A Closer Look at the Details on Mammography, in Between the Lines

Recently I wrote a review of Between the Lines, a helpful handbook on bio-medical statistics authored by an acquaintance and colleague, Dr. Marya Zilberberg. In that post, I mentioned my concern about some of the assumptions and statements on mammography. One thing I liked the book, abstractly, is the author’s efforts to streamline the discussion so that the reader can follow the concepts. But simplification and rounding numbers, “for ease of presentation” (p. 29) can mess up facts, significantly in ways that some primary care doctors and journalists might not appreciate. And so I offer what I hope is a clarification, or at least an extension of my colleague’s work, for purposes of helping women understand the potential benefits and risks of mammography.

In the section on mammography (pp. 28-31), the author rounds down the incidence of breast cancer in women between the ages of 40 and 50 years, from “1 in 70” (1.43%) to “1 in 100” (1%). As any marketing professional might remind us, this small change represents a 30% drop (0.43/1.43) in the rate of breast cancer in women of that age group. This difference – of 30%, or 43%, depending on how you look at it – will factor into any calculation of the false positive (FP) rate and the positive predictive value (PPV) of the test.

For women ages 40-49 Have breast cancer Don’t have breast cancer
If estimate 1 in 100, 1.0 % 100 9,900
If estimate 1 in 70, 1.43 % 143 9,857

Keep in mind that these same, proportional difference would apply to any BC screening considerations – in terms of the number of women affected, the potential benefits and costs, for the 22,996,493 women between the ages of 40 and 49 counted in the 2010 U.S. Census,

My colleague estimates, fairly for this younger age group of women (who are relatively disposed to fast-growing tumors), that the screening technology (mammography) only picks up 80% of cases; 20% go undetected. In other words – the test is 80% sensitive; the false negative, FN, rate is 20%. In this same section, she considers that the FP rate as 10%. Let’s accept this (unacceptably high) FP rate for now, for the sake of discussion.

As considered in Between the Lines:

If FP rate is 10%, prevalence 1 in 100 Really have BC Don’t have BC Total
Mammography + 80 990 1,070
Mammography – 20 8,910 8,930
Total 100 9,900 10,000

But the above numbers aren’t valid, because the disease affects over 1 in 70 women in this age bracket. Here’s the same table with a prevalence of 1 in 70 women with BC:

If FP rate is 10%, prevalence 1 in 70 Really have BC Don’t have BC Total
Mammography + 114 986 1,100
Mammography – 29 8,871 8,900
Total 143 9,857 10,000

In this closer approximation to reality, the number of true positives is 114, and false positives 986, among 1,100 abnormal screening results. Now, the PPV of an abnormal mammogram is 114/ (114+986) = 10.4%. So the main statistical point – apart from the particulars of this discussion –  is that a seemingly slight rounding down can have a big impact on a test’s calculated and perceived value. By adjusting the BC rate to its prevalence of approximately 1 in 70 women between 40 and 49 years, we’ve raised the PPV from 7.5% to 10.4%.

Here I must admit that I, too, have rounded, although I did so conservatively very slightly. I adopted a 1 in 70 approximation (1.43%) instead of 1 in 69 (1.45%), as indicated on the NCI website. If we repeat the table and figures using a 1 in 69 or 1.45% prevalence rate and 6% FPS, the PPV rises a tad, to 10.5%.

Now, we might insert a different perspective: What if the false positive rate were 6%, as has been observed among sub-specialist radiologists who work mainly in breast cancer screening?

If FP rate is 6%, prevalence 1 in 70 Really have BC Don’t have BC Total
Mammography + 114 591 705
Mammography – 29 9266 9,295
Total 143 9,857 10,000

As you can see, if we use a FP rate of 6% in our calculations, the total number of FPs drops to 591 among 10,000 women screened. In this better-case scenario, the PPV of the test would = 114/ (114+591) =16%. Still, that’s not great – and I’d argue that public health officials, insurers and patients should be pushing for FP rates closer to 2 or 3% – but that’s irrelevant to my colleague’s point and her generally instructive work.

