Low-positive controls for monitoring progesterone receptor immunohistochemical staining.

Aims: Progesterone receptor (PR) is a crucial prognostic marker in breast cancer. However, achieving consistent results in PR immunohistochemistry (IHC) remains challenging due to the lack of well-defined low-positive controls. This study aimed to identify benign tissues with consistent low-level PR expression to serve as ideal controls for IHC.

Methods: We evaluated PR expression in the squamous epithelium of the uterine cervix, nipple smooth muscle and pancreatic islets. QuPath digital image analysis was employed to compare the intensity and quantity of PR staining in target cells within a 2×2 mm area.

Results: The squamous epithelium of the secretory phase cervix, nipple smooth muscle and pancreatic islets displayed appreciable weak PR expression, with mean values of 73, 55 and 60 cells, respectively. Notably, 62% (8/13) of the 2×2 mm areas in the atrophic cervix were completely negative for PR expression. The coefficients of variation for weak PR-expressing cells in pancreatic islets (57.4%) and nipple smooth muscle (65.0%) were lower than those observed in the cervix (96.2%-222.0%). The squamous epithelium of the cervix, especially during the secretory phase, exhibited weak positivity confined to the basal layers, providing another viable control option. However, variations in PR expression may be influenced by physiological factors, such as hormonal fluctuations.

Conclusions: Pancreatic islets and nipple smooth muscle, with their consistent low-level PR expression, offer a promising solution to the challenges associated with PR IHC. This approach may help minimise variations resulting from differing staining methods across laboratories.