Answer:
Most breast lumps are benign cysts, particularly in younger women. Size and tenderness may vary with the menstrual cycle. When lumps are tender, they are more likely benign, but about 10 percent of breast cancers do cause breast pain or tenderness.
Cancers tend to feel much harder than benign cysts and fibroadenomas. Both benign and malignant masses can be rounded and mobile. Only when cancers are quite advanced are they fixed to skin or the underlying chest wall, and not moveable.
Any new, persistent, or changing lump in your breast should be evaluated by your physician. Sometimes mammograms and ultrasound can differentiate between benign and malignant breast masses. But, often just a needle aspiration of the lump can quickly prove that it is just a cyst when fluid drains.
The best approach is to get regular screening mammograms on a schedule defined by the American Cancer Society - starting early if there is a family history of breast cancer or other risk factors. With very dense breasts that are difficult to evaluate with exam and mammogram, targeted or 3D ultrasound may be added.