It seems like a no-brainer that catching a breast tumor when it’s miniscule is better than finding one that’s larger. After all, wouldn’t early detection lead to treatment that can stop small tumors from growing into larger, life-threatening ones?

Not necessarily. Mammogram detection of small tumors often leads to needless testing and treatment—without any health benefit. Now a startling new analysis has shed new light on why this happens: Many small breast tumors grow so slowly that they would never threaten a women’s life.

These tumors are often much less dangerous than larger ones—and likely to stay that way.

Background: If small breast cancer tumors tended to progress rapidly to large ones, then finding and treating them when they’re small should mean that there would be fewer large tumors later. But that hasn’t happened. Since the widespread adoption of screening mammography (looking for cancer in women with no signs or symptoms) decades ago, small breast cancer tumors have been detected three times more than the decrease in the diagnosis of large breast cancer tumors.

Yale University investigators designed a research study to find out why.

Study: Researchers analyzed data from the National Cancer Database from hundreds of thousands of cases of invasive breast cancer. These are breast tumors that have broken out from the milk ducts into nearby breast tissue. They concern doctors because they have the potential to spread throughout the breast and possibly beyond.

Some tumors, though, are much more likely to spread than others. In this study, the cancers were divided into three prognostic categories based on biological features of the tumor—”biologically unfavorable” (worst chance of survival), “biologically favorable” (best chance of survival) and “intermediate.” Then the researchers analyzed how tumor size stacked up against these categories.

Results: There was a dramatic difference in underlying tumor biology for women over 40 based on tumor size…

  • Among tumors one centimeter across or smaller—a lump that’s generally too small to be felt by hand—38% were favorable and only 14% unfavorable.
  • Among tumors five centimeters across or larger, only 9% were favorable and 36% were unfavorable.

By definition, a favorable tumor is highly unlikely to turn into an unfavorable tumor. When they do grow, it’s very slowly—they often take 15 to 20 years to grow large enough to be noticed. Indeed, the authors of this analysis wrote, these results provide evidence that many small tumors with favorable biologic features will not progress to large tumors within the lifetime of the patient. Plus, when a “favorable” tumor does progress, the prognosis for the patient after treatment is excellent, so there is little benefit to detecting them early.

Small “unfavorable” tumors are a different story—while these tumors can metastasize even when they are small, they are more likely to do so when they are larger. So the smaller these are when they are found, the better the prognosis. Unfortunately, these tumors tend to grow very aggressively, so they are rarely diagnosed while they are still small. They are often too small to be detected by one mammogram but have grown large and dangerous by the time of the next regular mammogram a year or two later.

Bottom line: Many small breast tumors picked up by mammograms aren’t likely to threaten women’s health in their lifetimes. Mammograms still matter—they identify many tumors that are life-threatening and that require immediate treatment. But if your next mammogram identifies a lump of one centimeter or less…even if it turns out to be cancerous…it’s important to realize that there’s a good chance it’s not a life-threatening emergency. Work with your doctor to determine how serious a cancer it really is.

The new research opens the possibility that some women with small “favorable” breast tumors may benefit from an approach that has become widespread for men with prostate cancer—active surveillance. That is, keeping an eye on a tumor to see if it changes and becomes a medical problem. (That is also an option for a common form of non-invasive breast cancer called ductal carcinoma in situ—a tumor that is confined to the milk duct.)

Meanwhile, researchers are seeking new approaches that will help them identify small “unfavorable” tumors as early as possible so they can be treated aggressively.

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