Assessment of estrogen receptor low positive status in breast cancer: Implications for pathologists and oncologists

Estrogen receptor (ER) status assessment by immunohistochemistry (IHC) is the gold standard test for the identification of patients with breast cancer who may benefit from endocrine therapy (ET). Whilst most ER+ breast cancers have a high IHC score, about 3% of cases display a low positivity, with 1% to 10% of cells being weakly stained. These tumors are generally classified within the luminal-like category; however, their risk profile seems to be more similar to that of ER-negative breast cancers. The decision on ET for patients with a diagnosis of ER-low breast cancer should be carefully considered in light of the risks and possible benefits of the treatment. Potential pitfalls hinder pathologists and oncologists from establishing an appropriate threshold for "low positivity". Furthermore, several pre-analytical and analytical variables might trouble the pathological identification of these clinically challenging cases. In this review, we sought to discuss the adversities that can be accounted for the pathological identification of ER-low breast cancers in real-world clinical practice, and to provide practical suggestions for the perfect ER testing in light of the most updated recommendations and guidelines.