What is the difference between benign and malignant tumors?

Whether it can spread. Malignant tumors invade nearby tissue and can travel to distant parts of the body to form new tumors. Benign tumors do not.

A tumor is a lump of tissue that forms when abnormal or damaged cells grow and multiply when they should not. Malignant tumors are cancerous. Not all cancers form solid tumors, though: blood cancers such as the leukemias generally do not.

What "spread" actually means

The travel process is called metastasis. Cancer cells break away from where they first formed and start new tumors elsewhere, which is why cancer becomes so much harder to treat once it moves.

Behind that behavior is a set of specific abilities normal cells do not have. NCI lists them, and they are worth reading as a checklist of what makes a cell cancerous. Cancer cells grow without being told to grow. They ignore signals to stop dividing or to die, a process called apoptosis. They invade nearby areas, while normal cells stop growing when they meet other cells. They signal blood vessels to grow toward the tumor, supplying oxygen and nutrients. They hide from the immune system, and some even talk immune cells into protecting the tumor instead of attacking it.

Benign tumors do none of this. When removed, they usually do not grow back, whereas cancerous tumors sometimes do.

Benign does not mean harmless

This is the part people get wrong.

Benign tumors can be quite large. Some cause serious symptoms or are life threatening, and NCI gives benign brain tumors as the example. A growth that cannot spread can still press on something that matters.

So the question after "is it benign" is not "am I finished." It is "does it need removing, and what happens if it grows."

The states in between

Not every abnormal tissue change is one or the other. Three sit on a path between normal and cancer, and knowing the words helps when you read a report.

Hyperplasia means cells in a tissue are multiplying faster than normal and extra cells have built up, but the cells and the tissue still look normal under a microscope. Chronic irritation is one cause.

Dysplasia is further along. There are extra cells, the cells look abnormal, and the tissue's organization has changed. In general, the more abnormal it looks, the greater the chance cancer will form. Some dysplasia needs monitoring or treatment and some does not. An abnormal mole, called a dysplastic nevus, is an example; it can turn into melanoma, though most do not.

Carcinoma in situ is further still, and it is sometimes called stage 0 cancer. NCI is precise here: it is not cancer, because the abnormal cells do not invade nearby tissue. But because some do become cancer, it is usually treated.

How the answer gets made

Only a pathologist looking at tissue can say which one you have. That is why a biopsy usually comes before a diagnosis, and why imaging alone cannot settle it.

If you have a report in front of you, three questions get you most of the way. Is this benign, precancerous, or malignant? If it is precancerous, is the plan to monitor or to remove? And if it is being monitored, what would trigger a change of plan? Our overview of what cancer is covers how cancer develops, and the piece on how cancer is diagnosed explains how tissue is examined.

Sources

• https://www.cancer.gov/about-cancer/understanding/what-is-cancer
• https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis