By Lynne Eldridge, MD 

 Medically reviewed by Doru Paul, MD

The term malignant can be used in different ways, either to describe a cancerous tumor or a very serious medical condition. There are several similarities as well as differences between malignant and benign tumors. Learn about the characteristics and behavior of malignant tumors, the important ways in which they differ from benign tumors, and why it can sometimes be difficult to tell the difference between the two.

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Medical Definition of Malignant

In medicine, the word malignant is a term referring to a condition that is dangerous to health. While it is often used interchangeably with cancer, the term is also used to describe medical and psychological conditions other than cancer that are dangerous or ominous.

Synonyms of malignant in medicine include cancer, virulent, or malevolent. In contrast, antonyms (opposites) of malignant in medicine refer to processes that are not dangerous to health or well-being and include terms such as benign, noncancerous, or harmless. That said, some malignant conditions are very treatable, whereas some benign conditions may be life-threatening.

Malignant Tumors

A malignant tumor (cancerous tumor) is one that is invasive and can spread to other parts of the body.¹ In contrast, tumors that stay localized and don't spread are called benign. Benign tumors may grow quite large and can do damage, but they do not usually spread through the bloodstream or lymph vessels to other parts of the body.²

Malignant Conditions

Not all malignant conditions are cancerous. For example, the phrase malignant hypertension is used to describe blood pressure that is dangerously high, but in this context, it has nothing to do with cancer. Likewise, the condition malignant hyperthermia describes an emergency situation in which a dangerously high fever develops during surgery with general anesthesia. Malignant otitis externa is a complication of swimmer's ear that was very serious (and sometimes still is) before the advent of antibiotics.

Malignant Mental Health Conditions

The term malignant may also be used when describing mental health conditions, such as a side effect of psychiatric drugs referred to as neuroleptic malignant syndrome or the description of a particularly damaging (to others) form of narcissism called malignant narcissism.

Characteristics of Malignant Tumors

In describing the characteristics of malignant or cancerous tumors, it's easiest to do so by discussing both the similarities and differences (sometimes surprising) between these tumors and benign or noncancerous tumors.

Similarities to Benign Tumors

Some ways in which benign and malignant tumors are similar include:

• Size: Both benign and malignant tumors can grow to be quite large. For example, uterine fibroids, a benign tumor, can grow to become as large as a basketball³ .

• Ability to Cause Damage: When benign tumors occur in the closed space of the brain, or in sensitive regions such as near nerves, the eye, or the heart, they can be very damaging, even though they do not spread to other regions of the body. Benign tumors can also be disfiguring depending upon their location.⁴

• Local Recurrence: Both benign and malignant tumors may come back after they are treated. The difference is that malignant tumors may come back in different regions of the body to which they have spread, whereas benign tumors will only recur in the location where they were initially found.

How Malignant Tumors Differ from Benign Tumors

Some of the ways that malignant (cancerous) tumors differ from benign tumors include:

• Invasion of Nearby Tissues: Malignant tumors have poor boundaries. Unlike benign tumors which can press on nearby structures, malignant tumors can penetrate into nearby structures.¹ The term "cancer" comes from the word crab or claw, which refers to these finger-like projections invading tissues near the tumor.

• Ability to Spread (metastasize): Unlike benign tumors, malignant tumor cells have the ability to break away from the tumor and travel (metastasize), either locally, or through the bloodstream or lymphatic system. Many believe that most deaths from cancer (roughly 90%) occur due to this ability of malignant tumors to spread, although the statistic has been questioned.⁵ (Learn more about how cancer spreads.)

• Likelihood and Location of Recurrence: Benign tumors may recur after removal, in the region where they were first located. In contrast, malignant tumors recur more often, and may recur locally (as with benign tumors), regionally (for example, in lymph nodes near the original tumor), or distantly (in organs or regions far from the original tumor)⁶ .

• Cells: There are many important differences between benign and malignant cells. Cancer cells can be distinguished from normal cells by microscopic examination. They are usually less well differentiated than normal cells or benign tumor cells. In a specific tissue, malignant cells usually exhibit the characteristics of rapidly growing cells, that is, a high nucleus-to-cytoplasm ratio, prominent nucleoli, many mitoses, and relatively little specialized structure. The presence of invading cells in an otherwise normal tissue section is the most diagnostic indication of a malignancy.

Difficulty Distinguishing Between Benign and Malignant Tumors

If your healthcare provider isn't certain whether a tumor is benign or malignant you may be surprised. Isn't that obvious? On scans such as a CT scan, MRI, or even PET scan, benign and malignant tumors can sometimes look very similar. Yet it can sometimes be difficult to tell the difference even under the microscope. While there are many differences between cancer cells, precancerous cells, and normal cells there is also a lot of overlap. In addition, in many tumors, there is a mixture of normal, precancerous, and cancer cells. Even among cancer cells, there can be differences in the appearance of these cells in different parts of a tumor (something referred to as "heterogenicity").

Pronunciation: mu-leeg-nant

Examples: Rodney was saddened to learn his tumor was malignant and he would have to go through treatment for cancer.

6 Sources

National Cancer Institute. What Is Cancer?

Institute for Quality and Efficiency in Health Care. How do cancer cells grow and spread?

Harvard Health Publishing. Harvard Medical School. Fibroids.

National Institute of Neurological Disorders and Stroke. Tuberous Sclerosis Fact Sheet.

Dillekås H, Rogers MS, Straume O. Are 90% of deaths from cancer caused by metastases?. Cancer Med. 2019;8(12):5574–5576. doi:10.1002/cam4.2474

Gabriel E, Skitzki J. The Role of Regional Therapies for in-Transit Melanoma in the Era of Improved Systemic Options. Cancers (Basel). 2015;7(3):1154–1177. Published 2015 Jul 1. doi:10.3390/cancers7030830

Additional Reading

U.S. National Library of Medicine. MedlinePlus. Malignancy.

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."