Pathology dictionary -
PR stands for progesterone receptor. Progesterone receptor is a protein that allows a cell to respond to the actions of the hormone progesterone. After the progesterone receptor protein is made, it stays in the nucleus of the cell. Progesterone hormone that enters the cell can change the behavior of the cell by activating the progesterone receptor.
Not all cells make progesterone receptor. Cells that normally make progesterone receptor are found in the breast, ovaries, uterus, and cervix. Some types of cancer including those of the breast, ovary, uterus, and cervix also make progesterone receptor.
Pathologists can perform a test called immunohistochemistry to see if a group of cells are making the progesterone receptor. Cells that make progesterone receptor will be called positive or reactive while those that do not will be called negative or non-reactive.
Pathologists test all breast cancers to see if the tumour cells are making progesterone receptor. This test is performed because tumour cells that make progesterone receptor require the action of the progesterone hormone to help them grow.
Why is this important? A patient with a progesterone receptor producing breast cancer is more likely to benefit from hormone blocking medications. The response to the medication depends on the amount of progesterone receptor being produced. Tumours with higher levels of progesterone receptor are more dependent on progesterone for growth and respond best to treatment.