My second concern has to do with language, and making the consequences of false positives seem worse than they really are. On page 29, the author writes: “ So, going back to the 10,000 women being screened, of 9,900 who do NOT have cancer… 10%, or 990 individuals will still be diagnosed as having cancer.” The fact is, the overwhelming majority of women with positive mammograms won’t receive a cancer diagnosis. Rather, they’ll be told they have “an abnormal result, or a finding that suggests the possibility of cancer and needs further evaluation,” or something along those lines. It would be unusual in practice to jump from a positive mammogram straight to a breast cancer diagnosis. There are steps between, and every patient and journalist should be aware of those.


Finally, if I were to write what I really think, apart from and beyond Between the Lines – I’d suggest the FP rate should be no higher than 2 or 3% in 2012. This is entirely feasible using extant technology, if we were to change just two aspects of mammography practice in the U.S. First, require that all mammograms be performed by breast radiologists who get extra training and focus in their daily work almost exclusively on breast imaging. Second, make sonograms – which, together with mammograms, enhance the specificity of BC screening in women with dense breasts– universally available to supplement the radiologists’ evaluations of abnormal mammograms and dense breasts in younger women.

By implementing these two changes, essentially supporting the practice of sub-specialists in breast radiology, we could significantly lower the FP rate in breast cancer screening. The “costs” of those remaining FPs could be minimized by judicious use of sonograms, needle biopsies and other measures to reduce unnecessary surgery and over-treatment. Over the long haul, we need to educate doctors not to over-treat early stage disease, but that goes far beyond this post and any one woman’s analysis of mammography’s effectiveness.

All for now,
ES

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What Does a Bikini Parade Have to Do with Breast Cancer?

A recurring question on this blog is this: Is there a limit, in terms of appropriateness or “correctness,” in fundraising for causes that would help put an end to breast cancer?

My blogging colleague and friend, fellow BC ~survivor/advocate/NBCC summit attendee and former chemo recipient, AnneMarie Ciccarella, @chemobrainfog wrote about an upcoming bikini parade planned by a tanning salon owner in Madison Lake, MN. Proceeds from the march will go toward a nonprofit group called the Breast Cancer Natural Prevention Foundation (preventbc.org). This true story is problematic at many levels, as AnneMarie points out.

But sometimes an extreme case of something – here what’s billed as a BC fundraiser – can be instructive. A few months ago I wrote about Boobstagram – a French website that asks women to submit pictures of their breasts to increase awareness of the value of healthy breasts. The site, vaguely and with few words, tries connecting the barely clad images with “the fight against cancer.” Although I’m still not convinced that the concept utterly lacks merit in principle, and maintain that some of the voices raised here were, perhaps, too quickly dismissive and uptight about the possibility of fundraising or BC activism by this method, I acknowledge that the men running that company seem to be doing nothing useful in terms of reducing breast cancer or its complications.

The Minnesota bikini march will take place on July 28. The line-up starts at noon. The walk will begin at 1PM. According to the announcement on the Electric Beach Mankato website, “only females in bikinis will be counted toward the world record.” The organizer and salon owner, Cynthia Frederick, needs 451 participants to break the Guinness World Records mark for largest bikini parade. That site lists the record as 357 women, based on a 2011 event in Queensland, Australia. But that achievement was recently surpassed in Panama City, FL. What’s different about the prior demonstrations is that there was no pretense of raising money or awareness to help fight, prevent or cure breast cancer.

Minnesota bikini parade participants will pay $20 or $25 for tee shirts. Net proceeds will to go the Breast Cancer Natural Prevention Foundation. The foundation’s site suggests that sunlight prevents BC by increasing vitamin D levels (which is total BS, to be perfectly clear). Taking too much vitamin D can do damage, as can excessive sun exposure.

As I read this, a tanning salon – a business that causes melanoma and other skin cancers – is promoting a walk of bikini-wearing women in midday summer sun to break an amusing world’s record. The parade will, if anything, harm those women who, naively or otherwise, believe they’re supporting a legitimate effort to prevent breast cancer. Any funds raised will support a foundation that promotes what’s tantamount to snake oil for the disease.

So there is a line, in the sand… And it’s been crossed!

If I were an investigative journalist, I’d want to know more about the organization that calls itself the “Breast Cancer Natural Prevention Foundation.” Does it get tax breaks? If so, why?

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