Tumor: Definition

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A tumor is an abnormal growth of cells. Normal cells grow and divide in a controlled way. Tumor cells multiply without that control, forming a lump or mass. A tumor can be benign (not cancer) or malignant (cancer).

This article explains what a tumor is, how pathologists tell benign tumors from malignant ones, and what your report may say.

What is a tumor?

A tumor is a growth made of cells that keep multiplying when they should stop. Tumors can start almost anywhere in the body. They differ in size, appearance, and behavior. Doctors also use the word neoplasm, which means “new growth.” The two words mean the same thing. Reports tend to use neoplasm, while doctors often say tumor when they talk with patients.

A tumor is not the same as a mass. A mass is any lump that a doctor can feel or see on a scan. A tumor can cause a mass. A cyst, an infection, or inflammation can also cause a mass.

Does tumor mean cancer?

A tumor does not always mean cancer. Tumors fall into two main groups:

Benign tumors. A benign tumor is not cancer. They usually grow slowly and stay in one place. It does not grow into nearby tissue or spread to other parts of the body. A benign tumor can still cause pain or press on nearby organs. Surgery can often remove it.
Malignant tumors. A malignant tumor is cancer. It can grow into nearby tissue. It can also spread through the blood or lymph vessels to other parts of the body. Doctors call this spread metastasis.

Some tumors fall between these two groups.

What are the main types of tumors?

Doctors name tumors by the cell type they come from and whether they are benign or malignant. Common examples include:

Benign tumors. Examples include lipomas (fatty growths) and uterine leiomyomas, also called fibroids. Moles and adenomas are other examples.
Carcinomas. Carcinomas start in the cells that line the skin and organs. Most breast, lung, prostate, and colon cancers are carcinomas.
Sarcomas. Sarcomas start in connective tissue, such as bone, fat, or muscle.
Lymphomas and leukemias. These start in immune cells or blood-forming cells. See lymphoma and leukemia.

What causes a tumor?

Tumors start when changes in a cell’s genes disrupt normal growth. Doctors call these changes mutations. The cause differs by tumor type and location. Sun exposure contributes to many skin tumors. Human papillomavirus (HPV) causes most cervical tumors. Tobacco smoke causes many lung tumors. Chronic inflammation can also contribute. For many tumors, doctors cannot find a single cause.

How do pathologists tell if a tumor is benign or malignant?

The pathologist examines tissue from a biopsy or surgery under the microscope. A biopsy is the most reliable way to tell a benign tumor from a malignant one. The pathologist looks for these features:

Abnormal cells. Cancer cells often vary in size and shape. Pathologists call this atypia or describe the cells as pleomorphic.
Growth into nearby tissue. Malignant tumors grow into the surrounding tissue. Pathologists call this invasion. Growth around nerves is perineural invasion. Cancer cells inside blood or lymph vessels are lymphovascular invasion.
Dividing cells. Malignant tumors often have many dividing cells. Pathologists call them mitotic figures.
Spread. Tumor cells in a distant site, such as a lymph node or another organ, show that the tumor is malignant.

For a malignant tumor, the pathologist also assigns a grade. The grade describes how abnormal the cells look.

What does it mean if a tumor is borderline or of uncertain malignant potential?

Some tumors have features between benign and malignant. Pathologists may call them borderline tumors or tumors of uncertain malignant potential. These tumors usually behave like benign tumors. A small number come back after removal or spread. They occur most often in the ovary, thyroid, uterus, urinary tract, and soft tissue.

Surgery alone cures most of these tumors. Your doctor may still recommend regular checkups or imaging to watch for changes.

What might my report say about a tumor?

Your report may use several terms to describe a tumor. Common ones include:

Benign or malignant. The report states whether the tumor is cancer.
Neoplasm. The report may use this word instead of tumor. It has the same meaning.
Tumor size. The report gives the tumor size, usually in centimeters.
Tumor capsule. A thin layer of tissue that separates a tumor from the normal tissue around it. Both benign and malignant tumors can have one. See tumor capsule.
Tumor necrosis. Areas of dead cells inside a tumor. See tumor necrosis.
Tumor bed. The area where a tumor was before surgery or treatment. See tumor bed.
Tumor stage. For a cancer, the report may give a tumor stage, which is part of the TNM system.

Questions to ask your doctor

Is my tumor benign or malignant?
What type of tumor is it, and where did it start?
Which tests did the pathologist use to decide?
If the tumor is cancer, what is its grade and stage?
Was the whole tumor removed?
Do I need more tests or more treatment?
If the tumor is borderline or uncertain, what follow-up do I need?
What symptoms should I watch for?