After surgery, how do you know that all of the cancer has been removed?

When a cancer is removed during surgery, it is sent to the pathologist, who will study it extensively to determine the type of cancer, whether it has spread to lymph nodes, and various characteristics of the cancer that may help the medical oncologist and the patient choose the best method of fighting it and preventing a recurrence.  The pathologist will also examine all of the edges of the tissue that has been removed, the “margins”, under a microscope, to make sure it doesn’t look like there is cancer at the edge of the specimen, which could indicate that some tumor was left behind.