Screening is more effective in reducing colorectal cancer than previously thought

CANCER DIGEST – Jan. 5, 2024 – Colorectal cancer screening is substantially more effective at reducing rates of the disease than previously thought, a new analysis of existing data shows.

Studies following up with patients after undergoing screening for colorectal cancer had consistently shown that the number of participants who subsequently developed colorectal cancer was in the neighborhood of .1 percent. A new analysis of multiple screening trials has shown the number of subsequent cancers was more like .5 percent, or about twice the effect previously thought.

The difference says lead study author Josh Angrist, an econometrician at the Massachusetts Institute of Technology is due to not accounting for 'nonadherence'.

"Nonadherence is widespread in randomized clinical trials, especially those offering relatively unpleasant interventions like CRC (colonoscopy or sigmoidoscopy) screening," Angrist said in a press release. "Offers of a free colonoscopy are not always taken up with enthusiasm."

Angrist shared the Nobel Prize in Economics with David Card of the University of California at Berkeley and Guido Imbens of Stanford University for their work on econometric tools in 2021.

Angrist’s re-analysis of the data from five major colorectal cancer screening trials appears in the Dec. 15, 2023 journal, Proceedings of the National Academy of Sciences.

By taking into account the number of participants who for whatever reason chose not to get screened, and including them in the statistical calculations, Angrist found the actual percentage of people who developed cancer after screening over a 10-year period fell to just 0.5 percent.

The new analysis also resolved a puzzle of why the cancer rates post screening varied so much from study to study. Non-adherence caused the number of participants who actually got screened to vary from 42 percent to as high as 87 percent. Taking that into account showed that the rates actually aligned across studies.

Angrist and Hull concluded their paper by calling on researchers to make their type of analysis a routine part of clinical research.

“If you want to encourage a reluctant colonoscopy patient, you shouldn’t tell them the effect of being invited to screen, you should tell them the effect of actually being screened,” Angrist said. “And that’s a much bigger number.”

Sources: MIT News and the Proceedings of the National Academy of Sciences.