Tumor invasion and metastasis: are they the same thing?

Here are a couple great questions from one of my lovely students regarding invasiveness and metastasis.

Q. I have a quick question on today’s lecture. There is a slide near the end that has a picture of non-invasive carcinoma. For a tumor to be malignant, should it not be invasive?

A. Great question! I think you may be referring to the image above, which shows a gland with either severe dysplasia or carcinoma in situ.

Cancers are usually invasive, as opposed to benign tumors, which grow with pushing borders and are typically encapsulated.

However, very early cancers are called “carcinoma in situ”, which means they have not broken through the basement membrane yet (and thus are non-invasive). Every cancer has to start somewhere!

The only really definitive quality of malignancy is metastasis. If a tumor has metastasized, that is definite evidence of malignancy.

Q. But is invasiveness different from metastasis? That is, can a cancer metastasize without first invading tissue? Or are we talking about a tumor that has the ability to metastasize, but has not yet metastasized?

A. I’ll answer your questions separately.

1. Yes – invasiveness is different than metastasis.

  • Invasiveness is the ability of a tumor to extend into the surrounding tissue, and it is almost always a sign of malignancy. Benign tumors (with very few exceptions), are encapsulated and grow simply by expanding and pushing the surrounding tissue aside. Malignant tumors (with very few exceptions), are unencapsulated and grow by reaching into the surrounding tissue.
  • Metastasis is the ability of the tumor to move to a different location in the body and set up shop (start growing) there. Benign tumors NEVER metastasize. Malignant tumors usually do, although if detected early, they may be removed before they have the chance.

2. No: a cancer cannot metastasize without first invading tissue. In order to metastasize, tumor cells must first invade tissue, then make their way into vessels (either blood vessels or lymphatics), and then make their way out of those vessels and into new tissue.

3. Yes, the image above shows a non-invasive malignancy (carcinoma in situ), which is a malignant tumor that has not yet metastasized (or even invaded) yet. Left to its own devices, carcinoma in situ almost always becomes invasive carcinoma. As the tumor grows, some cells will most certainly develop the ability to become metastatic. So it’s way better to detect a carcinoma when it is in the carcinoma in situ stage rather than the invasive stage.