What is a Tumor, Exactly?

  • A tumor is any abnormal swelling within the body, but the word most often refers to a mass of cancer cells.
  • Most lumps that can be felt are not cancerous, but they should be checked out by a doctor.
  • Methods for detecting tumors include lab tests, imaging scans, and visualization with clinical instruments.

Medically reviewed by Robert I. Haddad, MD

Strictly speaking, a tumor is an abnormal swelling or enlargement within the body, but the term most commonly refers to a growth of cancerous cells.

In the broadest sense, then, a harmless cyst or collection of fluid would be considered a tumor. But when a physician uses the word in speaking with a patient, he or she is likely to mean a mass of cells that are growing and dividing uncontrollably.

What is the difference between a tumor and cancer?

Although the words “tumor” and “cancer” are often used interchangeably, they aren’t necessarily synonymous.

A noncancerous tumor, also known as a benign tumor, does not metastasize, or spread, to distant parts of the body. Most such tumors are not life-threatening, but because they can press on and interfere with surrounding organs and tissues, they generally need to be treated or removed.

Cancerous, or malignant, tumors, by contrast, can invade nearby tissue and release cells to other parts of the body to form “secondary” tumors. Their potential to metastasize makes such tumors especially dangerous and necessitates treatment.

Some tumors are classified as “pre-malignant” — that is, they aren’t metastatic when diagnosed, but they have the potential to become metastatic in the future.

What does a tumor feel like?

Only tumors that are in the skin, in tissue just beneath the skin, or on the surface of organs palpable through the skin can be felt. The way a tumor feels depends on its size, location, type, stage, and other factors.

A cancerous lump in the breast, for example, tends to feel firm or solid and might be fixed to underlying tissue. Such lumps are often painless but do produce pain in a small percentage of patients. The vast majority of breast lumps prove not to be cancer.

Tumors of the thyroid gland, located at the base of the neck between the larynx and the collarbone, usually feel firmer than the rest of the gland and are usually painless. As with lumps in the breast, the overwhelming majority of thyroid lumps are not cancerous.

The most common symptom of testicular cancer is a lump or swelling in the testicle. Such lumps can be as small as a pea and may feel like an irregular thickening of the area. The lumps are often painless.

How can I tell if a lump is a tumor?

People who feel a suspicious lump in any part of their body should have it examined by a physician. Often, an imaging scan such as a mammogram for breast lumps or ultrasound for thyroid lumps can indicate whether a lump is benign or whether further tests, such as MRI or a biopsy are necessary. In a biopsy, a clinician uses a small needle to collect fluid or tissue from a lump so it can be examined under a microscope for the presence of cancerous cells.

How are tumors detected?

Detection techniques vary depending on the type of tumor and its location. Tumors in the digestive tract, for example, may initially be detected with an endoscope, an instrument that can be passed through the tract and enables the physician the inner lining of various organs. Other detection techniques include imaging scans, such as:

  • Mammographies
  • Ultrasounds
  • Magnetic resonance imaging (MRI)
  • Positron emission tomography (PET)
  • Computed tomography (CT)
  • X-rays

There also are a variety of lab tests that analyze blood, urine, or other bodily fluids for substances associated with different types of cancer. Ultimately, a diagnosis of cancer is made after a pathologist has viewed the cells within tissue suspected of being cancerous.