IDC and DCIS in the same tumor?

Both can exist. At this point, it doesn't really matter as you will be treated for the IDC, not DCIS. best of luck to you

My tumor, 2.4 cm, held DCIS and IDC. I had radiation and 5 years of Arimdex, no chemo. I'm almost 7 years out.

I had a total 1.8 cm mixed mass of DCIS and IDC. It was described as the center being 0.5 cm DCIS surrounded by IDC. Wish it was found when it was still DCIS.....

Flautadulce, my understanding is that it is believed that approx. 85% - 90% of IDC develops from DCIS. I wasn't aware that there was any disagreement about that. What isn't known, however, is what causes DCIS to evolve to become IDC. In my 11 years reading up on this, I've seen dozens of theories and research studies about this, none that have as yet come to a clear and definitive conclusion.

Because most IDC develops from DCIS, it is very common to find IDC tumors that include a very small amount of DCIS. This probably happens more often than not.

Similarly, it is very common to find DCIS tumors that include a very small amount of IDC. I had over 7cm of DCIS along with 1mm of DCIS. DCIS-Mi, my diagnosis, is in fact separately identified as the very first level of Stage I invasive cancer. The "Mi" stands for an invasive tumor that is 1mm or small in size.

For those who have a combination of DCIS and IDC, the staging is based on the size of the IDC. The DCIS is not added in. This is why I am Stage IA, despite having so much DCIS. The only thing that counts in my staging is that 1mm of invasive cancer.

Here's what I think is the big question, and which I have never yet seen researched:

Why do some tumors that start as DCIS almost immediately evolve to become IDC and then continue to develop and grow as IDC? (i.e. those tumors that are found to be mostly IDC with just a small amount of DCIS),

whereas

Why do other tumors that start as DCIS continue to develop and spread as DCIS but at some point also have a tiny breakthrough of invasive cancer? (i.e. those tumors that are found to be mostly DCIS with just a small amount of IDC).

I suspect that although both these types of cancer include DCIS and IDC, they might be very different in genetic make up and in terms of what triggers the development of the IDC and the continuing growth of the cancer.

I was diagnosed with DCIS and eventually was found to have "micro-invasions" of IDC. It seems common. Good to talk to people in a similar boat as it is a bit confusing.

letsgogolf, that's my understanding as well.

If DCIS (ductal in-situ, i.e. non-invasive) and IDC (invasive ductal) together in the same tumor was to be classified as "Mixed Type" (or ILC and LCIS) then the vast majority of cancers would be"Mixed Type".