Colon cancer screening more effective than previously thought

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A recent study, co-authored by MIT economist Josh Angrist, has revealed new insights into the effectiveness of colon cancer screening.

The study, which revisits data from five randomized trials, suggests that screening for colon cancer significantly reduces cancer rates, more than previously believed based on earlier analyses.

In these trials, about 1% of participants were found to develop colon cancer within a decade. However, the new analysis indicates that screening can reduce this rate by approximately 0.5%, double the impact estimated in earlier studies.

Earlier research suggested that the effect of screening was around a quarter of a percentage point, but Angrist’s study finds a more substantial reduction.

The critical difference in Angrist’s approach lies in focusing on getting screened rather than just being invited for screening.

This distinction is vital because many trial subjects choose not to participate in screening methods like colonoscopy or sigmoidoscopy, a phenomenon Angrist calls “nonadherence”. Prior studies did not adequately account for this factor.

The research, published in Proceedings of the National Academy of Sciences, was co-authored by Angrist and Peter Hull, a professor of economics at Brown University.

Their paper, titled “Instrumental Variable Methods Reconcile Intention-To-Screen Effects Across Pragmatic Cancer Screening Trials,” highlights the importance of understanding the difference between being offered a screening and undergoing the procedure.

The study analyzed data from five major randomized clinical trials that used either sigmoidoscopies or complete colonoscopies.

The trials had varying levels of actual screening among participants, ranging from 42% to 87%, highlighting the issue of nonadherence.

Angrist and Hull employed an econometric method known as “instrumental variables” to adjust for this nonadherence and to accurately measure the effect of screening on those who actually underwent it.

Their findings not only demonstrate a larger effect of screening but also resolve inconsistencies seen across different trials. The instrumental variable estimates from all five trials showed a fairly consistent decrease in cancer incidence among those screened.

Angrist, who won the 2021 Nobel Prize in Economics for his work in econometrics, advocates for the broader adoption of instrumental variable analysis in clinical research.

He emphasizes that the actual benefit of screening, as shown in their study, is a more compelling statistic for encouraging screening among individuals.

This study underscores the significant impact of colon cancer screening and the importance of considering actual participation in such screenings in clinical trials.

It offers a more optimistic view of the potential for colon cancer prevention and highlights the need for improved strategies to increase screening adherence.

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The research findings can be found in PNAS.

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