Breast cysts are a benign (not cancer) condition. They are one of the most common causes of a  breast lump, and can develop in either one or both  breasts. It is thought that they develop naturally as the breast changes with age, due to normal changes in hormone levels. It is common to have more than one cyst.
Breasts are made up of lobules (milk-producing glands) and ducts (tubes that carry milk to the nipple), which are surrounded by glandular, fibrous and fatty tissue. Sometimes, fluid-filled sacs develop in the breast tissue; these are breast cysts.
Although you can develop breast cysts at any age, they are most common in women over 35 who have not yet reached the menopause. They develop more often as women get closer to the menopause and usually stop once a woman has been through the menopause. However, women who take hormone replacement therapy (HRT) after the menopause may also develop cysts.
How are they found?
Cysts usually become noticeable as a lump in the breast or are found by chance during a routine screening mammogram (breast x-ray) or while having investigations at a breast clinic for another reason.
At the breast clinic you will have a breast examination. You may also have a mammogram and/or an ultrasound scan (which uses high-frequency sound waves to produce an image of the breast). If you are under 35 you are more likely to have an ultrasound scan only. This is because younger women’s breast tissue can be dense, which can make the x-ray image less clear, making normal changes or benign conditions harder to find.
When a mammogram shows a possible cyst, a breast ultrasound is usually done. An ultrasound shows whether the lump is a fluid-filled cyst or a solid lump. It also shows whether a cyst is a "simple cyst" or "complex cyst." Breast ultrasound is the best way to identify and diagnose breast cysts because it is accurate 95 to 100% of the time.
Do cysts lead to cancer?
Almost all breast cysts identified as "simple cysts" by ultrasound are benign and never become cancerous. It is estimated that one in 1,000 cysts contain a tumor (but they are usually benign). These tumors can be identified by ultrasound in most cases. Women with cysts are not at greater risk for cancer although this risk may be slightly higher if there is a family history of breast cancer (mother, sister, or daughter).
Treatment and follow-up
If you do have a breast cyst or cysts, you will not usually need any treatment or follow-up. Most cysts go away by themselves and are nothing to worry about.
If the cyst is large or causing discomfort, your specialist may draw off the fluid using a fine needle and syringe. Sometimes this is done using ultrasound to help find the cyst. The fluid drawn off from the cyst can vary in colour and range from clear to very dark. It will only be sent to a laboratory for testing if the fluid is blood-stained.
Once the fluid has been drawn off, the cyst usually disappears. You might feel some discomfort as the fluid is being drawn off, and the area may feel bruised and tender for some days afterwards. If so, taking pain relief like paracetamol should help.
Breast cysts can come back, or you may develop new cysts. The treatment for cysts is the same each time.
Breast cysts can feel soft or hard and can be any size, ranging from a few millimetres to several centimetres. They can develop anywhere in the breast. For some people, cysts can feel uncomfortable and even painful. Before a period, cysts may become larger and feel sore and tender. However, many women can have cysts and not be able to feel them at all.
What will happen if a cyst is left alone?
Breast cysts often change in size. Breast cysts can look different or disappear entirely on mammograms from one year to the next. This is why we recommend  like many other renowned top breast clinics in the world, diagnostic rather than screening mammograms for our patients with "cystic breasts" and changing lumps. Diagnostic mammograms allow the doctor to complete an ultrasound at the same appointment as the mammogram to make sure the changes are actually due to cysts and not solid lumps.

The writer is an onco-plastic breast surgeon.