What does the receptor (ER and/or PR) percentage mean?

treelilac

I met with my second-opinion oncologist and she explained to me that I'm 40% ER/PR (+), Her2 (-). I said but I was previously told to be triple-negative. She said well, 60% of the cells from my specimen didn't stain (+) for the ER/PR, if someone happened to biopsy on those cells, they would have concluded that I'm triple negative.

I thought about what she said at home and now have new questions. What she said basically mean my tumor is a heterogeneous cancer. But isn't that rare? My impression from my first oncologist five years ago is that most tumor is homogeneous and even though it may show say only 60% ER (+), we still treat all the tumor cells as the same.

I find myself suddenly confused about what the percentage truly mean despite having been dealing with the terminology for 5 years. Anyone who is knowledgeable is greatly appreciated!

Background story: Core biopsy: TNBC, estimated stage IIa based on MRI. After MX, the pathologist couldn't find invasive carcinoma. Cancer is concluded to exist only in the biopsy ( max. size 0.4 cm). Second opinion: ER/PR+ 40%, tumor size 1.2 cm. My head is spinning.

ingersollnic

It means out of 100 cells tested how many of those cells are sensitive to oestrogen or progesterone