**3. Results**

*3.1. Impact on Eligibility for B-MRI through the HROBSP Program*

During the study interval, 140 women age 40–69 (average 51.4 years) with no prior history of BC underwent HROBSP assessment. One was excluded from this study as she was a known carrier of a *BRCA* gene pathogenic variant and automatically eligible for HR OBSP. Eight of the remaining 139 women (5.8%) never had a mammogram in the past 10 years (7) or only had a mammogram from another country (1) (Table 1).


**Table 1.** The impact on risk assessment and determination of clinical eligibility on addition of breast tissue density according to age groups 40–49 and 50–69.

MG = mammogram. \* Women age 40–49 y had never undergone a mammogram while those 50–69 had mammograms in another province or country and the report was not available. \*\* eligibility was determined if calculated lifetime risk ≥25%. ˆ Increase of calculated risk ≥1%. ˆˆ Decrease of calculated risk ≤1%.

Excluding the genetic mutation carrier, 139 women were assessed, 94.2% (131/139) of whom had a mammogram available and 13% (17/131) of whom required review by a radiologist to determine BTD on the mammogram. The incorporation of BTD impacted 16.8% (22/131). When reported BTD was incorporated into the risk assessment, 6.9% (9/131) became eligible for HROBSP and MRI screening; 9.9% (13/131) became ineligible, for a net 3.1% (4/131) fewer eligible patients (Table 2, Figure 2).



\* Eligibility determined if calculated lifetime risk ≥25%.

**Figure 2.** 42-year-old woman with a strong family history of BC shown to have BI-RADS category C breast tissue on screening mammograms. When BTD was incorporated into IBISv8 calculation, the lifetime risk increased from 24.2% to 32.4%, and she became eligible for HROBSP screening with B-MRI.

When comparing the number of patients who were eligible with reported BTD versus the number of patients who were eligible without including density, no significant difference was identified (*p* = 0.3938); even when considering the McNemar's exact test to account for the relatively small sample size (*p* = 0.5235). Based on these results, the proportion of eligible individuals was not significantly different between the two assessment methods.

As predicted, the 10 women who became eligible had the highest BTD as measured by BI-RADS (C or D) while 61.5% (8/13) who became ineligible had non-dense breasts (A or B). Five women who had never had a mammogram were 40–49 years old. Three women who had no mammogram available were 50–69 years and had it done outside of Ontario or remotely.

#### *3.2. Feasibility of Incorporating BTD into the IBISv8 Calculation*

When it became routine practice for the HROBSP NN to include the mammogram report in the referral to Genetics, it took no extra time for the GC to add BTD (BI-RADS A, B, C or D) with the other risk factors and family history information routinely collected from the patient in the risk calculation. This occurred in 87% (114/131) of assessed patients. For 13% (17/131) patients that required contact with the radiologist to help obtain the BI-RADS density score, both the GC/nurse navigator and radiologist estimated that it took about 5 min each, or 10 min in addition to the assessment, because the images were readily available for viewing. In total, for 17 patients at 10 min each, took 170 min.
