The Clark scale and Breslow thickness describe how deeply the melanoma has gone into the skin.
This is different to the number staging system that doctors also use. The Clark scale and Breslow thickness only look at the depth of melanoma cells in the skin. The number stages look at the melanoma depth, and also whether the melanoma has spread to lymph nodes or another part of the body.
Breslow thickness is the main measurement your doctor uses to give information about your outlook. They use the Breslow thickness in another staging system for melanoma called the TNM staging system.
Staging a cancer helps doctors decide on the treatment you need.
The Clark scale is a way of measuring how deeply the melanoma has grown into the skin and which levels of the skin are affected. You can see the main layers of the skin in this diagram.
The Clark scale has 5 levels:
Level 1 is also called melanoma in situ – the melanoma cells are only in the outer layer of the skin (the epidermis)
Level 2 means there are melanoma cells in the layer directly under the epidermis this is known as the papillary dermis (superficial dermis)
Level 3 means the melanoma cells are touching the next layer down known as the reticular dermis (deep dermis)
Level 4 means the melanoma has spread into the reticular dermis
Level 5 means the melanoma has grown into the layer of fat under the skin (subcutaneous fat)
Breslow thickness is the measurement of the depth of the melanoma from the surface of your skin down through to the deepest point of the tumour.
It’s measured in millimetres (mm) with a small ruler, called a micrometer. A specialist doctor called a pathologist uses the special ruler with a microscope when looking at your tissue sample in the laboratory.
Doctors use the Breslow depth in another staging system for melanoma called the TNM staging system.