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First, a definition* -

False positives happen in screening mammography when the images suggest the presence of a malignancy in a woman who doesn’t have cancer in her breast.

Here’s my proposed model –

Categories of False Positives in Screening Mammograms

False positives can arise during any of three conceptual segments of the testing process:

1. False positives occur during the test itself.

This happens when a radiologist inspects a film or digital image and labels the result as abnormal, but no cancer is present. This sort of problem is interpretive.

A common scenario goes like this – a spot in a mammography image suggests the presence of a possible tumor and the radiologist correctly notes that abnormality; later on, a doctor determines by sonogram, biopsy or another method that there is no malignancy in the breast.

(Other, uncommon problems in this category would include faulty equipment that reduce image quality, mislabeling or accidental switching of films; in principle, these kinds of errors should be non-events.)

2. False positives stem from miscommunication or misunderstanding of test results.

If a clerk accidentally phones the wrong patient and tells her she needs another procedure because the results of her mammogram are abnormal, that call might instigate an untoward, false positive result. If the error is corrected early on, so that affected woman worries only for a period of hours and has difficulty sleeping for one night, she might experience some psychological and/or small financial cost from the matter. But if the mistake isn’t caught until after she’s had a sonogram or MRI, and consulted with a surgeon or another physician, the costs grow.

False positives also arise if a patient misunderstands a test result. An essentially normal mammography report, for example, might mention the appearance of benign calcifications. Upon reading that result, a woman or her husband might become upset, somehow thinking that “benign” means “malignant.” This type of false positive error, based in poor communication and lack of knowledge, can indeed generate extra doctors’ visits, additional imaging tests and, rarely, biopsies to relieve misguided fears.

3. False positives derive from errors or misinterpretation of results upon follow-up testing.

This category of false positives in screening mammography is by far the biggest, hardest to define and most difficult to assess. It includes a range of errors and confusion that can arise after breast sonograms, MRIs and breast biopsies.

3a. false positives in subsequent breast imaging studies such as sonograms and MRIs:

Many women in their forties and early fifties are premenopausal; their estrogen-stimulated breasts tend to be denser than those of older women. Reading their mammograms may be less accurate than for postmenopausal women. For this reason, a doctor may recommend a sonogram or MRI to further evaluate or supplement the mammography images.

These two radiology procedures – sonograms and MRIs – differ and, for the most part, are beyond the scope of this discussion except that they, too, can generate false positive results. A sonogram, for instance, may reveal a worrisome lump that warrants biopsy. MRIs are more expensive and sensitive; these tend to pick up subtle breast irregularities including a relatively high proportion of benign breast lesions.

3b. false positives in breast biopsy:

A breast biopsy is an invasive procedure by which a piece of the gland is removed for examination under the microscope. Sometimes pathologists use newer instruments to evaluate the genetic, protein and other molecular features of cells in the biopsy specimen. Years ago, surgeons did the majority of breast biopsies. Now, skilled radiologists routinely do a smaller procedure, a core needle biopsy, using a local anesthetic and a small albeit sharp instrument that’s inserted through the skin into the breast. Some doctors do a simpler procedure, fine-needle aspiration, by which they remove cells or fluid from the breast using a small needle attached to a syringe.

In principle, a false positive biopsy result would occur only when a pathologist, a physician trained to examine tumors at the cellular and molecular levels, misreads a case, meaning that he or she reports that the cells appear cancerous when they’re not. Fortunately, this is not a frequent issue in breast cancer diagnosis and management.

The real issue about false positives – and what may be the heart of the issue in mammography screening – has to do with how pathologists describe and define some premalignant conditions and low-grade breast tumors. This concern extends well beyond the scope of this tentative outline, but a few key terms should facilitate future discussion:

Lobular Carcinoma in Situ (LCIS) is not considered a malignancy by most oncologists, but rather an abnormality of breast glands that can develop into breast cancer.

Ductal Carcinoma in Situ (DCIS) is a Stage 0 breast tumor – a tiny cancer of breast cells that have not penetrated through the cells lining the ducts of the breast gland.

Indolent or “slow” tumors – The idea is that some breast cancers grow so slowly there’s no need to find or treat these.**


*This definition warrants some discussion, to follow in a separate post.

**As a physician and trained oncologist, I am uncomfortable with the published notion of some breast tumors being “so slow” that they needn’t be found or evaluated. I include these tumors only for the sake of completeness regarding theoretical types of false positive results upon screening mammography, as there’s been considerable discussion of these indolent tumors in recent news.

Slow-growing breast tumors are quite rare in young women. In my view, their consideration has no bearing on the screening controversy at it pertains to women in their forties and fifties.


As outlined above, the first two categories of false positives seem relatively minor, in that they should be amenable to improvements in quality of mammography facilities and technology; the third category is huge and where lies the money, so to speak.

Clearly there’s more work ahead -

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