Introduction Mammography screening aims at early detection of breast cancer so that adequate treatment will eventually lower breast cancer mortality. In a mass screening programme, it is therefore especially important to reach women who have the highest chance of being diagnosed with advanced stage or have the lowest survival rates. 1 4 1 5 6 7 9 10 11 12 We studied whether survival according to SES was affected differentially by the implementation of the screening programme. Methods The Eindhoven Cancer Registry records data on all patients newly diagnosed with cancer in the south–eastern part of the Netherlands, an area with now 2.4 million inhabitants (about 15% of the Dutch population) and only general hospitals. Trained registry personnel actively collect data on diagnosis, staging, and treatment from the medical records after notification by pathologists and medical registration offices. 13 For our analyses we included all patients age 50–69 years diagnosed in 1983–2002 with invasive breast cancer in the eastern part of the registration area (about 1 million inhabitants). This population has been followed-up for vital status up to 1-1-2005. Information on the vital status of all patients was obtained initially from the municipal registries and since 1998 from the Central Bureau for Genealogy. These registers provide virtually complete coverage of all deceased Dutch citizens. 12 11 We calculated distribution of age and stage of disease according to period of diagnosis. 14 T Crude survival analyses were performed. The log-rank test was used to evaluate significant differences between survival curves in univariate analyses. We used Cox regression models to compute multivariate rates. The proportional hazard assumption of the predictor was evaluated by applying Kaplan–Meier Curves. The predictor satisfied the assumption of proportionality as the graphs of the survival function versus the survival time resulted in graphs with parallel curves as did the graphs of the log(−log(survival)) versus log of survival time. The independent prognostic effect of SES was investigated, adjusting for age and stage of disease, and stratified according to period of diagnosis (1983–1990, 1991–1996, 1997–2002). We also calculated the age and stage-adjusted effect of period of diagnosis stratified according to SES. Results Median age was similar for all three periods of diagnosis (59, 60, and 59 years, respectively). P 1 P Table 1 Characteristics of all women age 50–69 years diagnosed with invasive breast cancer between 1983–2002 in Southeastern Netherlands  1983–1990 1991–1996 1997–2002 Total n % n % n % n % TNM I 465 30 642 41 838 45 1,945 39 II 638 42 665 43 805 44 2,108 43 III 278 18 152 10 115 6.2 545 11 IV 115 7.5 68 4.4 65 3.5 248 5.0 unknown 38 2.5 31 2.0 24 1.3 93 1.9 Treatment* S alone 271 18 341 22 249 13 861 17 S + RT 766 50 628 40 658 36 2,052 42 S + RT + ST 305 20 422 27 673 36 1,400 28 S + ST 108 7 123 8 231 13 462 9.4 ST alone 26 1.7 23 1.5 23 1.3 72 1.5 Other 58 3.8 21 1.4 13 0.7 92 1.9 Socio-economic status 1 (low) 336 22 285 18 262 14 883 18 2 325 21 319 20 342 19 986 20 3 308 20 279 18 355 19 942 19 4 154 10 274 18 358 19 786 16 5 (high) 302 20 315 20 414 22 1,031 21 institution# 0 0.0 23 1.5 33 1.8 56 1.1 unknown 109 7.1 63 4.0 83 4.5 255 5.2 Total 1,534  1,558  1,847  4,939 100 * S = Surgery, RT = Radiotherapy, ST = Systemic therapy #institution = care-providing institution such as a nursing home P P P 1 P P Fig. 1 Stage distribution according to socio-economic status and period of diagnosis of patients age 50–69 years with invasive breast cancer in Southeastern Netherlands  2 P P P Fig. 2 Trend in survival according to socio-economic status for all women age 50–69 years diagnosed with invasive breast cancer in Southeastern Netherlands  2 P Table 2 Multivariate regression analysis of survival of breast cancer patients age 50-69 years according to period of diagnosis, Southeastern Netherlands  1983–1990 1991–1996 1997–2002 a 95% CI a 95% CI a 95% CI Age (continuous) 1.03 1.0–1.0 1.04 1.0–1.1 1.01 1.0–1.0 Socio-economic status 1 (low) 1.01 0.8–1.2 1.29 1.0–1.7 2.01 1.3–3.0 2 1.03 0.8–1.3 1.28 1.0–1.7 1.54 1.0–2.3 3 0.95 0.8–1.2 1.18 0.9–1.6 1.53 1.0–2.3 4 0.99 0.8–1.3 1.39 1.0–1.8 1.33 0.9–2.0 b 1.00  1.00  1.00  2 P P P TNM stage b 1.00  1.00  1.00  II 1.75 1.5–2.1 2.33 1.9–2.9 2.00 1.4–2.8 III 3.11 2.5–3.8 4.67 3.5–6.2 5.39 3.6–8.1 IV 9.85 7.6–13 16.0 12–22 16.5 11–24 unknown 2.25 1.4–3.5 2.56 1.4–4.6 4.81 2.3–10 * HR = Hazard Ratio, CI = Confidence Interval a b 3 Table 3 Multivariate regression analysis of survival according to socio-economic status (SES) of breast cancer patients age 50–69 years in Southeastern Netherlands SES 1 (low) 2 3 4 5 (high) a 95% CI a 95% CI a 95% CI a 95% CI a 95% CI Period of diagnosis b 1.00  1.00  1.00  1.00  1.00  1991–1996 0.87 0.7–1.1 0.72 0.6–0.9 0.80 0.6–1.0 0.84 0.6–1.1 0.60 0.5–0.8 1997–2002 0.77 0.6–1.1 0.49 0.4–0.7 0.61 0.4–0.8 0.49 0.3–0.7 0.36 0.2–0.5 2  P P P P P HR = Hazard Ratio, CI = Confidence Interval a b Discussion We found that the proportion of breast cancer patients with a low SES has decreased since the introduction of a mass biennial mammography screening programme with high response rates. Although stage distribution improved for all socio-economic groups, the proportion with advanced disease decreased the most in the highest socio-economic group. In the 1980s survival was similar for all socio-economic groups, but since the introduction of screening the survival of women with a high SES has improved more than that for low socio-economic classes, also after adjustment for age and stage. 15 17 18 2 3 4 13 19 20 21 3 22 20 23 24 6 P 25 26 27 28 29 30 Since the Eindhoven Cancer Registry has recorded comorbidity for all newly diagnosed patients since 1993, we checked whether the prevalence varied across socioeconomic strata. Indeed, the proportion of patients with comorbidity was higher among those with a lower SES (70% of patients in the lowest SES group had one or more concomitant conditions compared to 60% of the high SES group). In particular, the prevalence of diabetes and cardiovascular disease was highest in the low SES groups (diabetes in 10% with low SES and 4% with high SES, cardiovascular disease 7% and 4%, respectively). 31 35 36 36  37 In conclusion, despite a very high participation rate women from lower socio-economic strata clearly benefited less from the introduction of the breast cancer screening programme than those with a lower SES, probably due to a higher prevalence of comorbidity and suboptimal treatment (for both the cancer and the concomitant disease).