What is a Benign Tumor? What is a Malignant Tumor?

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A tumor is an abnormal mass of tissue that has formed a lump. It’s called benign if it grows slowly and is self-limiting; that is, if it doesn’t have the capacity to invade nearby tissues and spread beyond its original site.

A malignant, or cancerous, tumor, on the other hand, is innately dangerous because its cells can divide uncontrollably and produce virtually immortal daughter cells. Malignant tumor cells can penetrate and destroy adjacent tissue, and can metastasize, or travel through the circulation to distant parts of the body and form new tumors.

By examining biopsy samples of a tumor using a microscope and molecular tests, pathologists can classify the tumor as benign or malignant, says Keith Ligon, MD, PhD, of the Center for Molecular Oncologic Pathology at Dana-Farber/Brigham and Women’s Cancer Center. They further classify the tumor cells according to “grade,” or degree of aggressiveness. “The overall goal is to try and predict how the tumor will behave and what treatment the patient will need,” explains Ligon.

Often, benign tumors need no treatment, but they can become dangerous if they grow large enough to press on vital organs, blood vessels or nerves. In such cases they are generally removed through surgery, which also allows pathologists to confirm that they are not malignant.

Some lesions — sores or plaques of abnormal cells — and benign tumors have the potential to become malignant. These are known as “precancerous” or “dysplastic” tumors, and contain large numbers of primitive, abnormally shaped cells. Examples are precancerous polyps in the colon, growths in the cervix caused by the human papilloma virus (HPV), and dysplastic growths in the mouth and lungs. These may progress to cancer when some of the tumor cells undergo genetic changes such as mutations that give them malignant properties.

Meningiomas are a type of brain tumor that are generally benign but can become malignant. Dana-Farber researchers recently found mutations in meningiomas that are potential targets for drug therapy.

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