Both are measurements of the vertical height of the tumour in the skin.
Tumour thickness is directly related to prognosis because thicker tumours have greater access to lymph capillaries which is the usual method of spread.
Breslow thickness is measured in mm whereas the Clarke’s level describes depth relative to other skin structures and is a less commonly used measure nowadays.
In the diagrams there are 2 melanomas (depicted M).
The right hand melanoma has a greater Breslow depth than that on the left.
Note that it lies in much closer proximity to the deep structures of the skin such as the lymphatic vessels. Here, invasion is much more likely, allowing the melanoma to spread (metastasize) to other sites through lymph flow.
This explains why Breslow depth is of great interest to clinicians and can help predict prognosis in those newly diagnosed with melanoma, once a biopsy of the tumour has been taken.