Answer: Cysts are very common and rarely turn into cancers. But it's extremely important to find out whether what you have is just a cyst or something else.
If you feel a lump in your breast and you don't know what it is, have your doctor check it. Some women get what are called cyclical lumps—cysts that appear sometime before the start of the menstrual cycle, and later shrink or disappear. These are generally harmless. But, again, if you have them, you should be monitored by a doctor with breast expertise.
Even if you've had cysts in the past, if you find a new lump in your breast, don't assume it's a cyst. If the lump doesn't go away or gets larger, it's important to check it out further.
Ultrasound is the easiest way to see if a lump is a cyst or not. Your doctor may insert a needle into a lump to see if it's a cyst. In a simple cyst, the needle will drain off the fluid. A pathologist will check the fluid to make sure there are no cancer cells in it. If the lump is made up of breast cells, a tissue sample can be taken out with the needle and checked by a pathologist under the microscope.
In some women, the examination under the microscope may reveal epithelial hyperplasia, which is just a harmless overgrowth of normal breast cells. Another finding, atypical hyperplasia, is an increase in breast cells that have started to look different from normal cells. The atypical form is associated with a slightly increased risk of developing breast cancer later on.
Routine mammograms are important for all women, regardless of your history of cysts or other biopsy results.