One aspect of cancer tumors I understand would be that the cancer sends out a chemical signal to the brain. Telling the brain it is a normal cell and it needs nourishment. A type of stem is linked to the cancer and it receives blood (nourishment). This is what causes the cancer to grow and spread.

Speaking about the UV rays, have you ever gotten a sun burn and afterward you got brown spots or marks on your back or anywhere you got sun burned? That can also cause skin cancer if you are not careful. One way to figure out if it is or can become cancerous, would be to check to see if the brown spot is a uniform color and not where half of the spot is dark brown and the other half if light brown. If it isn't a uniform color have a doctor check it.

If I was a off topic, I'm sorry.

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"Bad Ganon, No Triforce!"- Myself

"You are getting slow. Old man, take the money. Talk to Sam Douglas. Get him out of my life. Or I'll rip out your heart!"- Schinder (actor pending) from 3 Ninjas