Thanks for the question.
Melanoma in situ is an early form of melanoma where the cancerous cells are confined to the epidermis and have not breached the basement membrane. The basement membrane lies between the epidermis and the dermis. As the melanoma in situ has not spread to the deeper dermal layers, there is no risk of metastasis or systemic spread of the cancer cells and wide excision of melanoma in situ has an excellent prognoisis and is often curative.
Superficial speading melanoma is a form of melanoma where the cancer cells tends to grow and spread along the skin surface. However, although it favours a radial or superficial spread, it can invade the deeper layers and breach the basement membrane into the dermis. When the basement membrane is breached, it carries a risk of metastasis and systemic spread. This risk of spread does increase with the depth of the invasion. In general, superficial speading melanomas do have a better prognosis than nodular melanomas as the depth of invasion is usually less than that of a nodular melanoma at the time of diagnosis.
It is therefore always important to get a mole that is changing (growing thicker, larger, or changing colour) checked as early diagnosis has a much better prognosis.
