Invitation for FOBt screening and colorectal cancer mortality: A prospective analysis in the Million Women Study cohort

Using linked data from the Million Women Study (MWS) cohort and the NHS Bowel Cancer Screening Programme (NHS BCSP) offering biennial guaiac faecal occult blood test (gFOBt) screening from 2006, we examined factors associated with screening acceptance, and differences in colorectal cancer (CRC) mortality by screening invitation status. Characteristics of attenders and non-attenders were compared among 752,007 MWS participants born 1940-1950, who were all invited for at least one round of routine screening. Women declining screening had higher deprivation and smoking levels, and a 2-fold risk of all-cause and CRC mortality compared with women who accepted. Of 246,160 women born in 1935-1939, 111,956 were assigned to the "no intention to invite" group, and 134,204 to the "intention to invite" group based on year of birth and postcode sector, with an average of 0.01 and 2.40 screening invitations in each group, respectively. During a mean follow up of 11.9 years, there were 858 and 791 CRC deaths in the "intention to invite" and "no intention to invite" groups, respectively. In the period 4 or more years after study entry there was no significant reduction in risk of death from CRC associated with invitation for screening (RR = 0.94, 95%CI 0.83-1.06), but evidence of differences in associations by anatomical sub-site, with a reduction in deaths from distal colon cancer (0.64, 0.47-0.88), but not proximal (1.02, 0.83-1.25) or rectal cancer (0.97, 0.79-1.20) (p-value for heterogeneity by subsite = 0.05). Investigation of the effectiveness of current bowel screening methods using faecal immunochemical testing (FIT) by sex and cancer sub-site is warranted.