Invasive interval cancers in the Ostergötland Mammographic Screening Programme: radiological analysis

The aim of this study was to find out by radiological analysis whether it is possible to prevent interval cancers in screening programmes without a concomitant increase in false positives. Re-scrutiny of all mammograms of 544 interval cancers was implemented. Blinded re-scrutiny of screening mammograms of 390 interval cancers mixed with other cases (prevalence of detectable cases 37 %) by two outside radiologists was also carried out. True interval cancers constituted 49 %, missed tumours 25 %, occult tumours 10 % and unclassifiable cases 16 %. There were no significant differences in survival between true interval cancers, overlooked or misinterpreted cancers, and mammographically occult cancers. In the blinded re-scrutiny both examiners reached a positive predictive value for malignancy of 67 %, and a sensitivity of 69 % and 81 %, respectively. In a screening programme such as this, it was estimated that 100-400 additional women would have to be recalled from screening for further workup in order to detect each additional cancer which would otherwise emerge as an interval cancer. The occurrence of interval cancer increased with age and was influenced by the design of screening. The proportion of patients with potential iatrogenic delay in diagnosis (overlooked and misinterpreted cancers) was equivalent to figures from other studies. There were no significant differences in survival between the various categories of interval cancer. The study confirmed the association of criteria for referral for further investigation with the number of false positives. Efforts to reduce the number of interval cancers by lowering the threshold for recall are likely to be counterproductive.