Background
Early detection of breast cancer (BC) through organized screening reduces mortality and improves treatment outcomes. In Flanders (Belgium), the population-based breast cancer screening program reduces BC-specific mortality by 51%, yet participation remains suboptimal, particularly among socioeconomically underserved groups, with participation rates averaging below 40%.

Objective
To address this participation gap, a culturally and linguistically tailored reminder letter was developed and tested as an intervention to increase BC screening participation.

Methods
A reminder letter was co-created through a three-month iterative process involving focus group discussions with women of low socioeconomic status living in Flanders (n = 33) and a Delphi panel of sector experts (n = 10). Compared with the standard invitation, the final reminder featured simplified, culturally adapted text and visuals, and a QR code providing translations in 12 languages. The effectiveness of the reminder among non-responders was evaluated in a randomized controlled trial (RCT) conducted between April and June 2024. In total, 7,922 socioeconomically non-attenders were randomized 1:1 to receive either the standard invitation alone (control) or the standard invitation plus the tailored reminder (intervention).

Results
Overall, 1,071 participants (13.5%) attended screening within 40±4 days. According to the Intention-to-Treat analysis, there was a 6.4% absolute increase in participation in the intervention group (624/3,954; 15.8%) compared to the control group (373/3,968; 9.4%), OR 1.81 (95%CI 1.58-2.07), p<0.001). Within this population, 27% (n = 2,037) were classified as socioeconomically underserved, as defined by eligibility for increased reimbursement of medical expenses. Stratified results indicated a larger increase in participation in the intervention group among low SES individuals (+8%, from 4.0% to 12.0%; OR 3.24, 95% CI 2.27–4.61; p < 0.001) compared to non-low SES individuals (+5.2%, from 11.7% to 16.9%; OR 1.53, 95% CI 1.31–1.79). These effects were consistent across age groups, provinces, and radiology units, and were robust in both intention-to-treat and per-protocol analyses.

Conclusions/Implications
Individuals living in socioeconomically disadvantaged circumstances often face competing demands and structural constraints that limit participation in preventive screening. In this context, even well-designed reminder interventions should be viewed as one component of a broader, multifaceted strategy to reduce socioeconomic inequities in cancer screening uptake. Nevertheless, despite pragmatic limitations in the study design, the present reminder strategy demonstrated strong potential to address socioeconomic disparities in organized breast cancer screening, offering a scalable and feasible approach to increasing participation. Importantly, screening uptake also increased overall, highlighting that equity-oriented strategies can generate benefits across the entire screening population.