An international cohort study of breast cancer survival in young women

There is large variation in case-fatality rates of breast cancer in young women across the world. The objective of this study was to estimate the ten-year overall survival of women diagnosed with breast cancer before age 40 in six countries, by clinical presentation and by treatments received.

Data were supplied by seven centers in six countries, including the USA, Canada, Poland, Iceland, Iran and Thailand, totaling 36,861 women diagnosed with breast cancer before age 40, from 1977 to 2020. American patients from the SEER registry were sub-divided into (Non-Hispanic) White and Black. The following factors were compared across countries: year of diagnosis, tumor size, lymph node status, estrogen receptor (ER) status, use of chemotherapy, use of tamoxifen and use of radiotherapy. The Kaplan-Meier method was used to estimate ten-year overall survival for patients in each center. Cross-center comparisons are provided. The Cox Proportional Hazards model was used to estimate the impacts of the various factors on death for each center separately and for all centers combined, for all patients and for patients with stage I breast cancer. For these comparisons, SEER white women were considered the reference group.

The mean age of diagnosis was 35.4 years. The percentage of patients with node-positive breast cancer ranged from 48.5% in Poland to 53.4% in Iceland. Mean tumor size ranged from 2.1 cm in Poland to 3.5 cm in Iran. The percent of patients with ER-negative breast cancer ranged from 34.0% in USA White women to 47.4% in Canada. The use of chemotherapy ranged from 65.4% in Thailand to 78.8% in Poland. Ten-year survival rates ranged from 52.5% (95%CI: 50.4%-54.7%) in Thailand to 77.9% (95%CI: 75.5%-80.3%) in Poland. A two-fold variation in case-fatality across centers persisted after adjusting for year of diagnosis, stage, tumor features and treatment. The difference in case-fatality across countries was greatest for patients with stage I breast cancer (four-fold variation).

There is a wide disparity around the world in the long-term survival of women diagnosed with breast cancer before age 40, which cannot be accounted for by differences in screening, tumor stage at presentation or other tumor features. It is possible that differences in survival are due to inherent ethnic differences and/or differences in cancer management, but more research is required.