Overall survival and prognostic factors in young women with breast cancer: a retrospective cohort study from Southern Thailand.

[BACKGROUND] Breast cancer diagnosed at a young age is often associated with aggressive tumor characteristics and poorer survival outcomes. In Southeast Asia, data on overall survival and prognostic factors among young women with breast cancer remain limited, particularly from real-world clinical settings with complete mortality ascertainment. This study aimed to describe survival outcomes and identify independent prognostic factors among women aged 45 years or younger with breast cancer at a major provincial referral hospital in Southern Thailand.

[METHODS] We conducted a retrospective cohort study of women aged 45 years or younger diagnosed with histologically confirmed breast cancer at Hat Yai Hospital, Southern Thailand, between 2013 and 2018. Mortality was ascertained through linkage with the Thai National Civil Registration database. Molecular subtypes were classified using immunohistochemistry surrogate criteria per the St. Gallen 2013 consensus. Missing data were handled using multiple imputation by chained equations (MICE, m = 10). Overall survival (OS), disease-free survival (DFS), and distant disease-free survival (DDFS) were estimated using the Kaplan-Meier method. Prognostic factors were evaluated using univariable and multivariable Cox proportional hazards regression with the revised model including disease stage, tumour grade, molecular subtype, surgery, and hormone therapy.

[RESULTS] A total of 293 young women were included. Median follow-up was 93.3 months (95% CI: 89.8-98.2). The 3-year and 5-year OS rates were 75.8% (95% CI: 71.0-80.8%) and 66.2% (95% CI: 61.0-71.9%), respectively. Five-year OS by stage ranged from 87.1% (Stage I) to 33.3% (Stage IV). The 5-year DFS and DDFS were 58.0% and 61.8%, respectively. The most common molecular subtype was HR+/HER2- (52.0%), followed by HR-/HER2- (22.3%), HR+/HER2+ (16.0%), and HR-/HER2+ (9.7%). In multivariable analysis, HR+/HER2 + subtype was independently associated with poorer overall survival compared with HR+/HER2- (adjusted HR = 2.08, 95% CI: 1.07-4.07, p = 0.032). Hormone therapy receipt was independently associated with better overall survival (adjusted HR = 0.20, 95% CI: 0.10-0.42, p < 0.001). Receipt of surgery was strongly associated with overall survival but largely reflected disease operability rather than an independent treatment effect.

[CONCLUSIONS] This study provides systematically ascertained, registry-linked survival benchmarks for young-onset breast cancer in Southern Thailand. HR+/HER2 + subtype and hormone therapy receipt were independent prognostic factors. The relatively lower stage-specific survival rates compared with high-income country benchmarks highlight the need for improved access to HER2-targeted therapy and endocrine therapy in this resource-limited setting.