Surgical margins

When a surgeon removes an entire tumor, they usually don't cut right up against it. They try to take a little healthy tissue around it as well, and that rim is called the surgical margin. The question the margin answers is simple: do the tumor cells reach all the way to the edge of what was taken out, or not?

That's where the pathologist comes in, the doctor who studies cells and tissues under a microscope. The removed tissue is cut into very thin slices, called sections, and the pathologist carefully looks at what's at the edges. Think about cutting a bruise out of an apple: you don't slice right along the brown spot, you take some of the white flesh around it, and then you look at the edge of the piece you cut out to see whether the brown reaches that far.

If no cancer cells show up at the edge, the margin is called negative, or "clean," and that suggests all of the cancer was removed. If they do show up, it's called positive, or "involved," and that suggests some of the cancer was not removed. Notice the word: suggests. What gets examined is the edge of the piece that came out, not what's still inside the body.