Q. How is comedocarcinoma malignant but also still in situ, that is, confined to the ducts and noninvasive? Is not invasiveness one of the criteria for malignancy?

A. Comedocarcinoma of the breast is indeed confined to the ducts and, therefore, non-invasive. It is a form of ductal carcinoma in situ of the breast in which there is a lot of necrosis within the tumor – so much so that you can kind of squeeze it out of a gross specimen (hence the prefix comedo-). You can see the necrosis under the microscope too, obviously – it looks like grainy pink junk (there’s a ton of it in the photo above).

Carcinoma in situ is early stage of malignancy in which the cells have not yet breached the basement membrane. Invasiveness is common in carcinoma – but it is not a requirement for diagnosis. There are in situ variants of many carcinomas (like breast carcinoma, cervical carcinoma and squamous cell carcinoma). The cells in in situ carcinomas are definitely malignant, and if left alone, will become invasive in short order. They are simply at a very early stage of malignancy.

It’s great if you can catch a tumor at this stage – it’s much more treatable than invasive carcinoma, and the prognosis is much better. That’s the idea behind the pap smear. Cells in the cervix undergo predictable changes prior to becoming invasive carcinoma cells. They first appear mildly dysplastic, then moderately dysplastic, then severely dysplastic, then malignant (but in situ) – and the changes take many years, so there’s plenty of time to treat before invasive carcinoma sets in.

Beautiful photo of comedocarcinoma by permission from the Department of Pathology at the Center for Comparative Medicine at the University of California, Davis: