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breast health, cancer, Contraception

Does the Mirena IUD increase the risk of breast cancer? New data

A new study has been published suggesting a link between breast cancer and the Mirena IUS (intrauterine system). The Mirena IUS is used commonly in many European countries and is increasing in use in the United States (with good reason, it is highly effective and has a very high satisfaction), so I think a review of the study and the data on any connection between the Mirena and breast cancer is deserved before any connection becomes urban myth. This is especially important given all the great disservice done by the Supreme Court in the Hobby Lobby case as the Mirena IUS does not cause abortions or affect a fertilized egg.

The short answer: The worst case scenario is there may be a 19% increased risk of breast cancer for women who had a Mirena IUS inserted between the ages of 30 and 49. In reality this study only suggests a correlation and provides no proof whatsoever of causation. There are many reason women with a Mirena IUS could have a higher risk of breast cancer when compared with all other women and be completely unrelated to the hormone in the IUD. The most obvious reason is that
women who have an IUD may have other significant risk factors for breast cancer not accounted for in the study such as: fewer pregnancies, later first pregnancy, or they may

Image courtesy of Margaret McCartney

Image courtesy of Margaret McCartney

have had a strong family history of breast cancer and chose the Mirena because they believed it to be safe than taking hormones by mouth or a shot.

I had a Mirena inserted at the age of 41 and had it for thee years and I’m not losing any sleep over the study. I am somewhat peeved at the design (more on that below. This study actually contributes very little to what we know on the subject.  

Real life verdict: studies designed not to answer the question (i.e. does the  Mirena IUS cause breast cancer) can’t answer the question. We need the right study because we all know how unsubstantiated medical beliefs about IUDs play out in America.    

 

For those of you who want more than a summary keep on reading, but it’s cool if you don’t want to geek out on a more in-depth analysis!

 

Depo provera, an injected progestin (same family as levonorgestrel) birth control, is associated with an increased risk of breast cancer for women who use it for more than 1 year. Once the medication is stopped the risk returns over time to baseline. None of the studies that have identified this risk are randomized trials, but given there are at least five that support a connection between one progestin and breast cancer it is valid to ask, about other methods so the idea behind the new study is valid.

It is important to keep in mind that the amount of levonogestrel released in the blood stream is so minute that a new ultra sensitive assay had to be developed to detect it, so we certainly cann’t extrapolate data from depo provera to the Mirena. Different drug and way different dose.

This latest study looked at women in a national registry in Finland. It is an observational study, so they identified all women who had received a Mirena between the ages of 30 and 49 (> 93,000 women) and then looked to find who had a diagnosis of cancer (they apparently have registries for about everything in Finland, they must have more epidemiologists per capita than anywhere else except Framingham). There were no specific controls, but a percentage of the women had given some personal data as part of another study and from that they derived basic demographic data that they extrapolated to the entire cohort. The cancer incidence that was used as to compare the IUD users was derived from the corresponding cancer incidence of the population, not from a specific control group. What the investigators found was a 19% increase in the risk of breast cancer among women who had used the Mirena IUS between the ages of 30 and 49 and a decrease in endometrial (lining of the uterus), ovarian, pancreatic and lung cancers. The pancreatic and lung cancer reduction was completely unexpected.

There is a major issue with this study, the lack of control group. Using an IUD inserts all kinds of confounders, some very important ones include age at first delivery, breastfeeding data, and number of pregnancies. Women using a highly effective method of contraception are likely to have fewer children and may have them later and these are two major risk factors for breast cancer. We have no idea if Mirena users are more likely to breast feed or not. The issue of family history is also huge. What if women with a family history self selected for Mirena because they were worried oral contraceptive pills might raise their cancer risk? Maybe women with more medical issues chose the Mirena over other methods? Maybe…maybe…maybe. Arggh, too many maybes are a concern!

