Interval cancers in the Norwegian breast cancer screening program: Frequency, characteristics and use of HRT

Abstract

Key words: mammography screening; interval cancer; breast density; HRTAs a result of the mortality reduction found in several randomized controlled clinical trials, 1,2 breast cancer screening programs have been implemented in several western countries. 3 The Norwegian Breast Cancer Screening Program is a population-based program that offers women aged 50 -69 years biannual mammography screening. The program started as a pilot project in 4 counties in 1996, and the results from the first screening round were reported earlier. 4 Interval cancers can be defined as breast cancer diagnosed in screened women during the interval between 2 screening rounds, where the diagnosis does not follow from the screening examination. 5 The benefit of attending a screening program is reduced for women diagnosed with interval cancer. Thus the effect of a screening program in terms of mortality reduction can diminish if the frequency of interval cancer is high. This frequency depends, among other factors, on sensitivity of the test and length of the screening interval.Most interval cancers are diagnosed because of symptoms, hence interval and screening-detected cancers might be expected to have different characteristics. High breast density and use of hormone replacement therapy (HRT) have been discussed as possible contributors to reduction of screening sensitivity, and this may be related to occurrence of interval cancers. 6 -11 The purpose of our study was to quantify the interval cancers in the pilot project and to explore characteristics and factors that may be associated with interval cancer. To study the relations between use of HRT, breast density and interval cancer, data from a blinded review (breast density) and a questionnaire (use of HRT) were linked. MATERIAL AND METHODS Screening populationThe Norwegian Breast Cancer Screening Program is population based and covers women aged 50 -69 years. The screening population is identified through the Central Population Register, in which each inhabitant in Norway has a unique 11-digit number. Women are invited into the Norwegian Breast Cancer Screening Program irrespective of previous breast cancer history. The screening interval is 2 years. Two-view mammograms and independent double reading are standard. The pilot project included 4 counties with about 40% of the Norwegian female population aged 50 -69 years. During the first round (1996-97), 159,887 women born from 1927-46 were invited and 127,064 women were examined (79.5%). 4 Study subjectsWomen who had their first screening examination within the pilot project after July 31, 1998 (n ϭ 411) and women who experienced recurrence in the same breast as a previous lesion (n ϭ 6) were excluded from the study. Women who received a cancer diagnosis in the first screening round (n ϭ 854) were not included as being at risk of interval cancer. However, women who were screen-positive but cleared after assessment were included. This left 125,793 women at risk of developing interval cancer. The women were followed until the date an in...

Methodological issues in international comparison of interval breast cancers

“…[7][8][9][10][11][12][13][14][15][16] A metaanalysis of population-based programmes showed a 6-fold variation in the level of IC in the first year after mammographic screening. 17 However, several important issues affect the estimation of IC frequency and, consequently, their comparability and interpretation.…”

Different types of postmenopausal hormone therapy and mammographic density in Norwegian women

“…Wang et al found a significant relationship between ever-use of postmenopausal hormone therapy (HT) and mammographic density, but no information on the type of HT was available. 15 On the basis of the differences in HT formulas described, more knowledge about how different types of HT effect mammographic density could increase the understanding of the etiology of breast cancer.…”

Episode sensitivity in association with process indicators in the Finnish breast cancer screening program

“…Only few service screening programs have reached performance and sensitivity figures reported from the randomized studies. [6][7][8][9] There has also been variation in the performance between and within screening programs in different countries. [10][11][12] In Finland, mortality impact of the mammography program has been demonstrated earlier.…”