Ductal carcinoma in situ

Ductal carcinoma in situ is a breast cancer, and what defines it is where it still is. The abnormal cells remain sealed inside the duct, never having crossed the basement membrane, the thin wall that separates the tube from the surrounding tissue. That is why it is called noninvasive: the cells are inside the tube, and the tube's wall is intact.

What sits inside is a clonal proliferation in the epithelium lining the ducts and lobules, meaning cells that all descend from a single cell that went out of control. What keeps them shut in is the basement membrane built by the myoepithelial cells, the layer that wraps around the outside of the duct.

That boundary decides the name of the disease. If the basement membrane is breached, it is no longer called in situ: it becomes invasive breast cancer.

And this is how it usually turns up. Nine out of ten cases of ductal carcinoma in situ are found on a screening mammogram, the one done when there are no symptoms at all. But mammography alone does not settle the diagnosis: a tissue biopsy is needed.