GISMa analyses the interval cancer (IS) topic providing guidelines and reference standards in addition to CE recommendations. IC identification is based on Cancer Registries (CR), if existing, or on hospital discharge records, in alternative. The optimal measure of IC frequency (inversely correlated with sensitivity) is the IC proportional incidence (observed IC/carcinomas expected in absence of screening). Other formulas (IC/IC + screen detected cancers; IC rate per 1000 negative screens) look less reliable. IC stage at diagnosis (if available through CR) must be compared with screen detected cancer and cancer detected in non-attenders. Review of mammograms preceding the IC (coded as screening error, minimal signs, or occult) should be done mainly with a blind procedure (IC mixed with negative controls), as this procedure is more representative of the original scenario and more respectful of radiologist's rights.