Objective: Breast cancers detected between screening examinations can influence the sensitivity of a screening programme. Studies of the prognosis of these so-called interval breast cancers show diverging results. We investigated the course of interval breast cancer over time in the Malmö Mammographic Screening Trial (MMST) 1976-86 and the Malmö Mammographic Service Screening Programme (MMSSP) 1990-99.
Material and methods: Stage distribution and survival of interval cancers in MMSSP were compared with screen-detected and non-attender cancer cases in MMSSP, with interval cancers in MMST and with breast cancer cases in a non-screened population five years before the start of MMSSP (pre-screening cancer cases).
Results: In MMSSP 1990-99, the interval cancers did not differ in stage distribution or survival compared with cancer cases in non-attenders, while screen-detected cancer cases had more favourable stage distribution and rate of survival than had the interval cancer cases. The MMST interval cancer cases, 1976-1986, had more favourable stage distribution but higher overall case fatality rate, relative risks (RR) 1.78 (1.00-3.20), and breast cancer case fatality rate, RR 2.05 (1.05-4.00), compared with the more recent MMSSP interval cancer cases. No significant difference in five-year survival was seen in the MMSSP interval cancer cases compared with pre-screening cancer cases not exposed to screening.
Conclusion: In this urban population invited to mammographic screening, the survival rate for women with interval cancer has improved over a period of 20 years. Further studies are needed to assess what factors might explain changes in the course of interval breast cancer.