How much of early cancer detection is “overdiagnosis”?
May 4, 2010
The Advocate Says:
When the conversation turns to the potential harms of cancer screening, the concern I most often hear expressed in the breast cancer world is that if women are told that screening has downsides, and that overdiagnosis is a possibility, they will avoid getting their mammograms. This is seen as a step backwards. By comparison with the alarming possibility of a missed or delayed diagnosis, anxiety, unnecessary biopsies, and even surgery and adjuvant treatment for a mass that may never become life-threatening seem to some (many?) as a worthy price to pay.
What doctor is likely to recommend against intervention in the face of a finding that just might turn out to be a cancer? In today's litigious world, treating aggressively is often inevitable. The problem is, the very process of looking for a cancer sets this process in motion.
If you test, you find, and then you have to treat.
Consider this: A consensus conference last fall discussed whether DCIS should even be referred to as "cancer." DCIS is treated with either mastectomy or lumptectomy with radiation, and often, years of hormonal treatment. Yet it's believed that most of this DCIS will never go on to become an invasive cancer. But because some will, all of it is treated aggressively. Moreover, DCIS is most often an incidental finding of mammography screening, with an incidence that has increased from 5 to 30 times what it was in the early 1980's.
Here's another example. Recently, an old friend diagnosed five years ago with Stage II breast cancer had an incidental finding of a lung nodule and a couple of nearby swollen nodes as part of a work-up for chronic obstructive pulmonary disease (COPD) that led to an extremely invasive biopsy that involved having to collapse and re-inflate one lung and a several days hospital stay. After being examined by the pathologist, this "incidentaloma" turned out to be neither lung nor breast cancer, and recovery from the surgery has been difficult for my friend. And now her oncologist wants to follow her with PET scans every 6 months!
She is wondering if further testing is wise, and I can't say I blame her.
For a very interesting read that I think every cancer advocate should read, check out H. Gilbert Welch's book, "Should I be tested for cancer? Maybe not, and here's why."
-Musa Mayer, Project LEAD® 2000, Clinical Trials Project LEAD® 2006, Quality Care Project LEAD® 2006
Study Design: A review of randomized trials, observational studies, and population-based statistics to estimate the magnitude of cancer overdiagnosis
Overdiagnosis is when a condition is diagnosed that would not have gone on to cause symptoms or death. Cancer overdiagnosis can happen because either the cancer never progresses (and in some cases, regresses) or because the cancer progresses so slowly that the person dies from other causes before the cancer causes symptoms. In order for cancer overdiagnosis to occur, the following two factors are required: a large number of asymptomatic cancers, and activities (such as screening) that lead to early detection.
Fifteen-year follow-up data from the Malmö breast cancer mammography trial conducted in Sweden was used to estimate the magnitude of breast cancer overdiagnosis. The number of breast cancer diagnoses among the screened group and the non-screened group were compared both at the end of the ten-year trial then again, fifteen years later. If screening detected only cancers that would eventually progress and become symptomatic, then one would expect that among the non-screened group, if one waited for enough time, the number of cancer diagnoses would eventually be the same as in the screened group. However, the long-term follow-up data showed that at the end of the trial there were 150 more breast cancers diagnosed in the screened group than in the non-screened group, and after 15 additional years of follow-up there was still a difference of 115 more breast cancers in the screened group. Based on these numbers, it was estimated that almost 25% of breast cancers detected by mammography constitute overdiagnosis.
Similar long-term follow-up data from randomized screening trials was presented estimating that 50% of screen-detected lung cancers constitute overdiagnosis and 60% of PSA-screened prostate cancers are overdiagnosis.
Observational and population-based studies were also included in the review to illustrate that overdiagnosis has occurred in several other cancers as well. Large cohort studies conducted in Japan have shown that spiral CT screening has led to overdiagnosis of lung cancer, and pediatric screening for neuroblastoma has also resulted in overdiagnosis. By comparing rapidly increasing rates of testing and diagnosis to relatively stable cancer death rates, population-based data was also presented indicating the overdiagnosis of thyroid cancer, melanoma, kidney cancer, and renal cancer.
This review highlights overdiagnosis as a significant harm associated with early cancer detection. And while early detection does help some people, the potential harms for others are not well communicated to patients for informed decisionmaking. Patients and their doctors need to better understand the trade-off and magnitude of potential benefits with potential harms for cancer screening. Overdiagnosis could also be reduced by raising the diagnostic threshold for common screening tests.
Click here for access to the original journal article.
Citations
Welch HG, Black, WC. Overdiagnosis in cancer. J Natl Cancer Inst 2010; 102(9):605-613 http://dx.doi.org/doi:10.1093/jnci/djq099
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