Abstract PO5-22-10: Are contralateral prophylactic mastectomy rates impacted by ASBrS guidelines to offer germline genetic testing to all patients with breast cancer? Results from a large, prospective, single-institution cohort
Authors: Weiss Anna , Knapp Stephen , Braun Danielle , Barton Brenna , McGrath Monica , Stokes Samantha , Laws Alison , Warren Laura , Morganti Stefania , Lynce Filipa , Bychkovsky Brittany , Rana Huma , Davis Dillon , Stopfer Jill , Garber Judy , King Tari 
Published: May 2024

Abstract
Background: It is unclear whether the release of the American Society of Breast Surgeons’ (ASBrS) guideline to offer germline genetic testing to all patients with breast cancer impacted contralateral prophylactic mastectomy (CPM) rates. The objective of this study was to describe trends of germline testing and CPM rates, and to determine predictors of CPM uptake.
Methods: This is a retrospective review of two prospectively maintained single institutional databases: Dana-Farber/Brigham Cancer Center’s surgical database, merged with Dana-Farber Cancer Institute’s Genetics and Prevention database. Trends in germline testing and CPM rates were described, in relation to the February 2019 ASBrS guideline release, and a multivariable logistic regression model was used to determine factors associated with CPM utilization. Patients with unilateral stage 0-III breast cancer who underwent surgery between January 2016 and July 2020 were included. Patients were considered “tested” if they underwent germline genetic testing and results were disclosed before their index operation. CPM was defined as contralateral mastectomy performed at index operation or anytime thereafter, excluding mastectomy for a new contralateral breast cancer event.
Results: Among 6,064 women in the cohort, 2,455 (40.5%) had germline genetic testing before their index surgery. Rates of testing significantly increased over the study period from 33.7% in 2016 to 64.9% in 2020 (p&lt; 0.001). A total of 783/6,064 (12.9%) patients underwent CPM which was 40.7% (783/1,926) of the patients who underwent mastectomy. The CPM rate was 13.3% before the guideline release (2016 – January 2019) and 12.5% after (February 2019 – 2020). There was no significant change in CPM rate over time (p=0.527), including before (p=0.380) and after (p=0.220) the 2019 guideline release. The following factors were associated with increased CPM rates on multivariable logistic regression: identification of a pathogenic/likely pathogenic variant in a breast cancer predisposition gene (Adjusted Odds Ratio [Adj. OR] 24.43), genetic testing with a negative test result (Adj. OR 1.52), number of relatives with breast/ovarian cancer (Adj. OR 1.18 and 1.39, respectively for each additional relative), younger age (Adj. OR 0.97 if pre-menopausal, 0.91 if post-menopausal, for each year decrement), and cT2-3 tumors (Adj. OR 1.53 and 1.94, respectively as compared to other cT stages); all p&lt; 0.05.
Conclusions: Despite increasing germline genetic testing rates, CPM rates were stable over time, indicating that offering genetic testing to more patients does not increase CPM rates in our experience. Selection bias is most certainly contributing to the apparent association between CPM and genetic testing, even if there is a negative test result, as the classic indications for genetic testing are similar to those for CPM.
Citation Format: Anna Weiss, Stephen Knapp, Danielle Braun, Brenna Barton, Monica McGrath, Samantha Stokes, Alison Laws, Laura Warren, Stefania Morganti, Filipa Lynce, Brittany Bychkovsky, Huma Rana, Dillon Davis, Jill Stopfer, Judy Garber, Tari King. Are contralateral prophylactic mastectomy rates impacted by ASBrS guidelines to offer germline genetic testing to all patients with breast cancer? Results from a large, prospective, single-institution cohort [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-22-10.