Abstract 4212: Comparison of thirty-year population-based incidence rates of invasive lobular vs ductal vs mixed breast carcinoma in Ontario, Canada
Authors: Lim David Wai , Giannakeas Vasily , Narod Steven , Metcalfe Kelly A. 
Published: April 2023

Abstract
Purpose: We calculated crude, age-adjusted and age-specific incidence rates for invasive lobular, ductal and mixed ductal-lobular breast carcinoma from 1990 to 2020 in the province of Ontario, Canada. We further examined incidence relationships between clinical stage, age at diagnosis and time.
Methods: We used population-based administrative healthcare datasets from the Institute of Clinical Evaluative Sciences (ICES Ontario), including the Ontario Cancer Registry, to identify all women diagnosed with breast cancer between 1990 and 2020. We calculated crude, age-adjusted and age-specific incidence rates for invasive lobular (ILC), ductal (IDC) and mixed ductal-lobular (IDC-ILC) breast carcinoma. Incidence rates were adjusted to the 2011 Canadian female standard population. We further examined the incidence relationships between clinical stage and age at diagnosis over time.
Results: From 1990 to 2020, the 5-year crude incidence rates of ILC increased from 53.1 to 73.4 per 100,000 (+38%), while IDC increased from 501 to 746.5 per 100,000 (+49%). The crude incidence of mixed IDC-ILC peaked at 45 per 100,000 between 2005 and 2009 and is currently 29 per 100,000. The age-adjusted 5-year incidence rate of ILC has slightly increased from 64 to 70 per 100,000 (+9%) while that of IDC has increased from 598 to 726 per 100,000 (+21%). The age-adjusted 5-year incidence of mixed IDC-ILC peaked at 47 per 100.000 between 2005 and 2009 and has declined to 29 per 100,000. Age-specific 5-year incidence rates for ILC has decreased over time in women &lt; 40 years of age and increased in women over the age of 65. In contrast, age-specific 5-year incidence rates for IDC have remained stable in women over 75. For mixed IDC-ILC, age-specific 5-year incidence rates have increased over time in all age categories. Among women with ILC, there is a greater proportion of women under the age of 50 diagnosed with stage III disease (30%) compared with women diagnosed over the age of 50 (16%). Women between the ages of 50 and 74 have higher rates of being diagnosed at stage I (43%) compared with 35% and 31% for women diagnosed between the ages of 40-49 and over 75, respectively.
Conclusions: The incidence of invasive lobular breast carcinoma in increasing, particularly in women over the age 65. Consequently, the burden of lobular breast carcinoma is expected to increase, as the proportion of women over the age of 65 is expected to rise exponentially in the foreseeable future, highlighting a need for further study of this not uncommon breast cancer subtype. While representing a smaller proportion of breast cancer diagnoses, the incidence of mixed invasive ductal-lobular subtype is increasing in all age groups.
Citation Format: David Wai Lim, Vasily Giannakeas, Steven Narod, Kelly A. Metcalfe. Comparison of thirty-year population-based incidence rates of invasive lobular vs ductal vs mixed breast carcinoma in Ontario, Canada. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4212.