Screening mammography was introduced slowly in the late 1960s, some momentum in the 1970s, and full steam ahead in the 1980s. Sad to say, along with its clinical introduction came a boatload of misconceptions about its efficacy. For the past 33 years, I’ve been trying to clarify the overstated benefit of screening mammography, but it has been a moving target with new bells and whistles added, such that the efficacy has nearly reached the original exaggerated numbers from decades past.
The original “Mammograms will detect 90% of breast cancers” was a deeply flawed misconception. To be as brief as possible, this 90% applied to cancers that could also be felt. That’s NOT screening. If you have a lump, then the mammograms are “diagnostic,” while screening is a term reserved for identifying breast cancer that is NOT palpable. The original prospective, randomized trials (most started in the 1960s through 1980s) were measuring “mortality reduction” in a screened population vs. no screening. So, for instance, a “20% mortality reduction” in the screened group would have fewer deaths than occurred in the non-screened group.
For further illustration, if the trial included 20,000 women over a 5-year period of mammographic screening wherein 400 women develop breast cancer — 200 in the screening group, 200 in the non-screened group — while deaths specifically from breast cancer are 100 in the non-screened group and 80 in the screened group, the “20% mortality reduction” is from 100 down to 80. (beg pardon on this oversimplification that ignores the 4 major epidemiologic biases). But 80 women still died of breast cancer even though they were in the screened group?! What gives? Mammograms appear to have saved lives, but not nearly as many as one would guess. This 20% mortality reduction is a conservative number in use today, while large observational studies indicate a 40%d mortality reduction, a better number, but not as easily accepted by critics.
It gets worse. Of the 7 prospective randomized trials (depends on how you count them), only one (Swedish 2-county) had a mortality reduction which reached statistical significance! Other trials trended toward significance, but the decision to proceed with major screening programs was a borderline recommendation from the beginning, largely made possible by combining these trials together (into meta-analyses) which then allowed us to reach statistical significance. The full story is complicated, but if you’re interested, a good resource (if I do say so myself) for a deep dive is the book, “Mammography and Early Breast Cancer Detection: How Screening Saves Lives” by Alan B. Hollingsworth (McFarland Publishers). Slightly outdated at the end by the decision to return to screening in women starting at age 40 (it was 50 when I wrote the book, appealing for age 40 in 2016). It’s a fascinating story, and I wrote it with the intent of providing insider info, personal anecdotes, historical trickery, uncontrolled human bias, and on and on, but the Editors moved all the “fun stuff” to the end, in a Chapter Notes section (rather than leave it in place next to the appropriate vignette). Still one reviewer said, “It’s more entertaining to buy the book and then just read Chapter Notes at the End.
Back to screening mammography. Why was the mortality reduction so weak in many of the trials? At the time, activist critics of screening mammography said it was because breast cancer outcome is decided very early in the course of disease and screeningsimply doesn’t find cancer early enough to make a big difference. In fact, this “biologic predestination”was the dominant belief by many academics, extending into the 2000s. But I believe there is a different explanation.
In determining whether or not screening saves lives, there are only TWO variables. The first is BIOLOGY — the cancer must lend itself to early detection. In the discussion above, critics laid the blame for weak results here. They said that early detection doesn’t matter in breast cancer because the tumor is biologically predestined as to outcome (in keeping with Fisher Theory that drove the development of lumpectomy). However, for a variety of reasons, I came to believe that this was not the case, but instead, it was variable Number TWO — SENSITIVITY — that determines outcome of screening. Sensitivity is, in lay terms, “the percentage of cancers identified.” So, the 90% sensitivity originally attributed to screening mammography was about as good as it gets, until people started to ask, “How do you know that you missed only 10%?” Therein lies the rub.
As it turned out, dense tissue mammograms are missing more than half of the detectable cancers. In the past, these women were told their mammograms were “all clear” then sent out the door with “see you next year” while their life-threatening cancer remained in place, well-hidden. Once things shifted from BIOLOGY (where there’s not much you can do) to SENSITIVITY, the race was on to improve Sensitivity. These cancers that are encased in dense tissue are best detected using MRI (or molecular imaging). But the problem arises as to how we select patients for MRI when there is no way to screen the entire population with this modality. My answer here has always been to develop a blood test that is reasonably sensitive for cancer detection, and if mammograms are negative, then a blood test positive, an MRI is indicated. Some blood tests are on the market already, but these multi-cancer options with circulating tumor DNA are not very sensitive for early-stage breast cancer, for some reason. We don’t know why. One study failed to detect a single breast cancer out of 14 detected by mammography. Needless to say, this is not good and runs counter to marketing efforts by these companies. Still, we are seeing results now in RNA-based testing that appear close to clinical use.
