Abstract P1-08-12: Trends in incidence of bilateral breast cancer: A Population-based comparative study of the United States and Japan
Authors: Sakai T , Ozkurt E , Desantis S , Wong S , Rosenbaum L , Zheng H , Ohno S , Golshan M 
Published: May 2019

Abstract
Background: Previous studies demonstrated that the incidence rate of invasive contralateral breast cancer (CBC) was 5% within the first 10 years after the primary breast cancer (BC). However improving long-term breast cancer survivorship and advancements in diagnostic imaging have resulted in an increased detection of bilateral breast cancer (BBC), and trends of bilateral invasive and in situ breast cancer are not well established. The aim of this study was to assess national trends of BBC incidence of the United States (US) and Japan.
Methods: The Surveillance, Epidemiology, and End Results (SEER) database (1973-2014) and the clinical database of Breast Oncology Center of Cancer Institute Hospital in Tokyo, Japan (Ganken) database (1946-2015) were used to identify nSEER=11,771 and nGanken=1,499 women diagnosed with BBC, respectively. BBC was defined as invasive BC and/or ductal or lobular carcinoma in situ diagnosed in both breasts simultaneously or after primary breast cancer diagnosis. BBC was grouped into synchronous or metachronous BBC by the interval between first BC and contralateral BC; synchronous cases were defined as CBC diagnosed at the same time or within an interval of 1 year from primary BC diagnosis whereas metachronous cases were defined as a diagnosis occuring 1 year following the primary BC. We assessed trends of BBC incidence and characteristics of BBC cases between the two countries. To determine temporal trends in the incidence of BBC and proportion of the characteristics, we compared them using the Cochrane-Armitage test for trend.
Results: The rates of BBC have significantly increased in both countries (Table 1, 2) [1975: 2.6%; 2014: 7.5% in SEER (p&lt;0.001), 1946-1980: 3.3%; 2011-2015: 10.7% in Ganken (p&lt;0.001)]. The increase was identified in both synchronous and metachronous BBC. In SEER, 40% of synchronous BBC were found as in situ BC and about 15% of BBC presented as invasive lobular carcinoma. More recently, CBC was more likely to be diagnosed at early stages (in situ and local disease) than in previous years [1975: 65%; 2014: 85% in SEER (p&lt;0.001)]. The interval between first BC and contralateral BC have shortened, and CBC were more likely to be operated simultaneously in both countries [1985: 40%; 2014: 51% in SEER, 1946-1980: 24%; 2011-2015: 74% in Ganken].
[Table 1]Crude rates of BBC in all breast cancer in SEER 19751985199520052014All breast cancer941813618258766016471505BBC and rates (%)249 (2.6%)790(5.8%)1421(5.5%)3336(5.6%)5381(7.5%)Rates of synchronous BC2.1%2.8%2.3%2.6%3.5%Rates of metachronous BC0.5%3.0%3.2%3.0%4.0%BBC: Bilateral breast cancer
[Table 2]Crude rates of BBC in all breast cancer in Ganken 1946-19801981-19851991-19952001-20052011-2015All breast cancer47772162280637915241BBC and rates (%)157(3.3%)110(5.1%)188(6.7%)298(7.9%)559(10.7%)Rates of synchronous BC1.0%1.9%2.0%2.7%4.9%Rates of metachronous BC2.3%3.2%4.7%5.2%5.8%
Conclusion: In the modern era, the number of BBC cases have increased and are more likely to be found at an early stage. Further studies are needed to demonstrate the usefulness of early detection of CBC and to define the best means to tailor therapy for patients with bilateral disease.
Citation Format: Sakai T, Ozkurt E, Desantis S, Wong S, Rosenbaum L, Zheng H, Ohno S, Golshan M. Trends in incidence of bilateral breast cancer: A Population-based comparative study of the United States and Japan [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-08-12.