*Article* **Feasibility Study and Clinical Impact of Incorporating Breast Tissue Density in High-Risk Breast Cancer Screening Assessment**

**Alison Rusnak 1, Shawna Morrison 2, Erika Smith 2, Valerie Hastings 2, Kelly Anderson 2, Caitlin Aldridge 2, Sari Zelenietz 2, Karen Reddick 3, Sonia Regnier 3, Ellen Alie 4, Nayaar Islam 5, Rutaaba Fasih 6, Susan Peddle 6, Erin Cordeiro 7, Eva Tomiak 8 and Jean M. Seely 6,\***


Correspondence: jeseely@toh.ca

**Abstract:** Breast tissue density (BTD) is known to increase the risk of breast cancer but is not routinely used in the risk assessment of the population-based High-Risk Ontario Breast Screening Program (HROBSP). This prospective, IRB-approved study assessed the feasibility and impact of incorporating breast tissue density (BTD) into the risk assessment of women referred to HROBSP who were not genetic mutation carriers. All consecutive women aged 40–69 years who met criteria for HROBSP assessment and referred to Genetics from 1 December 2020 to 31 July 2021 had their lifetime risk calculated with and without BTD using Tyrer-Cuzick model version 8 (IBISv8) to gauge overall impact. McNemar's test was performed to compare eligibility with and without density. 140 women were referred, and 1 was excluded (*BRCA* gene mutation carrier and automatically eligible). Eight of 139 (5.8%) never had a mammogram, while 17/131 (13%) did not have BTD reported on their mammogram and required radiologist review. Of 131 patients, 22 (16.8%) were clinically impacted by incorporation of BTD: 9/131 (6.9%) became eligible for HROBSP, while 13/131 (9.9%) became ineligible (*p* = 0.394). It was feasible for the Genetics clinic to incorporate BTD for better risk stratification of eligible women. This did not significantly impact the number of eligible women while optimizing the use of high-risk supplemental MRI screening.

**Keywords:** breast screening; high-risk breast screening; dense breasts; supplemental breast screening; breast MRI
