Mode of detection categorization

Mode of detection was constructed using both CMR and CBCS data and was used to classify how breast cancer was detected. We initially categorized mode of detection into three groups: screen-detected, interval-detected, or clinically detected, based on standard

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definitions for mode of detection. However, due to likely missing data and heterogeneity within the clinically-detected group, this group was later renamed as “unknown” mode of detection and excluded from all analyses. In this section mode of detection categorization will be defined as it was originally planned.

The date of the last screening mammogram before diagnosis in combination with the date of breast cancer diagnosis was used to assign mode of detection. The date of breast cancer diagnosis was taken from CBCS data. A screening mammogram was defined using the definition constructed by the BCSC. The BCSC considered a mammogram to be

screening if the indication for the exam is routine screening, a mammogram exam was done, the first exam sequence of the day, the woman was 18 or older, had no breast implants or prior mastectomy, bilateral screening views were done, there was no history of breast cancer cased on self-report or in the analytic cancer file, there was no imaging in the previous 9 months in the database or based on self-report, radiologist report, or comparison film, and the overall assessment code was not BI-RADS 6.

The mammogram findings are recorded in CMR, using BI-RADS assessment

categories202, as shown in Table 3.4; it is important to note that these categories are different from the BI- RADS categories that are used to describe mammographic density. Our

definitions for classifying how breast cancer was detected use the outcome of a screening mammogram, more specifically, whether it was positive or negative. A positive screening mammogram will be defined as a screening mammogram with a BI-RADS assessment code of 4 (suspicious abnormality), 5 (highly suggestive of malignancy) or 0 (incomplete) or 3 (probably benign finding) with a recommendation for biopsy, fine needle aspiration (FNA),

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or surgery. A negative screening mammogram will be defined as a screening mammogram with a BI-RADS assessment category of 1, 2, or 3 with no recommendation for biopsy, fine needle aspiration (FNA), or surgery.

As described earlier and as shown in Appendix A, screening

recommendations greatly varied from organization to organization and from year to year over all 3 phases of CBCS;

recommendations were for 1 year, 2 year, and 1-2 year screening intervals. The 2- year interval was chosen for constructing the main mode of detection variable for

this analysis in order to increase comparability with other studies and to reflect current screening recommendations, although a 1 year interval was used to construct the mode of detection variable that was used in sensitivity analyses. Figure 3.3 visually demonstrates the classification scheme that was originally used to categorize women by mode of detection. The following definitions were used to classify cancers:

Screen-detected. Cancer diagnosed within 24 months after a positive screening

mammogram.

Interval-detected. Cancers diagnosed within 24 months after a negative screening

mammogram and prior to the next screening mammogram, among women with no self- reported symptoms at time of screening mammogram.

Category Description Likelihood of Malignancy 0 Incomplete Unknown 1 Negative 0 2 Benign finding 0 3 Probably benign finding <2% 4 Suspicious abnormality 12-25% 5 Highly suggestive of malignancy >95% 6 Known biopsy- proven malignancy 100% Table 3.4 BI-RADS assessment categories.

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Clinically detected. Women in this group are women who did not have a screening

mammogram within 24 months of breast cancer diagnosis, and were not classified as screen- detected or interval-detected. This category includes women whose breast cancers were detected by themselves or by a clinician. This group was renamed “unknown” mode of detection and excluded in final analyses.

To check the coding of the mode of detection variable, the variable constructed for this study was compared against the BCSC computed variable. The classification algorithm that the BCSC used is shown below, in Figure 3.4. One notable difference between the classification schemes used to construct the main mode of detection variable in this study vs. in BCSC is BCSC’s use of a “peri-cancer” mammogram for further classification of some interval and clinically detected cancers. In addition, the BCSC classification schema also includes an

“unknown” group. Table 3.5 shows the frequencies of the BCSC variable along with frequencies Figure 3.3 Mode of detection classification flowchart

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for the mode of detection variable constructed using the 1 year screening interval and the 2 year screening interval. The frequency of screen-detected cancers is similar for all 3 variables (11- 12%). The interval cancer counts from the 2 year interval variable are very similar to the BCSC variable, which is based on a 1 year interval, but they should in fact be approximately double the BCSC count since the time interval is twice as long. The BCSC definition includes an additional way to classify interval cancers, using the peri-cancer mammogram; when examining the full breakdown of the BCSC “interval” group, the interval cancers that were classified using the peri- cancer mammogram accounted for about 50% of interval cancers. Since the peri-cancer

mammogram was not used for the variable used in this study, this would account for why the study variable interval cancer counts are approximately half of those from the BCSC.

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Table 3.5 Comparison of frequencies of mode of detection variable.

1 year interval N (%) 2 year interval N (%) BCSC (1 year) N (%) Screen-detected 161 (11) 165 (11) 176 (12) Non-screen-detected 1336 (89) 1332 (89) 295 (20) Interval 107 (7) 205 (14) 196 (13) Clinically detected 1229 (82) 1127 (75) 99 (7) Unknown 1026 (69) CBCS