Owing to imperfect information, overdiagnosis cannot be predicted with certainty before the fact . Overdiagnosis is inferred . The fact that overdiagnosis is inferred places unique challenges on the methodologies for its detection. A consequence of the nature of methodology to detect overdiagnosis is that overdiagnosis is inferred in populations, although overdiagnosis affects individuals. Overdiagnosis is viewing diagnosis from 29,000 feet over an extended period of time.
Overdiagnosis is suspected when a simple algebraic relationship is not met: Incremental increase in detection of disease = Incremental improvement in disease-specific outcomes.
Once secular trends for an increase in the incidence of disease have been accounted for, if we are seeing more disease than have improved outcomes by a commensurate degree, then the disease has been overdiagnosed.
The degree of evidence for overdiagnosis ranges from incontrovertible to circumstantial. The plausibility of overdiagnosis hinges on certain factors. There must be a disease reservoir, which is to say a pool of disease which is clinically asymptomatic. There should be a spectrum of disease. When there is a spectrum of aggressiveness, it follows that disease can be aggressive or indolent or that disease in some is more equal than in others.
A reservoir for asymptomatic cancer is known to be present for many cancers. Autopsies of the prostate in men who have not died from prostate cancer show cancer in many 80-year-olds and even in many 30-year-olds . Pathologic examination of the thyroid gland shows high prevalence of papillary carcinoma of the thyroid gland .
In the study, not only did pathologists uncover thyroid cancer but also found that they were detecting cancers smaller than the tissue slices examined . The tissue slices were 2.5-mm thick. The investigators correctly reasoned that the slices were missing cancer. If the slices were even thinner, the pathology equivalent of our spatial resolution, even more cancer could be picked up, which means that the yield for incidental thyroid cancer depends on the vigor of its search.
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