The Breslow scale (also sometimes referred to as the “primary tumor thickness scale”) measures the thickness of the tumor in terms of millimeters. Your doctor or pathology report may say something like “the tumor has a Breslow thickness of 0.5mm. This simply means that the depth of the tumor, as measured from highest to deepest part, is 0.5mm. Breslow thicknesses are often grouped into categories. This is because studies have shown that the thickness of the tumor is highly predictive of the risk of whether it has spread or will recur once removed. Generally, the categories used are the following:1,2
- Thickness of <1mm: At this level, it is very unlikely that the melanoma has grown deep enough for any of the cancer cells to break away from the primary tumor and spread. Hence, there is a low risk of the melanoma coming back once it has been removed. Prognosis (5 year survival rate): 95-100%
- Thickness of 1-2mm: At this level, there is some risk that some melanoma cells have broken off an begun to spread. Your doctor will likely take a wider excision area when removing the melanoma tumor, and may perform other tests, such as a lymph node biopsy to assess whether it has spread to your lymph nodes. Prognosis (5 year survival rate): 80-96%
- Thickness of 2-4mm: At this level, the risk that the melanoma has begun to spread from the primary tumor is higher, and your doctor will likely perform some additional tests (indicates a high risk of the melanoma coming back). Prognosis (5 year survival rate): 60-75%
- Thickness of >4mm: At this level, the risk that the melanoma has begun to spread from the primary tumor is highest, and your doctor will likely perform some additional tests. Prognosis (5 year survival rate): 50%
In addition, if the melanoma shows signs of ulceration (if the tumor is cracked or bleeding), which will be confirmed by the pathology report, then the 5 and 15 year survival rates go down more. The biggest drop, is in the >4mm thickness category, where survival rates are roughly 15-20% lower(1).
It is important to note, however, that while the thickness of the melanoma is an indication of the risk that is may have spread, it is not a diagnosis that is has spread. Some melanomas thicker than 4mm have not spread, or have not spread far (and hence are very curable). This is why further diagnostic testing is usually warranted by your doctor.
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