When I watch TV I often hear the doctors (actors) assure patients and their family that they got “all of the cancer” during removal surgeries.
In my mind… I always thought that cancer was a lump of cells… multiplying uncontrolled… and at some point along its margins, it borders normal tissue.
How can you possibly cut perfectly between normal tissue and cancerous tissue?
They examine the cut edges of the tissue removed and check whether there are any cancer cells on that cut surface. If there are, they know they left some behind. If there aren’t, they can be reasonably sure that they got it all (assuming surgery was possible for all known cancer sites). Monitoring will remain fairly intensive following surgery to try to catch any recurrence early.
Adjuvant treatment, usually radiotherapy or chemotherapy, will often be used after surgery to try to mop up any remaining cancer cells which cannot be macroscopically detected, especially in cancers which are prone to spread beyond the original site of the cancer.