My father had a tumor removed from behind his knee. The doctor took out out and said it wasn't cancer. How can he know for sure?

When a tumor is taken out of someone's body, it is sent to a lab for analysis. There, the doctors check for cancer cells, which are seen under a microscope. If there are no cancer cells, then there is no cancer in the tumor. If the tumor was large, they will likely check sections from different places just to make sure it is the same throughout.

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