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Overdiagnosis is the diagnosis of "disease" that will never cause symptoms or death during a patient's lifetime. Overdiagnosis is a side effect of screening for early forms of disease. Although screening saves lives in some cases, in others it may turn people into patients unnecessarily and may lead to treatments that do no good and perhaps do harm. Given the tremendous variability that is normal in biology, . For a large percentage of them, the most appropriate medical response is to recognize them as something that does not require intervention; but determining which action a particular finding warrants ("ignoring", watchful waiting, or intervention) can be very difficult, whether because the differential diagnosis is uncertain or because the risk ratio is uncertain (risks posed by intervention, namely, adverse events, versus risks posed by not intervening). Overdiagnosis occurs when a disease is diagnosed correctly, but the diagnosis is irrelevant. A correct diagnosis may be irrelevant because treatment for the disease is not available, not needed, or not wanted. Some people contend that the term "overdiagnosis" is inappropriate, and that "overtreatment" is more representative of the phenomenon. Because most people who are diagnosed are also treated, it is difficult to assess whether overdiagnosis has occurred in an individual. Overdiagnosis in an individual cannot be determined during life. Overdiagnosis is only certain when an individual remains untreated, never develops symptoms of the disease and dies of something else. Thus most of the inferences about overdiagnosis comes from the study of populations. Rapidly rising rates of testing and disease diagnosis in the setting of stable rates of the feared outcome of the disease (e.g. death) are highly suggestive of overdiagnosis. Most compelling, however, is evidence from a randomized trial of a screening test intended to detect pre-clinical disease. A persistent excess of detected disease in the tested group years after the trial is completed constitutes the best evidence that overdiagnosis has occurred. Although overdiagnosis is potentially applicable to the diagnosis of any disease, its origin is in cancer screening – the systematic evaluation of asymptomatic patients to detect early forms of cancer.〔Welch HG. Should I Be Tested for Cancer? Maybe Not and Here’s Why. University of California Press (2006 paperback, 2004 hardback)〕 The central harm of cancer screening is overdiagnosis – the detection of abnormalities that meet the pathologic definition of cancer (under the microscope) but will never progress to cause symptoms or death during a patient's lifetime. ==Overdiagnosis and the variability of cancer progression== Cancer screening is the effort to detect cancer early, during its pre-clinical phase — the time period that begins with an abnormal cell and ends when the patient notices symptoms from the cancer. It has long been known that some people have cancers with short pre-clinical phases (fast-growing, aggressive cancers), while others have cancers with long pre-clinical phases (slow-growing cancers). And this heterogeneity has an unfortunate implication: namely, screening tends to disproportionately detect slow-growing cancers (because they are accessible to be detected for a long period of time) and disproportionately miss the fast-growing cancers (because they are only accessible to be detected for a short period of time) – the very cancers we would most like to catch. For more information, see Screening (medicine)#Length time bias. This long-standing model has a hidden assumption: namely, that all cancers inevitably progress. But some pre-clinical cancers will not progress to cause problems for patients. And if screening (or testing for some other reason) detects these cancers, overdiagnosis has occurred. The figure below depicts the heterogeneity of cancer progression using 4 arrows to represent 4 categories of cancer progression. The arrow labeled "Fast" represents a fast-growing cancer, one that quickly leads to symptoms and to death. These are the worst forms of cancer and unfortunately often appear in the interval between screening tests. The arrow labeled "Slow" represents a slow-growing cancer, one that leads to symptoms and death but only after many years. These are the cancers for which screening has arguably the greatest beneficial impact. The arrow labeled "Very Slow" represents a cancer that never causes problems because it is growing very slowly. If a cancer grows slowly enough, then patients will die of some other cause before the cancer gets big enough to produce symptoms. This is particularly likely with small cancers in the elderly – prostate cancer in older men serves as the most prominent clinical example. The arrow labeled "Non-progressive" represents a cancer that never causes problems because it is not growing at all. In other words, there are cellular abnormalities that meet the pathologic definition of cancer but never grow to cause symptoms – alternatively, they may grow and then regress. Although the concept of non-progressive cancers may seem implausible, basic scientists have begun to uncover biologic mechanisms that halt the progression of cancer.〔Mooi WJ, Peeper DS. Oncogene-induced cell senescence--halting on the road to cancer. ''N Engl J Med''. 2006;355:1037-46〕〔Folkman J, Kalluri R. Cancer without disease. ''Nature''. 2004;427:787.〕〔Serrano M. Cancer Regression by Senescence. New Engl J Med 2007 356:1996-97.〕 Some cancers outgrow their blood supply (and are starved), others are recognized by the host's immune system (and are successfully contained), and some are not that aggressive in the first place. Overdiagnosis occurs when either "Non-progressive" cancers or "Very Slow" growing cancers (more precisely, at a slow enough pace that individuals die from something else before the cancer ever causes symptoms) are detected. These two forms of cancer are collectively referred to as pseudodisease - literally false disease. Since the word "disease" implies something that makes, or will make, a person feel sick (something that causes symptoms), pseudodisease is an appropriate word for describing these abnormalities. Thus, another definition of overdiagnosis is simply the detection of pseudodisease. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「overdiagnosis」の詳細全文を読む スポンサード リンク
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