What is the posterior nipple line (PNL), and what percentage of CC views should include the pectoralis muscle?

The PNL is the perpendicular measurement from the nipple to the muscle on a well-positioned MLO view. (Convex muscle border goes to the level of the nipple.) The depth on the CC view (nipple to the back of the film, whether or not muscle is included) should be within 1 cm of the PNL.