Abstract PO2-11-02: Incidence, treatment patterns and outcomes of breast cancer during the first year of the COVID pandemic: A population-based study
Authors: Leone Jose , Leone Julieta , Hassett Michael , Freedman Rachel , Avila Jorge , Vallejo Carlos , Tayob Nabihah , Tolaney Sara , Lin Nancy 
Published: May 2024

Abstract
Background: The COVID pandemic created significant challenges in breast cancer (BC) care which were most pronounced during 2020 when screening and treatment access were reduced. The aims of this study were to evaluate and compare the incidence of BC, treatment patterns, and outcomes during the year 2020 versus the immediate pre-pandemic (2018 and 2019) period.
Methods: Using data from the Surveillance, Epidemiology, and End Results (SEER) program, we calculated age-adjusted incidence rates of ductal carcinoma in situ (DCIS) and invasive BC during 2018, 2019 and 2020, with a 95% confidence interval (CI) for each. In the invasive BC cohort, we compared chemotherapy and radiation utilization by year. To account for potential misclassification of cause of death (COD) during the pandemic, we evaluated outcomes by overall survival (OS) in the invasive cohort, defined as time from BC diagnosis until death from any cause. To assess the impact of COVID, OS was truncated at 12 months from diagnosis for each year and we compared OS at 12 months for patients (pts) diagnosed with invasive BC in 2020 versus 2018 and 2019. OS was compared in a multivariable Cox model with diagnosis year 2020 as the reference. The Cox model was adjusted for age at diagnosis, race and ethnicity, sex, tumor grade, histology, stage, tumor subtype, surgery, radiation, chemotherapy, marital status, median household income, and rurality.
Results: There were 37,834 DCIS diagnoses, and 199,594 invasive BC diagnoses between 2018-2020, with the distribution by year shown in the table. In 2020, the age-adjusted incidence of female DCIS decreased to 31.0 cases per 100,000, and the age-adjusted incidence of female invasive BC decreased to 166.6 cases per 100,000. As shown in the table, the incidence of male BC did not change. Among females, the relative reductions in incidence from 2019 to 2020 were: 15% for DCIS, 12% for stage I, 6% for stage II, 3% for stage III, and 2% for stage IV. Comparing 2020 to 2018-2019, we observed small absolute decreases in the proportions of pts who had surgery (2%) or radiation therapy (2.8%), and a small absolute increase in chemotherapy (1.4%) (p&lt; 0.001 for each). These treatment changes were present across each individual stage. The table shows the 12-month OS rates for each year of diagnosis. COVID was the COD for 77 pts in 2020, 43 pts in 2019 and 0 pts in 2018. In the Cox model, there were no significant differences in the risk of death between pts diagnosed 2018 and 2020 (adjusted Hazard Ratio: 0.95; 95% CI, 0.90 – 1.01; p=0.13) or between pts diagnosed 2019 and 2020 (adjusted Hazard Ratio: 0.95; 95% CI, 0.89 – 1.01; p=0.08).
Conclusions: During the first year of the pandemic, the incidence of BC decreased significantly, largely driven by a decrease in DCIS and early-stage diagnoses. Of interest, there was no stage shift observed with respect to the incidence of stage III or de novo stage IV disease, and no change in male BC incidence. We hypothesize that the observed trends are due to a reduction in screening, but this needs further study. It will be critical to evaluate future trends in cancer incidence beyond 2020, as it is possible that the reduction in early-stage diagnoses represent missed diagnoses that will be identified at a later stage in subsequent years. Finally, while there was no difference in 12-month OS by year of diagnosis, the true impact of the COVID pandemic on BC-specific outcomes will require further follow-up.

Citation Format: Jose Leone, Julieta Leone, Michael Hassett, Rachel Freedman, Jorge Avila, Carlos Vallejo, Nabihah Tayob, Sara Tolaney, Nancy Lin. Incidence, treatment patterns and outcomes of breast cancer during the first year of the COVID pandemic: A population-based study [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 PO2-11-02.