Abstract P4-03-29: Impact of the COVID-19 pandemic on breast cancer incidence and tumor stage in the Netherlands and Norway
Authors: Eijkelboom Anouk , de Munck Linda , Larsen Marthe , Nygård Jan , Hofvind Solveig , Siesling Sabine 
Published: March 2023

Abstract
Introduction. The SARS-CoV-2 infection rate and the COVID-19 death rate were relatively high in the Netherlands during the first wave of the COVID-19 pandemic (2.7 and 7.2 times higher than in Norway, respectively). Moreover, social measures differed between the two countries. This study aimed to compare the effect of the pandemic on breast cancer incidence and stage between the Netherlands and Norway. Methods. Women diagnosed with DCIS or invasive breast cancer between January 2017 and December 2021 were selected from the Netherlands Cancer Registry and from the Cancer Registry of Norway. The COVID-19 period was divided in three approximately equal periods: March-September 2020 (first wave), October 2020-April 2021 (second wave), May-December 2021 (post-second wave). Breast cancer incidence during the COVID-19 periods was compared with averaged data of the corresponding reference period: March-September 2017, 2018, 2019 (first wave_ref), October-April 2017, 2018, 2019 (second wave_ref), May-December 2017, 2018, 2019 (post-second wave_ref). Incidences were compared by age group, clinical tumor stage, and method of detection.
Results. The number of breast cancer diagnosis and the breast cancer incidence are shown in Table 1. Compared to the reference period, breast cancer incidence was lower during the first wave in the Netherlands and Norway (IRR: 0.72; 95%CI: 0.70-0.75; IRR: 0.83, 95%CI 0.78-0.88, respectively), and was higher post-second wave in Norway (IRR: 1.10, 95%CI: 1.04-1.16) (Table 1). During the first wave, breast cancer incidence was lower in all age groups in the Netherlands (age &lt; 50 IRR: 0.85, 95%CI: 0.79-0.91; 50-69 IRR: 0.64, 95%CIL 0.61-0.67; 70-74 IRR: 0.61, 95%CI: 0.56-0.67; &gt;74 IRR: 0.86, 95%CI: 0.80-0.93, respectively). During the first wave, incidence was lower in women aged 50-69 in Norway (i.e., women eligible for screening; IRR: 0.68, 95%CI: 0.62-0.74). Post-second wave incidence was higher in women aged 50-69 and &gt;74 in Norway (IRR: 1.09, 95%CI: 1.01-1.17; IRR: 1.13, 95%CI: 1.00-1.28, respectively). In the first wave the incidence of DCIS, stage I tumors, and screen-detected tumors was lower in the Netherlands (IRR: 0.55, 95%CI: 0.50-0.61; IRR: 0.62, 95%CI: 0.59-0.65, IRR: 0.36, 95%CI: 0.33-0.38, respectively) as well as Norway (IRR: 0.66, 95%CI: 0.54-0.79; IRR: 0.73, 95%CI: 0.66-0.81, IRR: 0.46; 95%CI: 0.40-0.52, respectively).
Conclusion. The current study showed that the incidence of early-stage tumors mainly decreased. Moreover, during the first wave of the pandemic breast cancer incidence decreased in all age groups in the Netherlands but only in women aged 50-69 in Norway. The relatively high infection and death rate in the Netherlands might have increased the fear of patients to visit the general practitioner (GP) and/or to overburden the healthcare system at the start-up of the pandemic. In addition, it might have reduced the capacity at the GP. As a result, appointments with the GP might have been postponed, resulting in a decrease in the number of breast cancer diagnoses in all age groups. A catch-up in breast cancer diagnoses was seen post-second wave in Norway, but not in the Netherlands. Incidence rates should therefore be monitored in the coming period.
Table 1: Number of breast cancer diagnoses (breast cancer incidence) during the COVID-19 pandemic and during the corresponding reference period (incidence is expressed per 100.000 women living in the Netherlands or Norway during the period of interest)
Citation Format: Anouk Eijkelboom, Linda de Munck, Marthe Larsen, Jan Nygård, Solveig Hofvind, Sabine Siesling. Impact of the COVID-19 pandemic on breast cancer incidence and tumor stage in the Netherlands and Norway [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-03-29.