Examining screening mammography participation among women aged 40 to 74

It is estimated that in 2016 breast cancer was the cause of death for 4900 Canadian women and was diagnosed in 25 700 women—representing more than a quarter (25.8%) of new cancer cases in women.¹ Women aged 50 to 69 are estimated to have accounted for a little more than half of new cases (51.0%).¹ Modifiable factors associated with breast cancer include alcohol consumption, smoking, hormone replacement therapy, and postmenopausal obesity.^2–4 Biologic factors associated with increased risk of breast cancer include older age, having a first-degree relative with breast cancer, reproductive factors, genetic predisposition, and clinically diagnosed denser breast tissue.^3–5

Screening mammography represents a secondary preventive strategy against breast cancer death and is indicated for asymptomatic women within a certain age range, often between the ages of 50 and 69.⁶ Its aim is to reduce breast cancer mortality.⁶ However, screening mammography can also lead to false-positive results, overdiagnosis, overtreatment, and psychological distress.⁷ The overall mortality benefits of screening mammography among women aged 40 to 59 were called into question by findings from the Canadian National Breast Screening Study. After 25 years of follow-up, women aged 40 to 59 who received an annual mammogram experienced no reduction in breast cancer mortality compared with those who underwent physical examination alone.⁸ However, after evaluating data from 20 cohort and 20 case-control studies, the International Agency for Research on Cancer concluded that for women aged 50 to 69, the benefits of screening mammography outweighed the harms, and that for women aged 40 to 49 the evidence was too limited to draw conclusions.⁹

In the literature, a 25% to 30% reduction in breast cancer mortality is often attributed to biannual screening mammography among women aged 50 to 69.¹⁰ Some authors contend that the literature on screening mammography exaggerates its benefits and minimizes the associated harms such as false-negative results, overdiagnosis, and overtreatment.^7,10,11 Two international studies found that women vastly overestimated mammography’s ability to reduce breast cancer mortality.^12,13 A lack of knowledge of the harms associated with screening mammography might unduly influence women in favour of mammography.^7,10,11

An analysis of Canadian Community Health Survey (CCHS) data showed that levels of mammography use among women aged 50 to 69 increased from 40% in 1990 to 72% in 2001—mammography use remained stable until 2008.¹⁴ Subsequent to 2008, the Canadian Task Force on Preventive Health Care (CTFPHC) revised its guidelines on screening mammography. The age for routine screening was extended from 69 to 74 years; the time interval between screening for women aged 50 to 69 was extended from 2 years to 2 to 3 years; and it was recommended that women aged 40 to 49 not be routinely screened (previous guidelines did not address this age group).¹⁵ The CTFPHC reported an unfavourable benefit-harm profile for women aged 40 to 49; this finding underscores the importance of quantifying this age group’s participation. Canadian literature reporting on factors influencing mammography participation has typically focused on women aged 50 to 69.^14,16–19 However, the CTFPHC’s revised guidelines point to the need to expand on the collection of information on participation levels to include women aged 40 to 49 and 70 to 74.

A search of the published literature to date yielded no study reporting Canadian mammography participation and factors that influence women’s participation based on population-based data collected after 2008. This study focuses on screening mammography, and thus differs from many of the previous analyses of the CCHS data, which did not differentiate between screening and diagnostic mammography.^14,16–18 Using data from the 2012 CCHS, this study examined participation in screening mammography among women aged 40 to 74 and identified the factors that are associated with those women who participate in screening.

METHODS

Data source

This study used survey data from Statistics Canada’s (a Canadian federal department) cross-sectional 2012 CCHS microdata file. The survey data are collected via telephone interviews. As the microdata file is for public use and contains anonymized data, ethics approval was not necessary. The CCHS includes Canadian residents 12 years of age and older from all provinces and territories, excluding people residing in institutions, full-time members of the Canadian Forces, and those living on First Nations reserves, in the Region of Nunavik, and in des Terres-Cries-de-la-Baie-James.²⁰ For calculations based on the survey data to be representative of the Canadian population, Statistics Canada assigns each CCHS respondent a weight that corresponds to the number of individuals in the Canadian population that she or he represents.²¹ The study sample consisted of 18 312 women aged 40 to 74, weighted to represent 7.6 million women aged 40 to 74 in the Canadian population (Figure 1).²¹

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Figure 1.

Selection of study sample from the 2012 CCHS survey respondents

CCHS—Canadian Community Health Survey.

Variables

The CCHS asked women aged 35 or older the following questions: “Have you ever had a mammogram, that is, a breast x-ray?” (yes or no); and “When was the last time?” (< 6 months, 6 months to < 1 year, 1 to < 2 years, 2 to < 5 years, and ≥ 5 years ago). Women whose responses were recorded as “don’t know,” “refused,” or “not stated” to the questions asking if they ever had a mammogram and when the last time was were excluded from the analysis of mammography participation. Women were also asked, “Why did you have it?” As multiple responses were accepted, a respondent could have legitimately reported reasons suggesting that she had received both screening and diagnostic mammograms. For example, a woman who had undergone screening mammograms over several years and then had undergone a diagnostic mammogram to investigate a lump might have responded “screening” and “follow-up of a breast problem.” However, for the purposes of this study, women who responded that their screening reasons included “part of regular checkup or routine screening,” “age,” “family history of breast cancer,” or “using hormone replacement therapy” and who indicated that they had had a mammogram less than 2 years ago were considered to have participated in screening mammography and are herein referred to as screening participants. Women who reported that the only screening reason was “previously detected lump” or “follow-up of breast cancer treatment” or “breast problem” were considered to have participated in diagnostic mammography. Finally, women whose only response to “Why did you have it?” was “other” were excluded from the analysis.

The CCHS participants were asked to provide their height and weight, from which we calculated their body mass index. Other variables were selected based on their association with breast cancer and their inclusion in similar studies; they included the following: age, marital status, number of years since immigration, household income, education attainment, having a regular doctor, number of years since last physical checkup and last Papanicolaou test, frequency of alcohol consumption, smoking status, and province or territory.^{2,4,14,16–19}

Frequencies, bivariate tabulations, and logistic regressions were performed to examine associations between independent variables and being a screening mammography participant in the 2 years before the 2012 CCHS interview using SPSS software, version 22.²² Odds ratios (ORs) and their associated 95% CIs were calculated, and then the model was adjusted for age, marital status, and household income.

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