In the meantime, what bells and whistles have been added to mammography to rise above the rather pathetic sensitivity in women with dense breast tissue.
First came DIGITAL MAMMOGRAPHY, but this was a head fake. The study to confirm its utility was actually intended to prove itself EQUAL to original film screen mammography….not “better than.” All radiology departments were converting to digital at that time (circa 2000) so this was part of the switch. But in the process of comparing film screen to digital, we learned that the overall sensitivity in the general population was remarkably low for either technique — 70% sensitivity for digital and 66% for film screen. The definition of “missed cancer” in this DMIST study was any tumor arising after a negative mammogram with 12-month follow-up. Shockingly, if this 12-month follow-up was extended to 15 months, both approaches to mammography showed a dismal 41% sensitivity (all densities included). Only one subgroup showed a statistically improved cancer detection rate wherein digital outperformed film screen — (under 50, premenopausal, dense breasts) — and it was this group where media focused entirely on the exciting new development of digital mammography. Yet, for this subgroup, even the digital sensitivity was bad at 59%, until you look at the film sensitivity of 27%s. (imaging these women reading their mammography reports where the disclaimer noted, “5 to 10% of cancers are not visitble on mammography,” when in fact, their personalized miss rate is 73%. Of the 20 subgroup analyses, NONE had a sensitivity of 90%, and only 3 were above 80%. And this includes older women with low density mammograms.
Nexa camce CAD (computer assitsted diagnosis), a primitive artificial intelligence (AI), proving to be of little or no help for expert breast radiolostis. Studies indicated a very slight improvement in cancer detection rates (CDRs), but this was confined to radiologists who were not full-time breast specialists. The computer circles suspecious areas, while the big problem is false-positivies. In the end, it offered little.
Then came 3D TOMOSYNTHESIS, this time with a demonstrable improvement in CDRs. This development was modest, but seen consistently in all studies. Marketing approaches used the relative increase in Sensitivity because the numbers sound so much better. As we all know, absolute risks are not nearly as impressive as the same study expressed in relative terms. Example for 3D mammography — a 4% CDR with 2D is elevated to 6% CDR with 3D, resulting in a 50% relative increase in Sensitivity. Is that a 2% increase or a 50% increase? Both are true. Oh, well, every little bit helps. Importantly, the cancers found by 3D tend to be skewed toward invasive cancer, a fact that indicates a better chance at a saved life with 3D tomosynthesis, and this technology should be standard everywhere.
Finally, came a true competitor to MRI in the form of contrast-enhanced mammography (CEM). With an injection of radiology dye (iodine-based) prior to the smammogram, the Sensitivity was initially reported by several groups as comparable to MRI. That said, not all studies agree, and there are reports of Sensitivity as low as 64%. There may be a learning curve that alters final outcomes, or some technical factor, but this will eventually be sorted out, and I believe there is a place for CEM.
Until more data accumulates, I think the woman with dense breast tissue has multiple options to avoid her cancer being missed with routine mammography. These range from adding whole breast Ultrasound to 3D mammography, all the wayt to MRI and dropping mammograms entirely. And, into this mix, we will eventually figure out how a blood test streamlines screening and, importantly, cuts the costs that are inherent with any large scale screening program.
We’re come a long way since that first clinical trial in the 1960s, and it’s been an unusual trip. I was a surgery resident in training when I saw my first mammogram.We residents didn’t have a clue what we were looking at, or why we would ever need an X-ray of the breast. Same for the faculty. The story of how those original technologies were grossly inflated as to their efficacy is a fascinating story of human nature, and it’s why I wrote a book that attempted to reveal both nuance and full-scale battles. An academic breast radiologist in the U.K., newly retired, purchased a copy then contacted me with interesting praise that went something like this: “I’ve taught breast imaging and screening my whole career, but until now, reading your book, I finally understand what the fuss is all about.”
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