Incidence and tumor characteristics of breast cancer diagnosed before and after implementation of a population-based screening-program

Background. Randomized controlled trials and service screening programs have shown that breast cancer screening reduces the mortality from the disease. Several years of monitoring are needed to prove such an effect. In the meantime attention should be paid to early surrogate measures, such as histopathological tumor characteristics. The Norwegian Breast Cancer Screening Program started November 1995. This study compares incidence, prognostic tumor characteristics and surgical treatment in breast cancer cases diagnosed in the pre-screening (1987-1995, n = 2 618) and screening period (1996-2004, n = 5417), in women aged 50-69 years, residing in the first four counties implementing the screening program. The screening period is divided into those invited versus those not invited to the screening program, and those exposed (participants) versus those not exposed to the program (non-participants). Results. The incidence of invasive breast cancer rose from 170 per 100 000 women years (wy) in 1987 to 355 per 100 000 wy in 1997. The proportion of DCIS was 5% in the pre-screening period, and 14% in the screening period. Tumors 20 mm or less were diagnosed in 56% of the invasive cases in the pre-screening period, in 74% of the invited, and in 77% of the exposed women. The relative risk of diagnosing breast cancer with metastases was 0.85 (95% CI 0.84-0.87) for invited and 0.82 (95% CI 0.81-0.84) for exposed women, relative to those diagnosed in the pre-screening period. Ablation was performed in 85% of the invasive cases diagnosed in the pre-screening period, and in 45% of the cases in the screening period. Conclusion. Breast cancer diagnosed in the screening period had prognostically favorable tumor characteristics compared to breast cancer diagnosed in the pre-screening period. Implementation of organized population based screening and the time trend are considered possible reasons.