Effect of organised mammography screening on stage-specific incidence in Norway: population study

Abstract

Background:We aimed to estimate the effect of organised mammography screening on breast cancer stage distribution by comparing changes in women eligible for screening, based on birth cohort, to the concurrent changes in younger, ineligible women.Methods:In an open cohort study in Norway, which introduced national mammography screening county-by-county from 1995 to 2004, we identified women (n=49 883) diagnosed with in situ or invasive breast cancer (ICD10 codes: D05 or C50) during the period 1987–2011 and born between 1917 and 1980. We estimated relative incidence rate ratios (rIRRs) comparing the development in the screening vs historic group to the younger vs younger historic group.Results:Including the compensatory drop, eligible women experienced a 68% higher increase in localised cancers (rIRR=1.68, 95% confidence interval (CI): 1.51–1.87) than younger women, while the increase in incidence of advanced cancers was similar (rIRR=1.11, 95% CI: 0.90–1.36). Excluding the prevalence round, eligible women experienced a 60% higher increase in localised cancers (rIRR=1.60, 95% CI: 1.42–1.79), while the increase in incidence of advanced cancers remained similar (rIRR=1.08, 95% CI: 0.86–1.35).Conclusions:Introduction of organised mammography screening was followed by a significant increase in localised and no change in advanced-stage cancers in women eligible for screening relative to younger, ineligible women.

The impact of mammography screening programmes on incidence of advanced breast cancer in Europe: a literature review

“…A stable trend was reported by one of the Italian studies [29]. A percent decrease of incident breast cancers with missing stage information was observed in other two Italian studies [15, 30], in the Swedish study [33], in three Norwegian studies [35, 38, 39], and in a study from Denmark [8]. In two of these, the resulting bias was adjusted for in the design [15] and, respectively, in the analysis [33].…”

“…Kalager et al observed a significant IRR (Stage III+ cancer) of 0.76, but the same figure was found in the not-yet invited population before screening [36]. Also, the reduction was confirmed by a second study but in association with an increase for Stage II cancer [39]. Others reported the opposite, that is, a decrease for Stage II cancer and an increase for Stage III cancer [19].…”

“…Because there is a latency for the effect of screening on ABCR to take place, at any point in time there are women (i.e., new quinquagenarians, new immigrants, and late attendees) with insufficient exposure time.The effect of screening on ABCR is underestimated, due to a disproportionate influence of prevalence screens.All studies3Pace of implementationPublic health screening programmes are implemented gradually, in a markedly stepwise fashion, since large populations are divided in distinct administrative units each targeted by an independent local plan of action.The effect of screening on ABCR is diluted. Until implementation is completed, there are women who are diagnosed with breast cancer before being invited, and who greatly contribute to ABCR.[8, 14, 15, 19, 29, 30, 32, 33, 36–39, 44]4Prevalence effectThe prevalence screen may be associated with a transient increase in ABCR [13]. During a stepwise implementation of the programme, when the time elapsed from the start is theoretically sufficient to see a decrease in ABCR, this is counteracted by an opposite effect due to newly enrolled women – especially if invitations increase over time.[8, 14, 15, 19, 29, 30, 32, 33, 36–39, 44]5Reference incidence (i)The reference (or underlying) incidence rate, with which to compare the rate observed after the introduction of screening, is not known with precision [49].…”

“…The increase in node-positive disease is likely to be population-specific and will depend on the rate of change of local surgical policy. However, reductions in node-positive disease as a results of screening are likely to be underestimated rather than overestimated due to the stage migration.[12–14, 19, 29–43]9Missing data on tumour stageWhatever staging system is being used, the introduction of a screening programme tends to bring an improved quality of breast cancer registration, with a sharp decrease in the proportion of unknown-stage cancers.Because more cases are increasingly placed in all known-stage categories, an apparent increase in all stage-specific rates occurs – including ABCR.[8, 15, 30, 32, 33, 38, 39]10Statistical approachThe statistical approach is not standardised, and includes the provision of purely descriptive information and the use of methods which are difficult to interpret, such as joinpoint analysis.Descriptive information does not allow evaluation of the magnitude and significance of observed changes in ABCR. Methods like the joinpoint analysis are useful for assessing the points in time when ABCR begins to decrease and when it stabilizes, but may be misleading when used to assess the significance of the trend.…”

Is the incidence of advanced‐stage breast cancer affected by whether women attend a steady‐state screening program?

“…[1][2][3][4][5] Observational studies have also looked for possible changes in the incidence of advanced-stage cancer after introducing a screening program; although early stage disease tends to have increased in these studies, advanced stage disease has variously been shown to decrease significantly, [20][21][22]37 slightly, 12,18 or not at all. 23,24,38 However, none of these studies has been able to link tumor stage to actual attendance at the individual patient level, and have instead relied on approximations. This lack of information on actual attendance means that tumors could have been detected in eligible nonattenders outside the screening program.…”