Introduction Interval cancer (IC) rate, risk factors and biological characteristics have been scarcely evaluated. In January 2010 started a multicentric project with the aim to estimate rates and determinants of IC and to compare their characteristics with those detected in the routine screening mammography.

Methods Information from women aged 45/50 to 69, participating in seven population-based screening programs in Spain from January 2000 to December 2006, was collected. The cohort was followed-up to June 2009 for cancer (IC and screening) identification. Three protocols were defined: (1) To describe the joint database format and architecture, and for the achievement of variables, including women and tumour-related data (prognostic factors and biomarkers expression); (2) To unify IC diagnosis criteria and identification process; (3) A radiological protocol for mammogram's review and IC classification, based on European Guidelines.

Results A database with information of 1 350 058 mammographies from 759 604 screened women has been built. 94.2% were analogical and 5.8% digital mammographies. A total of 4656 screen-detected cancers and 1479 IC have been identified. For IC identification, which is not finished, databases of screening programs have been linked with poblational and hospital-based cancer registries and data from active case search.

Conclusions The database will allow us to evaluate women's and screening determinants for IC (including true interval and false negatives cancers), and to compare tumour characteristics between IC and screen-detected cancers. This information will be useful to improve the efficiency of screening programs and the breast cancer classification, and could offer new insights on cancer prevention for specific subsets of women.