Andy Kaplan-Myrth
Andy is an internet policy advisor with the Government of Canada, with a background in technology law and an interest in how collaborative and social technologies are reshaping industries, governments, societies and the world. See more from me at http://kaplan-myrth.ca.
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Probabilities of Cancer for Jewish women
Canadian sources are reporting today that Jewish women have a much higher incidence of the two genetic mutations (BRCA1 and BRCA2) that are associated with an increase the risk of ovarian and breast cancer. The articles are talking about women getting prophylactic mastectomy and bilateral oophorectomy surgeries because of the increased risk. One woman featured in the article expresses her relief after her surgery:
"It took away a lot of the terror, of wondering, `Do I have it, don't I have it, will I get it, won't I get it,'" she said. "I never have to think about ovarian cancer again."
So, you're a Jewish woman, you get the test for the BRCA mutations and you test positive. Is it off to the operating room? Or do you get a second test? And if that's positive, then do you opt for surgery?
Let's look at the numbers.
While caucasian women in general have a 1:250 chance of having the mutation, Jewish women (by which researchers seem to mean Ashkenazi Jews, not Sephardic Jews, Kaifeng Jews, or other groups) have a 1 in 45 chance (2.22% -- although in this particular study they only found 22 in 2080, or 1.05%).
Interestingly, even though Jewish women have such an elevated risk of having the mutations, a Jewish woman who tests positive for the mutation still probably does not have the mutation. See, of the 2080 people tested, 46 would (in theory) have the mutations. Given a 5% false positive rate, of the 2034 who don't have the mutations, 5% would still test positive -- and that is 101 women. In other words, if you were among the 2080 women and got a positive test result, you would have only a 46:147 chance, or a 31% chance of actually having the mutation.
If the 1 in 45 chance is inaccurate and we instead use the numbers from the study that found 22 of 2080 have one of the mutations, then 2058 of that group don't have one of the mutations, but 5% or 103 of them will test positive. In that case, if you were among the 2080 and test positive, you would only have a 22:125 chance of actually carrying a mutation, or 17.6% chance of carrying one of the mutations.
Sounds like maybe that one test is not enough information to rely on and you want a second opinion? If the women who receive positive tests pay for a second test to confirm the result, about 6 of them will still test positive!
Imagine it -- you're a Jewish woman, with an increased risk of having one of the mutations, you test positive for it not once but twice... do you get treatment?
Think about it carefully, because the chance that you still do not have one of the mutations (5:2080 = 0.25%) is pretty close to the chance that the average caucasian woman does have one of the mutations (1:250 = 0.4%).
And that is all without getting into the chances, if you really do carry a BRCA mutation, that you will develop breast cancer or ovarian cancer -- not a certainty, to say the least.
Whether to get the surgery or not is a personal decision of course and, in a sense, the woman featured in the Health Zone article is absolutely right -- it's not so much that she found out early that she would get cancer, but that she doesn't need to wonder if she will.
It's just another example of how counterintuitive probabilities can be, and how that makes it difficult to make smart health choices on both personal and policy levels.
- Jewish women are high cancer risk (healthzone.ca)
- Give Jewish women genetic tests to detect cancer, doctors urge (nationalpost.com)
- Breast cancer gene test eligibility in Ont. misses some (cbc.ca)
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