Screening tests for early detection of cancer have undergone a lot of scrutiny lately. Just a few weeks ago the American Cancer Society (ACS) issued a press release reminding us that the Cancer Society may have overstated the benefits of screening and underemphasized the potential harm when it came to testing for early breast and prostate cancer.
The ACS went on to discuss the limitations of screening women for early breast cancer with mammograms and screening men for prostate cancer with the simple PSA blood test.
This week, the United States Preventive Task Force (USPTF) issued new guidelines for breast cancer screening. I suspect that explains why the ACS issued its recent comments, knowing these guidelines were about to be released.
The USPTF analyzed various mammography screening schedules and concluded that:
1) Women under the age of 50 should not be screened with routine mammograms because proven benefits are lacking and the risks of harm (such as unnecessary biopsies from false positive results, especially in women with dense breasts) are greater.
2) Screening women of average risk between the ages of 50 and 74 with mammography every two years will achieve most of the benefits of yearly screening, but with less harm.
3) The task force also recommends against teaching breast self-exam.
So why the new recommendations? What new information do we have on the behavior of breast cancers and mammograms?
Many scientists believe that there are two types of cancers.
First there are those that appear suddenly, grow rapidly, act aggressively, and often are diagnosed when it is already "too late" for curative treatment. Even a yearly mammogram might not diagnose this aggressive breast cancer in the early stages.
But for many women (and men when it comes to the prostate) their cancers are of a second type -- they are very slow growing, often not even showing up on mammograms for many years. These slow cancers are thought to be much less aggressive. Some experts are even suggesting that these cancers may either disappear on their own or would not cause injury or death to the person with them, even without treatment.
A recent study in the Journal of the American Medical Association (JAMA) pointed out the evidence for this. Breast cancer statistics for women in the Unites States has not shown a reduction of advanced breast cancers being diagnosed, despite the widespread use of mammography.
One would expect that if mammograms diagnosed breast cancers earlier and women were then treated for these cancers, over time there would be a reduction in the diagnosis of more advanced cancers from the "successful" screening. But this is not the case. Advanced cancers continue to be diagnosed with greater than expected frequency.
For men, the same seems to be true for prostate cancers. Many men will develop prostate cancer if they live long enough, yet many of these cancers will not cause problems and don't need to be treated. However the simple PSA blood test, like the mammogram for women, does not distinguish the slow growing from the more aggressive fast growing ones.