Vaginal intraepithelial neoplasia (VaIN)

Vaginal intraepithelial neoplasia, or VaIN, is a change in the cells that line the inside of the vagina. Neoplasia means a new growth of cells, and intraepithelial means that growth stays within the epithelium, the lining that forms the surface. In the vagina, that epithelium is squamous, which means it's made of flat cells. In VaIN, abnormal cells show up in that lining, but they don't invade the tissue underneath, and they aren't cancer.

What sets one grade apart from another is how much of the epithelium's thickness those cells take up. Picture a slice of cake seen from the side, and think of its full height as the thickness of the epithelium. If the abnormal cells fill a third of that height, it's VaIN 1; if they fill two-thirds, it's VaIN 2; if they go past two-thirds, it's VaIN 3. And when VaIN 3 takes up the whole thickness, from the surface all the way to the bottom of that layer, it's called carcinoma in situ. In situ is Latin for "in its place": the cells are still inside the epithelium.

This matters because VaIN may become cancer and spread into the wall of the vagina, meaning it breaks out of that surface lining into what lies beneath. But "may" doesn't mean "always": in VaIN the cells have already changed, and even so, they aren't cancer.