Education is everything when it comes to receiving a cancer diagnosis. For instance, understanding the differences between early and late-stage cancers, how pervasively specific cancers spread, and why solid tumor cancers such as breast cancer, lung cancer, skin cancer and colon cancer are more common than blood cancers like leukemia and Hodgkin's lymphoma. 

A whole new vocabulary of words is also introduced following the discovery of a tumor. These include words like biopsy, prognosis, chemotherapy, metastasis and carcinoma. Two of the other earliest and most important definitions that are often heard when cancer is first suspected in one's organs, blood, or tissue are malignant and benign.  

What does malignant mean? 

Though no one wants to learn that a tumor has been discovered in the first place, having a doctor tell you they've detected one is not the same as being told you have cancer. "Identifying that a tumor exists is only the first step towards determining if it's cancerous," explains Dr. Ryan Osborne, a surgical oncologist and the director of the Osborne Head & Neck Institute in Los Angeles.

That's where the word "malignant" is usually first introduced to a patient. “A malignant tumor is a cancerous tumor that can grow uncontrollably and invade other structures," explains Dr. Andrea Cercek, a gastrointestinal oncologist and co-director of the Center for Young Onset Colorectal and Gastrointestinal Cancers at Memorial Sloan Kettering Cancer Center. 

In other words, when a doctor talks about a tumor being malignant, it's the same as them saying cancer is present. Once discovered, "malignant tumors generally require treatment to avoid their spreading - treatment that can include surgery and possibly drug therapy or radiation therapy," says Dr. Julie Gralow, the chief medical officer at the American Society of Clinical Oncology.  

But sometimes, before any treatment becomes necessary, doctors refer to a tumor as being "pre-malignant," which means cancer cells aren't yet present, but the mass appears to have that potential or capability, so it needs to be monitored more closely.

What's the difference between malignant and benign?

In either case, "malignant is another way of saying a tumor is dangerous or harmful," says Gralow. Benign tumors, on the other hand, "are not cancerous and usually do not cause any harm," says Cercek. 

Benign tumors are distinct not only in that they usually don't spread around the body the way malignant tumors do, but are also known for having smooth, regular borders. Conversely, "a malignant tumor has irregular borders," notes Cleveland Clinic.

But just because benign tumors don't spread, doesn't mean they won't grow larger from where they started. In fact, if a benign tumor is left untreated, it's capable of growing significantly - though usually at a much slower rate than malignant tumors - and can even reach the point of weighing hundreds of pounds. 

If they don't grow very large and never end up impacting any vital organs or tissue, however, "benign tumors usually pose far less danger than malignant ones - and often none at all," says Dr. Scott Eggener, a urologic oncologist and the co-director of the UChicago Medicine High-Risk and Advanced Prostate Cancer Clinic. 

How to know if a tumor is malignant or benign

Determining whether a tumor is malignant or benign is where another cancer-related term is often introduced: biopsy. "Malignancy is usually determined through a biopsy, where a sample of the abnormal tissue is removed for examination under a microscope by a pathologist," explains Gralow. She says that other methods such as radiologic imaging like X-rays and CT scans can also potentially identify malignant markers. Blood draws can similarly show findings suspicious for cancer. But "removing tissue and studying it under a microscope is the only way to diagnose malignancy or cancer with 100% certainty," she says.

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The presence of malignant cancer cells is determined this way and defined through agreed-upon criteria as evaluated by a pathologist and shared with a clinician, Eggener adds. He explains that a biopsy also determines the type of malignancy one has and "how aggressively the cancer is likely to invade other organs and spread to other parts of the body."