What is overdiagnosis and why does it matter?
Overdiagnosis in cancer screening means finding a cancer that would never have caused symptoms or death during a person's lifetime if it had not been detected by screening. This is not a false positive—it is a real cancer, but one that is so slow-growing or indolent that it would have remained harmless. The harm comes from unnecessary treatment (surgery, radiation, chemotherapy) and the anxiety and side effects that go with it, without any benefit to the patient. For example, in lung cancer screening with low-dose CT, the odds of overdiagnosis for screened individuals compared to non-screened ranged from 7.6 to 17.1 times higher, depending on how strictly overdiagnosis was defined [5]. This means that for every person whose life is saved by early detection, many more may be treated for a cancer that never needed treatment.
Does overdiagnosis outweigh the benefits? The evidence is mixed and depends on the cancer.
For breast cancer screening, the balance is close. A 2023 cohort study of over 41,000 women in Australia found that for every 1,000 women screened from age 50, there were 20 fewer breast cancer deaths but 25 more cancers diagnosed (overdiagnosis) by age 85 [1]. That means about 21% of screen-detected breast cancers were overdiagnosed. The benefit and harm are nearly equal in number, though the benefit is in deaths prevented and the harm is in unnecessary diagnoses and treatment. A simulation study from Brazil found that the most efficient screening protocol (biennial from ages 60-69) prevented almost three times more deaths than screening from ages 50-59, but even then, the reduction in breast cancer mortality was only half of what was seen in the UK [4]. This suggests that the benefit-harm ratio can vary by country and screening protocol.
For prostate cancer screening, the harm from overdiagnosis is larger in absolute numbers. One study estimated that from 1986 to 2016, 1.5 to 1.9 million U.S. men were overdiagnosed with prostate cancer, and 0.9 to 1.5 million were overtreated [7]. However, the same study found that the harm-benefit tradeoff was more favorable for Black men: the number needed to diagnose to prevent one prostate cancer death was 8-12 for Black men versus 11-14 for all men [7]. This means that screening is more valuable in higher-risk groups. A Finnish trial estimated overdiagnosis rates from 2.3% to 15.4% depending on the birth cohort, but noted that long follow-up is needed to see the full impact [2]. Importantly, most patient decision aids for prostate cancer screening fail to communicate the risk of overdiagnosis clearly—only 17% reported its frequency [3].
For lung cancer screening, the evidence points to significant overdiagnosis, especially in certain groups. A Taiwanese study found that as low-dose CT screening volume increased, the number of stage 0 lung cancers (the strictest definition of overdiagnosis) rose sharply, while late-stage cancers (stage 4) did not decrease [5]. The impact was greater in women than men, especially women under 40 [5]. This suggests that screening is finding many early cancers that may never progress, without reducing the number of advanced cancers.
What does this mean for someone considering screening?
The key takeaway is that the decision to screen is not a simple 'yes, it saves lives' or 'no, it causes harm.' The balance depends on your personal risk factors, the type of cancer, and the screening method. For example, a 2022 review found that most prostate cancer screening decision aids do not include enough information on overdiagnosis—only 17% gave a frequency, and only 5% mentioned how common undiagnosed, harmless prostate cancer is in the general population [3]. This means many men are making decisions without full understanding. A 2021 survey of Norwegian women found that while 92% knew screening reduces breast cancer death, only 15% could correctly define overdiagnosis [6]. This knowledge gap is a problem because informed consent requires understanding both benefits and harms.
The evidence here suggests that for breast cancer, the benefit and harm are roughly balanced in number, but the harm is in unnecessary treatment, not death. For prostate cancer, overdiagnosis is more common, but the benefit is larger for Black men. For lung cancer, overdiagnosis is a real concern, especially in women and non-smokers. The best approach is to discuss your individual risk with your doctor and ask specifically about the chance of overdiagnosis for the screening test you are considering.
About These Sources
This answer is built on 7 peer-reviewed studies — published from 2021 to 2024, 2 from 2024 or later, 2 in Q1 journals, collectively cited 88 times — selected as the most relevant from 7 studies that passed quality screening, drawn from 42 papers retrieved from a database of over 500 million.
Sources used in this answer
Benefits and harms of breast cancer screening: Cohort study of breast cancer mortality and overdiagnosis
In a cohort of 41,330 women, for every 1,000 screened from age 50, there were 20 fewer breast cancer deaths and 25 more cancers diagnosed (overdiagnosis) by age 85; 21% of screen-detected cancers were overdiagnosed [1].
Estimating the rate of overdiagnosis with prostate cancer screening: evidence from the Finnish component of the European Randomized Study of Screening for Prostate Cancer
Using Finnish trial data from 80,149 men, overdiagnosis rates from prostate cancer screening ranged from 2.3% to 15.4% across birth cohorts, with long follow-up needed to see the full impact [2].
Including Information on Overdiagnosis in Shared Decision Making: A Review of Prostate Cancer Screening Decision Aids
In a review of 41 prostate cancer screening decision aids, only 17% reported the frequency of overdiagnosis, and only 5% mentioned the prevalence of undiagnosed prostate cancer in the general population [3].
Harms and benefits of mammographic screening for breast cancer in Brazil.
A simulation study in Brazil found that biennial screening from ages 60-69 was the most efficient protocol, with nearly three times more deaths avoided than screening from ages 50-59, but the mortality reduction was half that of the UK [4].
Impact of annual trend volume of low-dose computed tomography for lung cancer screening on overdiagnosis, overmanagement, and gender disparities
In a Taiwanese study of 4,971 lung cancer cases, increasing LDCT screening volume led to a rise in stage 0 cancers (overdiagnosis) without reducing stage 4 cancers; the odds of overdiagnosis for screened vs. non-screened ranged from 7.6 to 17.1, with a greater impact in women [5].
Women’s conceptual knowledge about breast cancer screening and overdiagnosis in Norway: a cross-sectional study
In a survey of 1,892 Norwegian women, 92% knew screening reduces breast cancer death, but only 15% could correctly define overdiagnosis, indicating a significant knowledge gap [6].
Harm-to-Benefit of Three Decades of Prostate Cancer Screening in Black Men
Estimating from U.S. data, 1.5-1.9 million men were overdiagnosed with prostate cancer by 2016; the number needed to diagnose to prevent one death was 8-12 for Black men vs. 11-14 for all men, showing a more favorable harm-benefit tradeoff for Black men [7].