Biomarker assays for ER, PR, and HER2 are used as surrogates for gene expression profiling and are measured with immunohistochemical methods or in situ hybridization. 1 While gene expression profiling is the defining feature of breast cancer subtypes, it is not often used clinically. Gene expression profiling has identified several molecular subtypes of breast cancer. These biomarkers, especially hormone-receptor and HER2, have a great impact on treatment and outcomes. The expert panel for the eighth edition felt that the peer-reviewed literature was now sufficient to permit inclusion of several of the most well-studied biomarkers. In the AJCC Staging Manual, biomarkers have previously been incorporated into other tumor systems, such as thyroid and prostate cancer however, until recently, there have been insufficient data to permit incorporation of biomarkers into the staging system for breast cancer. The eighth edition also maintains a firm footing in the past, enabling comparisons of outcomes to prior studies using the traditional TNM system as the basis, especially important in parts of the world where biologic markers are not routinely available. These factors-estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), grade, and multigene assays-are widely used in practice to define prognosis and determine therapy.

These biomarkers result in a modification of the TNM stage. The eighth edition of the AJCC staging for breast cancer is conceptually a radical departure from prior editions, because it incorporates contemporary biologic factors (biomarkers) into the traditional anatomic staging system.

Ajcc 8th ed