In Cancer 101, I gave some basics to understanding cancer. A commenter asked a good question, and our next lesson will attempt a simple answer.
The question regarded how a pathologist can tell if a cancer is “invasive” by looking at a specimen. Well, depending on the specimen, the answer changes, but let’s use the colon as an example. Most colon cancers start out as benign polyps. Eventually the cells in the polyp can become malignant, and after that, they can they can begin to grow through layers of normal cells.
Here is a diagram of a cancer of the colon at various stages. As you can see, at a certain point, the cancer begins to grow through each layer. The ability to grow through, or invade, other layers is one of the things that makes cancers behave in a nasty manner.
Here is a piece of colon cancer under the microscope. This could have been from a biopsy of a polyp, or from a tumor completely removed by a surgeon. The stripe of light purple cells labeled “normal muscle layer” should extend across uninterrupted. Instead, a glob of darker purple cells is growing through the muscle layer, destroying it. Once the cancer cells get to a blood vessel, they can go anywhere in the body (metastasize). That’s bad.
I hope that’s helpful.