Molecular Classification of Breast Cancer: A Retrospective Cohort Study

Objectives: The study was aimed to determine the distribution of various breast cancer molecular subtypes in Saudi Arabia. Further, association between these subtypes and different epidemiological features was assessed. Methods: A retrospective study was conducted between January 2012 and December 2018, at the King Abdul Aziz University Hospital. A total of 740 cases of breast cancer, using immunohistochemistry, were classified into 4 major molecular subtypes: luminal A, luminal B, HER2-positive, and triple negative. Chi-squared test was performed to evaluate the relationship between these subtypes and clinico-pathological features. Results: Luminal A (58.5%) subtype was the most prevalent, followed by triple negative (16%), luminal B (14%), and HER2-positive (11.5%). The average age of the patient at the time of diagnosis was found to be 49 years with an average tumor size of 3.2 cm. Out of all cases, 85% of cases were ductal, while 11.4% were lobular. 66.6% showed axillary lymph node metastases. While, 77% of lobular carcinomas were found almost exclusively in the luminal A and triple negative tumor subtype, 69.5% had modified radical mastectomy. Conclusions: Luminal A tumor was the most prevalent subtype, while HER2-positive was the least prevalent. Luminal A tumors were mostly associated with lobular carcinomas. HER2-positive and triple negative tumors showed higher histological grade and larger tumor size at the time of diagnosis. These tumors were commonly found in women below the age of 50 years. Carcinoma-in-situ was less prevalent in HER2-positive tumors. Furthermore, a strong association was observed between axillary lymph node status and molecular subtypes.