Abstract P5-07-13: Causes of death among breast cancer patients and their relationship to etiology of breast cancer
Authors: Katuwal Sushmita , Pukkala Eero 
Published: March 2020

Abstract
Background: With improved survival among women with breast cancer, the risk of dying from causes other than breast cancer has increased. Emerging studies suggest the possibility of shared modifiable risk factors between some of the causes of death among breast cancer patients and the occurrence of breast cancer. This study is aimed at understanding the etiology of breast cancer from identifying the mortality causes of women with breast cancer.
Methods: The study cohort consists of 50481 female breast cancer patients diagnosed between 1971 and 2000 and registered in the national population-based Finnish Cancer Registry. The cohort was followed up until the end of 2012 by data linkage with National Causes of Death Register. Standardized mortality rates (SMRs) for the different cause of deaths among breast cancer patients were calculated by age at diagnosis of cancer (&lt;50 years, 50+ years), histology (ductal, lobular), and by period of follow-up after the breast cancer diagnosis (&lt; 1 year, 1-4·99 years and 5-11·99 years, 12+ years). A Poisson regression model, with adjustment of age and calendar period, was used to further compare the difference in risk of each cause of death between breast cancer histology and between different follow-up time categories.
Results: 30841 deaths were recorded during the total follow up, of which 41% was due to causes other than breast cancer (SMR 2.71, 95% CI 2.68-2.74). SMRs for other malignant neoplasms such as stomach (SMR 2.25, 95% CI 1.67- 2.96), larynx, trachea and lung (SMR 1.84, 95% CI 1.49- 2.24), and ovary (SMR 1.49, 95% CI 1.12- 1.94) was significantly high among women &lt;50 years. For women 50+ years, SMR was significantly high only for stomach cancer (SMR 1.31, 95% CI 1.13- 1.49). Among other cancers, only stomach cancer had significantly increased SMRs for both ductal and lobular subtypes. Diseases of the circulatory system were significantly increased among women at age &lt;50 years and 50+ years, however, increased SMR was observed only for ductal breast cancer subtype. The SMR for the diseases of the respiratory system significantly increased only for women 50+ years of age and did not show any significant difference in the risk by histology. The risk of age-adjusted mortality due to other causes excluding breast cancer increased with follow up time for women &lt;50 years, however, for women 50+ years, the highest risk of all-cause excluding breast cancer was observed in &lt;1 year follow up (HR 19.7 95% CI 18.8-20.6) as compared to 5-11.99 years.
Conclusion: This study concludes that the causes of death among breast cancer patients vary by age at diagnosis, histology and follow up time and indicates that there might be shared genetic and environmental risk factors between breast cancer and mortality outcomes among breast cancer patients. The mortality patterns provide indirect information on what risk factors might have played on the development of breast cancer.
Citation Format: Sushmita Katuwal, Eero Pukkala. Causes of death among breast cancer patients and their relationship to etiology of breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-07-13.