Fortunately, someone did that study. They compared Mirena IUS users with an appropriate control, copper IUD users.  Interestingly, many of these patients were also from a Finnish registry. This study also had detailed demographic data on each patient such as body mass index (BMI), age when periods started, reproductive history, breastfeeding, hormonal exposure [contraception, hormone replacement therapy], period of IUD use, family history of breast cancer, and data on health behaviors that are associated with breast cancer (such as alcohol consumption, physical exercise, and smoking).  They had data on > 5,000 women who used the Mirena and > 20,000 who used the copper IUD. What did they find? 22% of 5113 women who had breast cancer cases had used the Mirena sometime before their cancer diagnosis and 1130 (22%) had used a copper IUD. The exact same percentage. They also found no difference when the first IUD insertion was 13–60 months prior to breast cancer diagnosis (so recent use) versus the use of IUDs preceding the breast cancer diagnosis by more than 60 months (remote use). The only issue with this study is that it was powered to find a 50% increase and the newer Finnish study only had a 19% increase. So, this other study can’t disprove the newer one, but it is a model of how the study should be done and can help women feel assured that if (and that’ s a big if) there is an increased risk it is indeed low.

I’m not sold on the new study. It is clearly biologically plausible (given what we know about progestins) that even minute doses could increase the risk of cancer. Whether the Mirena raises the breast cancer risk slightly, this study can’t tell us. It is really a shame that they did all that work and didn’t identify copper IUD users as controls. Even without demographic data comparing IUD users to IUD users  would have been so much better. I wrote a letter to the journal that published the Finnish study asking why a copper IUD control group wasn’t used. You simply can’t compare a Mirena IUD user to both the non-contracepting and contracepting general population without far more demographic and health data.

 

 

 

 

 

 

 

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Discussion

8 thoughts on “Does the Mirena IUD increase the risk of breast cancer? New data

  1. Fantastic analysis!

    Posted by Victo Dolore | July 23, 2014, 7:19 pm
  2. Dr. Gunter, I could only access the abstract to the article, but in the methods section of the abstract, it says that the women studied were using the Mirena IUD for the treatment of menorrhagia. I think that would be a major confounder. Wouldn’t you expect women with menorrhagia to perhaps have higher levels of endogenous estrogen? Also, I was surprised at the reductions in the incidence of pancreatic and lung cancer. The reduction in endometrial cancer and perhaps ovarian cancer makes sense, but those others, not so much in terms of biologic plausibility. And the rates that these cancers were reduced were much more significant than the increase in incidence of breast cancer. I know the media will focus on the increased breast cancer risk though.

    Posted by ellena | July 23, 2014, 7:45 pm
  3. Thank you for the astute analysis! I wish there were more journalists with the medical/scientific training to properly assess studies like this, rather than just blindly repeating what press releases say, or worse, misleading non-scientific readers with claims of causality.

    Posted by Megan F. - PatientPhD.com | July 24, 2014, 8:20 am
  4. I have always believed the Mirena IUD is what was the “trigger” that turned on breast cancer in my body at 34. I appreciate your perspective, but can only hope that more research and studies are done to identify if there truly is a link between the two. I rely on my gut instinct often and it usually doesn’t lead me astray — my gut tells me there is a link. Knowing this, my experience and those of others who were on the Mirena and also diagnosed w/ pre-menopausal breast cancer, I would be very skeptical of using the Mirena IUD. There are plenty of other options — it’s just not worth the risk..trust me.

    Posted by Allyson Wyles | August 26, 2014, 4:47 pm
    • Thank You for Your comment. I also think that there is a link between Mirena and cancer. Also it doesn’t make me feel well by keeping it for more than a year now. But my doctor (gynaecologist) is still insisting that this is the only solvation of my problem( constant spotting throughout the month) and wants me to keep it- otherwise hysterectomy.

      Posted by Desislava Gancheva | October 6, 2014, 2:38 am
    • Hi Allyson, my name is Berry, and i’m really interested by this subject. In fact I just have a mirena IUD placed. you are talking about plenty of other options. Can you please tell me about it?
      Thanks

      Posted by Berry | November 25, 2014, 11:55 am
  5. April of this year, I had my annual mammogram and at the same time had was planning to have a IUD (Mirena) put in. The week the IUD was to be place my doctor had my results for my mammogram back which they found something,that friday the doctor put the IUD.I just left the doctors office and they called and said that they found something in my mammogram and that I needed a sonogram. I started to have pain under my arm pit and down my left arm, getting dizzy spells head aches and different things going on with me. Later to find out that my breast cancer 1.5 cm went to my lymph notes being 6 cm in size. I do believe that this IUD brought on more estrogen to make that cancer spread to my lymph notes, because my gynaecologist was using the IUD to bring on more estrogen to slow down my monthly periods.

    Posted by Crystal Haines | October 19, 2014, 10:59 am

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