Most masses (lumps) in the breast are not visible because they are within the breast tissue rather than on the surface of the breast. Both benign and malignant (cancerous) masses can occur in your breasts, and it is sometimes difficult to tell the difference when palpating (using your fingers to examine) your breasts. You should both palpate your breasts and examine them in a mirror because---though you may not see a lump--often there are signs of hidden lumps.
Brunner & Suddarth's Textbook of Medical-Surgical Nursing outlines different characteristics of masses. Breast cysts are benign (non-cancerous), often multiple and in both breasts, and usually occur after age 30 and recede with menopause, when the body produces less oestrogen. They are also usually tender and may increase in size during menstruation, causing swelling and general tenderness of the breast. Cysts feel round and elastic and usually are movable.
Fibroadenoma is a benign tumour that can occur between the onset of menstruation and menopause. Usually a person will have only one tumour that is movable and is round, disk-shaped or lobed. A fibroadenoma is usually firmer than a cyst and is not tender.
Fibrocystic changes occur during reproductive years when you develop multiple cysts, overgrowth of some breast tissue and cycles of breast pain and swelling. The cysts in the beginning are small, but larger cysts form as the condition progresses, resulting in very lumpy and painful breasts, with symptoms worsening during menstruation. These lumps make self breast examination difficult, and fibrocystic changes increase your risk of developing cancerous tumours.
A malignant tumour (breast cancer) is most common between the ages of 40 and 80. A person will usually have only one mass, but if the cancer has advanced, it may have spread to other areas of the breast or lymph nodes. The tumour feels firm, often flat and rubbery, and is less movable because cancerous cells attach and invade the surrounding tissue. The tumour is usually not tender in the early stages, but there may be some noticeable changes in your breast.
According to MedicineNet, some indications of breast cancer include changes in the shape or size of your breast (especially when compared to the opposite breast), nipple discharge (blood-tinged or watery), nipple inversion (turning inward), dimpling or retraction (pulling inward) of the skin over the tumour site, discolouration of the skin (red-orange) and change in texture (hardening of skin, "orange peel" appearance).
If you feel or see a lump and/or any breast changes, you should immediately consult a physician for a mammogram and other tests, which may include a needle biopsy or surgical biopsy (taking a sample of the tissue). Identifying lumps in your breasts and determining the cause is critical because breast cancer should be treated as early as possible. You should do monthly self breast exams and have an annual exam by a health professional; including a screening mammogram, after the age of 40, or earlier if you have a family history of breast cancer.