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?
As you suspected, you can’t. That’s why a lot of cancer removal surgeries will remove a little potentially healthy tissue along the borders (if it’s not important), and err on the safe side.
There’s also some situations where the cancer may be benign, and you remove the entire body part it was attached to (like removing the relevant testicle in testicular cancer), which can give a greater likelihood of getting the whole tumour, but it’s not a certainty, and you would still need monitoring/treatment, just to check that there’s no relapse, or surprise cancer cells popping up elsewhere.
TV probably simplifies it for plot/drama reasons. Unless it’s specifically relevant to the plot, it adds unnecessary complications for the character to spend the next several years going back to the doctor for monitoring, or leaving the audience wondering whether the cancer had been removed or not.