HR stands for hormone receptor. HR+ means that tumor cells have receptors for the hormones estrogen or progesterone, which can promote the growth of HR+ tumors. HER2 stands for human epidermal growth factor receptor 2. HER2+ means that tumor cells make high levels of a protein called HER2/neu, which has been shown to be associated with certain aggressive types of breast cancer.1
Generally, breast cancer is divided into groups based on hormone receptor (HR) expression [including estrogen receptor (ER) expression and/or progesterone receptor (PR) expression], and human epidermal growth factor receptor 2 (HER2) expression or gene amplification.2 These 5 subtypes are the following:
(a) HR+/HER2− (ie, tumors expressing ER, PR, or both) that are HER2-negative
(b) HR+/HER2+ (ER+/PR−/HER2+, ER−/PR+/HER2+, and triple-positive [ER+/PR+/HER2+] disease)
(c) HR−/HER2+
(d) HR−/HER2− (or triple-negative: ER−/PR−/HER2−)
(e) HER2-low, defined as immunohistochemical detection of HER2 protein at a 1+ or 2+ level in tumor cells, and lacking amplification of the gene encoding HER2, encompassing both HR+ and HR− patients.Â
Generally, tumors classified as HR+/HER2− include those that are HER2-low and are the most common subtype, accounting for around 70% of all breast cancers.2,3 However, this rate appears to be increasing, with around 90% of new cases from 2017 to 2021 being HR+/HER2−.1Â
In terms of survival, stage at diagnosis may be the most powerful factor in determining survival outcome. For example, in those with localized disease, the 5-year relative survival was greater than 92% regardless of subtype.1
In each case above, the rates dropped significantly for metastatic disease. Not coincidentally, a variety of new treatments have been developed to treat the most common type of breast cancer (namely, HR+/HER2−) and continue to take aim at metastatic disease.3,4
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