Some people do end up having breast cancer in both breasts that are different types of cancer. It is also possible to have more than one kind of cancer in the same breast when initially diagnosed with cancer, but I don't know how common either of those scenarious are. It's also possible to be diagnosed with 1 kind of cancer in 1 of your breasts and after treatments to that breast it is possible to get a new and totally different cancer in the same breast. That's what happened to me. Had 1 type of breast cancer in 2017 and had a double mastectomy. In 2024 was diagnosed with cancer in the same breast for a 2nd time and it's totally different than the original cancer.

I would think it's common for some. My friend had two different cancers in each breast

Tumor markers work well for ER pos bc, but in my case, for HER2, they actually always stayed low and only ticked up one or two points during active disease. Many oncologists don’t even want to test them. However, if yours were elevated then indeed it’s a good marker. It measures a protein that gets shed from an active cancer (not CTCs). To get CTCs tested, it’s a much more sophisticated lab that specializes mostly in that kind of testing and is a large out of pocket expense.

In 2017 I had DCIS stage 1 in right breast caught early, had lumpectomy. Recently I had two different cancers in left breast. ILC and IDC.. I actually believe the ILC was there in 2017 but was undetected since a tumor has to be the size of the tip of an eraser before detected on mamo. My ILC was suppose to have been 5.3 cm but when taken out it was 3.5 cm. For this size tumor it had to be growing for at least 7 years. I think taking a cancer marker test every year during a physical would be more accurate than these mamo's. Every woman should have this test done yearly and insurance should be made to pay for the test. Looking back I wished I had know this in 2017 as I missed the last 4 years of mamo's due to covid and my doctor retiring and really didn't take the first cancer as serious as I should have, this was my fault and only mine. Tumor marker test will pick up any circulating tumor cells. Hope this helps.

Cancers over time can mutate and lose or gain different receptors. It doesn’t mean it’s a new cancer, it’s a recurrence with a mutated version of the original cancer. Even in remission, most patients have circulating tumor cells in blood that remain for years and cancer stem cells that can be asleep in the bone marrow. Under certain circumstances they wake up and migrate. That’s how recurrences happen.