This article summarizes the phenomenon of cancer overdiagnosis—the diagnosis of a “cancer” that would otherwise not go on to cause symptoms or death. We describe the two prerequisites for cancer overdiagnosis to occur: the existence of a silent disease reservoir and activities leading to its detection (particularly cancer screening). We estimated the magnitude of overdiagnosis from randomized trials: about 25% of mammographically detected breast cancers, 50% of chest x-ray and/or sputum-detected lung cancers, and 60% of prostate-specific antigen–detected prostate cancers. We also review data from observational studies and population-based cancer statistics suggesting overdiagnosis in computed tomography–detected lung cancer, neuroblastoma, thyroid cancer, melanoma, and kidney cancer. To address the problem, patients must be adequately informed of the nature and the magnitude of the trade-off involved with early cancer detection. Equally important, researchers need to work to develop better estimates of the magnitude of overdiagnosis and develop clinical strategies to help minimize it.
Early detection has forced clinicians and researchers to contemplate a more expansive and, to many, counterintuitive definition of the word “cancer.” What most of us were taught in medical school is captured by the terse definition contained in the medical dictionary—“a neoplastic disease the natural course of which is fatal”. It was a simple definition that was largely accurate in an era when patients were diagnosed with cancer because they had signs and symptoms of the disease.
But that all changed after we became technologically able to advance the time of diagnosis and detect cancer early—before it produces signs and symptoms. Now it has become evident that the word “cancer” encompasses cellular abnormalities with widely variable natural courses: Some grow extremely rapidly, others do so more slowly, others stop growing completely, and some even regress. Clinicians are left with the realization that the word “cancer” is less a prediction about disease dynamics and more a pathological description made at a single point in time. Continued adherence to the dictionary definition of cancer, however, can lead to harm—including overuse of anticancer therapies.
Although not yet contained in medical dictionaries, recently, a new word has appeared in the medical literature to describe a side effect of our technological progress: “overdiagnosis.” This article is intended to summarize the phenomenon.