What exactly is a mastectomy bra?

Individualized item. A mastectomy bra is usually fitted by a lady who is a trained specialist and is able to measure each patient and determine their size and options. It can hold a breast prosthesis for cosmetic purposes and can help in recovery after mastectomy surgery.
Has special inserts. After a mastectomy, the chest becomes flat like that of a male. To make it symmetrical, it usually needs a prosthesis in the breast cup on the mastectomy side. But the prosthesis would slip out. So the mastectomy bra has a back creating a pocket into which a prosthesis can be placed.