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In Stage 0 melanoma, the malignant tumor is still confined to the upper layers of the skin (epidermis). This means that the cancer cells are only in the outer layer of the skin and have not grown any deeper. The term for this is in situ, which means “in place” in Latin.
According to AIM at Melanoma Foundation - Stage 0 is defined as TisN0M0:
Tis (Tumor in situ): cancer cells are found only in the outer layer of skin (epidermis) and have not grown into any other layers. The cancer cells have not shown signs of spreading.
NO: means no evidence cancer has spread to the lymph nodes.
MO: means melanoma has not spread to distant sites (metastasized).