetration of the skin. The usage of Clark levels has been replaced by the updated melanoma staging system which uses more reliably predictive features such as ulceration, mitotic count and Breslow depth. Clark levels are discussed further in the Glossary
Ulceration of a skin tumour means that the epidermis (or top layer of the skin) that covers the melanoma is not intact. Ulceration may not be seen with the naked eye. Ulcerated melanomas pose a greater risk for metastatic progression.
Number of metastatic lymph nodes involved
If the melanoma has spread to the lymph nodes the risk of spread to other parts of the body is higher. The greater the number of lymph nodes containing melanoma, the less favourable the prognosis.
A sentinel node biopsy is a technique used to determine whether melanoma cells have spread to lymph nodes at the time of diagnosis of the skin primary lesion. The procedure involves the injection of a radioactive tracer by a radiologist (in the radiology department), to show where the site and lymph node where the lymph fluid from the skin at the primary melanoma will flow. Afterwards, at the same time as the extra surgery for the primary melanoma, a blue dye is injected around the site of the primary lesion. Using the guide from the radiologist a surgeon looks for the first lymph node to take up the dye. The lymph node is removed and sent to be examined by a histopathologist to determine if the node tests positive for melanoma. The procedure is considered when the Breslow thickness of the melanoma is more than 0.8mm.
Patients may develop lumps in the lymph node regions such as the neck, armpit and groin. This is lymph node metastasis.
Melanoma that has spread in the blood steam may grow in any site in the body.