Sebaceous cyst are caused by a blockage of the sebaceous glands. Sebaceous glands secrete 'sebum', which lubricates the hair and skin. There are sebaceous glands in many areas of the body, but they are most abundant on the face and scalp. A sebaceous breast cyst is a type of epidermal cyst, which forms as a collection of keratin-like material. Normally, a sebaceous breast cysts will have a small opening to the skin, but which may not be easily visible. Sebaceous cysts are most common on the face, back of the ears, and back, but also occur on the breast. Sebaceous breast cysts often appear as small bumps or lumps just beneath the skin. You can think of them as small, closed 'sacs', that contain keratin, which is a foul smelling 'pasty' buildup of protein.
On a mammogram a sebaceous breast cyst will likely appear as a well-circumscribed, round lesion close to the skin's surface. Usually a suspected breast cyst will be followed-up with ultrasound as this is one of the best ways to distinguish a liquid lesion from solid. (cysts are fluid-filled, cancers are solid). On ultrasound a sebaceous breast cyst appears as a small and hypoechoic round or oval shape, close to the surface of the skin. It is usually dark because it is fluid-filled allowing through-transmission of the ultrasound waves, and will show no evidence of vascular flow.
Sebaceous breast cysts do not have to removed unless they are undesirable cosmetically, or, if they are infected. When a sebaceous breast cyst is infected it will look swollen and red, and be quite painful. Sebaceous cysts are usually first treated with antibiotics to bring the inflammation down, and then surgically excised. When removed surgically, it is important to remove all of the cyst wall, or there is a high likelihood of recurrence. And of course, sebaceous cysts on the breast have nothing to do with breast cancer and do not increase the risk of breast cancer in any way.