Benign vs. malignant tumors: Their differences, explained

A tumor, or neoplasm, is a mass of abnormal cells in the body. Tumors are usually described as either benign or malignant. Benign tumors are not cancerous, and malignant tumors are cancerous.

It’s important to understand the differences between benign and malignant tumors. Their characteristics and how they behave help guide treatment.

Here, I’ll explain common types of benign and malignant tumors we see at UT MD Anderson, how they differ and explore the gray area between noncancerous and cancerous tumors.  

Differences between benign and malignant tumors

Benign and malignant tumors differ in many ways.

How their cells look

Benign tumors have cells that are well-contained and look mature and well-developed. They look similar to normal cells. They have well-defined borders or edges.

The cells in malignant tumors grow erratically and often do not look well-developed. They usually look abnormal, or very different compared to cells in nearby tissue. However, sometimes malignant cells can appear more well-developed. Malignant cells often have irregular borders.

When you get a biopsy, doctors can clearly see how a tumor’s cells look under a microscope and learn more diagnostic information after completing specific stains or tests on the cells. This makes it easier to identify benign and malignant tumors.

How they feel

Benign tumors are often mobile. So, you can push and move them around with your fingers.

Malignant tumors are often firm and fixed, so they can’t easily be moved.

How fast they grow

Benign tumors typically grow very slowly.

Most malignant tumors grow quickly, but some can be relatively slow-growing.

Ability to spread

Benign tumors do not invade nearby tissue or spread to other parts of your body.

Malignant tumors can potentially invade nearby tissue. They can also spread to other parts of your body. This process is known as metastasis. The ability to metastasize is considered malignant tumors’ most dangerous characteristic, as it allows cancer to reach vital organs.

It’s important to know that there are key differences between benign and malignant tumors, but some of these generalized features can vary with different tumor types.

What are the most common benign and malignant tumors?

Common benign tumors include:

• Lipomas, which form in the body’s fatty tissue
• Adenomas, which form in glandular tissue like the lining of the colon and adrenal glands
• Moles on the skin
• Fibroids, which are made of connective tissue and often form in the uterus
• Hemangiomas, which include skin growths made up of small blood vessels

The most common parts of the body where malignant tumors develop are the:

• Breast
• Prostate
• Lung
• Colon or rectum
• Skin

What are cysts?

Cysts are often mistaken for tumors, but they are not the same.

Whereas a tumor is a collection of abnormal cells, a cyst is a closed pocket of cells typically filled with fluid or other material like air. Cysts are very common and can appear on the skin or inside the body. Skin cysts can feel squishy. A cyst is different from an abscess, which is a collection filled with pus.

Most cysts are benign, but some may be precancerous. Doctors can use imaging tests to help tell if something is a cyst or a tumor. For example, mammograms and ultrasounds can show whether a growth is a tumor or a breast cyst by seeing if it’s filled with cells or fluid.

Can benign tumors ever become malignant?

People often think of tumors as either benign or malignant. But sometimes it’s a spectrum. And there’s a third type that’s somewhat in the middle.

Some benign tumors can also be precancerous. This means that even though they are not cancerous now, the cells have features that could turn into cancer later.

Colon polyps are benign growths found in the lining of the colon, but some can be precancerous. Over time, some colon polyps can turn into cancer. This is why regular colorectal cancer screening is important. During a colonoscopy, your doctor can locate and remove precancerous polyps before they have the chance to become cancerous.

Actinic keratoses are very common precancerous skin lesions caused by long-term sun exposure. If left untreated, over time, they can turn into skin cancer. Regular skin cancer screening can help detect actinic keratoses, so they can be treated before they turn into cancer.

What is dysplasia?

The term dysplasia is used to describe disordered cells, or cells that look abnormal under a microscope. These cells can often be precancerous. They are often found in parts of your body where there is a lining. One example is the lining of the cervix. Cervical dysplasia can later turn into cervical cancer. Another example is the lining of the anus. Anal dysplasia can later turn into anal cancer.

If you have dysplasia, your doctor will determine the grade. The higher the grade, the more likely that you will need treatment, so that it doesn’t turn into cancer later. Mild or low-grade dysplasia can often just be closely monitored for changes.

How do doctors know if a tumor is benign or malignant?

Your doctor may use any of the following to help diagnose a tumor.

• Physical exam
• Imaging tests
• Blood tests
• Biopsy with or without aspiration

A biopsy is usually the only way to truly confirm that a tumor is cancerous. During a biopsy, a doctor will remove a sample of the tumor’s tissue. A pathologist then examines the tissue sample under a microscope and makes an official diagnosis. They’ll send the report to your doctor to discuss it with you. 

Symptoms of benign and malignant tumors 

Benign tumors often don’t cause any symptoms. But if they grow large enough, they may press on nearby organs or structures and cause problems. Symptoms depend on the size of the tumor and its location. For example, an acoustic neuroma – also called a vestibular schwannoma – is a benign tumor that grows on the vestibular nerve. If it grows large enough, you may experience symptoms, such as hearing loss in one ear, tinnitus (ringing) in one ear, facial weakness or balance issues.

Sometimes, early-stage malignant tumors do not cause symptoms. In general, common symptoms of malignant tumors include:

• A new lump that may or may not be painful
• Fatigue
• Unintentional weight loss or muscle wasting
• Poor appetite

Certain types of cancer can cause specific symptoms.

• Some lymphomas can cause drenching night sweats
• Colorectal cancer can cause blood in your stool or changes in bowel habits
• Endometrial cancer can cause vaginal bleeding in postmenopausal women
• Cervical cancer can cause vaginal bleeding after sexual intercourse

See a doctor if you have any new or concerning symptoms. We all get occasional colds or allergies. But if a symptom lasts more than two weeks, get checked out. Your doctor can examine you, determine the cause of the symptoms and start you on treatment, if necessary.

Do benign tumors need to be removed?

In general, most benign tumors do not need to be removed. Your doctor may closely monitor a benign tumor to see if it grows, changes or causes any symptoms.

Benign tumors that are causing pain or other bothersome symptoms should be removed. And precancerous tumors like some colon polyps should be removed to keep them from becoming cancerous.

Regular cancer screening is important

Regular cancer screenings can help doctors find cancer early when it’s easier to treat. Screenings can also help prevent cancer by catching precancerous tumors before they have the chance to become malignant. This is what happens, for instance, when precancerous colon polyps are removed during a screening colonoscopy.

Cancer screening saves lives. Talk to your doctor, so they can develop a personalized screening plan for you.

Matthew Wright, M.D., holds a joint appointment as a medical oncologist in General Oncology and Breast Medical Oncology at UT MD Anderson. He is a breast medical oncologist at UT MD Anderson The Woodlands.

Request an appointment at UT MD Anderson online or call 1-877-632-6789.