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Audit, research and management
Published in Marwan Habiba, Andrea Akkad, Justin Konje, MRCOG Part 2, 2017
Marwan Habiba, Andrea Akkad, Justin Konje
C. Screening bias is a type of selection bias which occurs in screening programmes; those who comply with a screening programme tend to be generally healthier and at a lower risk compared to non-participants. There are a number of reasons for this, including socio-demographic factors. There is a risk that the better outcome noted in participants be wrongly attributed to the screening programme. Length, or length-time, bias occurs in diseases which present across a broad spectrum of biologic activity; some patients have aggressive, rapidly growing tumours with a short asymptomatic phase, whilst others have less aggressive tumours with longer latency and an inherently better prognosis. These less aggressive tumours will be more likely to be identified in a screening programme, and, even without therapy, the cohort identified by screening will have a better prognosis. Lead-time bias occurs when the disease has a long asymptomatic period which is not taken into account. Here, it is possible that screening may result in early diagnosis, but the natural history remains unaltered, for example because of the absence of effective intervention.
Lung Cancer Screening: Past, Present, and Future
Published in Phillip M. Boiselle, Charles S. White, New Techniques in Cardiothoracic Imaging, 2007
Charles S. White, Phillip M. Boiselle
Overdiagnosis occurs if cancers that are indolent are disproportionately detected by lung cancer screening. Patients with slow-growing tumors would have a prolonged disease course that would favorably affect 5-year survival data. Indolent cancers would not be as likely to be detected in the control group because they would remain asymptomatic for an extended period of time and the patient might succumb to other illness. If such indolent cancers were disproportionately found in the screened population, a survival advantage but no mortality benefit would be shown for this population. Length-time bias is a related bias but describes detection of indolent cancer over a more limited time frame than overdiagnosis.
Colorectal Cancer Screening
Published in Jim Cassidy, Patrick Johnston, Eric Van Cutsem, Colorectal Cancer, 2006
That screening is a useful strategy may seem obvious, but the process of screening is associated with inherent biases that inevitably make screen-detected disease appear to have a better prognosis than symptomatic disease whether or not the screening process has had a true effect on outcome. These biases are three in number: length-time bias, lead-time bias, and volunteer bias. Length-time bias arises from the fact that intermittent screening tests will tend to pick up slow-growing, indolent disease that is likely to have a better prognosis than the rapidly advancing disease, which is more likely to appear with symptoms between screening intervals (Fig. 1). Lead-time bias arises from early diagnosis itself; this will always lead to an apparent improved duration of survival merely by shifting the point of diagnosis forward and not necessarily by improving survival (Fig. 2). Volunteer bias is created by the fact that screening invitations tend to be accepted by health-conscious individuals who are likely to have a better outcome for reasons other than early detection of the tumor. The collective effect of these biases is to exaggerate the beneficial effect of screening, and to ensure that screening is producing a real benefit it is essential to carry out population-based randomized trials in which the group randomized to screening is analyzed as a whole and includes those who refuse the invitation to be screened and those who develop cancers that are not detected by screening. Only if the disease-specific mortality in the whole of this group is significantly lower than in the randomly selected group that is not offered screening can we be sure that the screening process is producing a truly beneficial effect.
Epidemiology and prevention of oesophageal adenocarcinoma
Published in Scandinavian Journal of Gastroenterology, 2022
Due to the increased risk of malignant transformation, BO patients are enrolled in surveillance programs to detect dysplasia or early adenocarcinoma [7–9,38]. If no dysplasia is found, the BO patients are followed every 3–5 years, while more rigorous follow-up or endoscopic treatment are indicated if dysplasia is present. BO patients in surveillance programs are diagnosed with an earlier tumour stage and has improved survival [39–43]. However, earlier detection (lead time bias) and detection of slow progressing tumours (length time bias) might explain some of the apparent survival benefit of surveillance [43,44]. Moreover, a large proportion (>85%) of patients diagnosed with OAC never had a diagnosis of BO, so surveillance have little impact on the overall incidence and prognosis of OAC [20,40,45,46].