Monday, September 13, 2010

Emerging Breast Imaging Technologies and Radiation Risk

A recent New York Times headline read:

Radiation, Risks Are Focus of Breast Screening Studies

The topic of the article was the risk for future cancer cause by radiation exposure from new breast cancer screening techniques, specifically, breast-specific gamma imaging (BSGI) and positron emission mammography (PEM).  These radiation risks are discussed in a recent study in the journal Radiology.

Dr. R. Edward Hendrick of the University of Colorado is a breast imaging specialist and the author of this new study.  His conclusion:  radiation from breast imaging techniques can cause radiation induced breast cancers BUT the number of excess breast cancer cases differs significantly based on the screening technique and the woman's age at screening.  For example, regular or digital mammography in 80-year-old women might cause at most one additional breast cancer per 1 million women screened; a very low risk.  Annual mammography obviously involves repeated small radiation doses and therefore an increase in radiation-associated risk.  Dr. Hendrick estimates annual mammography for 40 years beginning at age 40 might cause between 20 and 25 additional breast cancers per 100,000 women, again a small risk considering the lives saved by detecting early breast cancers through this screening method.

Dr. Hendrick then goes on to estimate the excess cancer risk of two new breast cancer screening technologies, positron emission mammography (PEM) and breast-specific gamma imaging (BSGI).  Both of these imaging techniques involve injecting a radioactive substance into the bloodstream and the imaging involves tracing the accumulation and/or metabolism of the radioactive substance in the breast tissue (details of both techniques are contained in Chapter 9 of Positive Results).  Dr. Hendrick estimates the risk of a radiation induced breast cancer is significantly greater for both of these techniques than for annual mammography, regardless of whether it is film or digital (digital mammography has a lower radiation dose than film mammography).  In fact, he estimates that the radiation induced risk from a single PEM or BSGI exam equals the risk of 40 years of annual mammography.  Additionally, mammography only exposes breast tissue to radiation whereas both BSGI and PEM expose all body tissues to the radiation because the radioactive substances travel through the blood stream, thereby potentially creating radiation-induced cancer risk in other tissues as well.

Many genes function to protect our cells and DNA from radiation injury.  Changes or mutations in several of these genes (ATM and CHEK2 are two examples) are common in the population, with as many as 1% of women carrying variations in these genes.  Both are part of a group of genes known as "DNA repair" genes and have been associated with increased susceptibility to radiation injury and radiation related cancers.  Current medical practice does not routinely test for these genes in the general population.  Nonetheless, the future medical practice could involve such testing, which could help determine those individuals who are most at risk from radiation based tests and those who really have no additional risk.   Better understanding of the interplay of our genetics with medical tests and treatment is the goal of "personalized medicine."  But practical decisions such as whether annual mammography is of benefit to any given woman given her genetic makeup is still in the future because, although a few of these genes can currently be tested, these genetic tests are expensive and the medical community lacks a clear consensus about who should be tested and how the information should affect decisions about mammography and other radiation based screening tests.

Dr. Hendrick's study is based on average-risk women, not women at high risk for breast cancer due to BRCA mutations or other hereditary factors.  Women with BRCA mutations may well be even more susceptible to radiation induced breast cancer, which has resulted in much discussion about the use of mammography in very young BRCA-positive women.  As we discussed in Positive Results, thus far only one study, published last year in the Journal of the National Cancer Institute (JNCI), has sought to quantify this additional risk through mathematical modeling.  The JNCI study found that for BRCA-positive women, more lives would be saved by early breast cancer detection than would be lost through radiation-induced cancers beginning annual mammography at age 35.  Under the age of 35, the net benefit of annual mammography was less clear.  Yet BRCA-positive women do develop breast cancer under age 35, and therefore require screening. Current  protocols recommend annual mammography AND breast MRI beginning at age 25.

So what is a BRCA-positive woman to do?  As we discussed in Positive Results, we believe the answer should be a pragmatic one.  The biggest reason why mammography is less effective at detecting cancer in young women is that they tend to have dense breasts, but not all young women have breasts that preclude effective screening.  Thus, all young BRCA-positive women should be screened with MRI, which is not affected by breast density, beginning at age 25 or ten years younger than the youngest breast cancer in the family.  A baseline mammogram should be done when screening begins and the decision on when annual mammography should continue for women under 30 should depend on the professional opinion of the breast experts.  By age thirty, most experts believe the benefits of mammography in BRCA-positive women outweigh the risks.

But what about breast screening with PEM or BSGI?  We discussed both PEM and BSGI in Chapter 9 of Positive Results and our feeling about the use of these techniques in high-risk women is that they are not appropriate for annual screening because of the significant radiation exposure.  This is not to say that a single PEM or BSGI might never be appropriate in a high-risk woman, but their use will likely be for specific circumstances when a breast lesion needs to be characterized and other techniques are inadequate.  Routine use of BSGI or PEM in BRCA-positive women is unlikely unless the radiation dose can be brought to a level that is comparable to mammography.

Studies quantifying the radiation risk of these new imaging techniques were limited at the time Positive Results went to press.  We welcome new studies such as the one done by Dr. Hendrick that help quantify the risk from the radiation exposure of different breast imaging modalities, especially studies that look at radiation-induced risk in BRCA-positive and other high-risk women who must begin breast cancer screening at younger ages than women in the general population.

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.

1 comment:

  1. Wow, great article, I really appreciate your thought process and having it explained properly, thank you!

    Breast cancer screening