Ducts, lobules and the terminal duct lobular unit

Each breast holds fifteen to twenty lobes, and each lobe is a branching system of ducts that ends in lobules, the grape-like clusters of acini that make milk. The junction where the smallest duct meets its lobule is the terminal duct lobular unit, and it is where almost all breast carcinoma and almost all of the benign lesions that mimic it arise. The rest of the breast is fat, fibrous tissue and skin, with lymphatic channels that drain mostly to the axilla and, from the inner part of the breast, to the internal mammary chain behind the ribs, which is why the armpit is examined and scanned at the first appointment.

The names of the two commonest types come from this anatomy, but they are misleading in an instructive way. Invasive carcinoma of no special type, still widely called invasive ductal carcinoma, does not arise in the large ducts, and invasive lobular carcinoma does not arise in the lobules: both begin in the terminal duct lobular unit, and the words describe what the tumour looks like down the microscope rather than where it started. The real difference is E-cadherin, the protein that glues cells to their neighbours. Lobular cancers have lost it, so they infiltrate in single files rather than forming a mass, which is why they are harder to feel, harder to see on a mammogram and harder to measure before surgery.

Men have ducts too, and a small amount of breast tissue behind the nipple, but generally lack the terminal lobules, which is the reason the male breast cancer page gives for male breast cancer being almost always of no special type and lobular carcinoma in a man being very rare. The breast changes throughout life: it is densest in young women, involutes after the menopause, and density itself both raises risk and hides cancers on a mammogram.