Breast Cyst on Ultrasound: What Do Simple, Complicated and Complex Mean?

A breast cyst on ultrasound is a fluid-filled sac in the breast tissue. Most are simple cysts, which are benign, very common and often need no treatment. The report wording, such as simple, complicated or complex, tells your care team whether anything more is needed.

Breast Cancer Now describes cysts as one of the most common causes of a breast lump. They form when a milk gland fills with fluid. Cysts are most common in women aged about 35 to 50 who still have periods, are uncommon after menopause, and may be more likely in people taking hormone replacement therapy (HRT). They are very rare in men.

This guide explains common report wording. It cannot tell you what your own scan shows or replace the review by the team that ordered it.

A known cyst does not explain every breast change. Cleveland Clinic advises never assuming a new lump is just a cyst. Contact your doctor or breast clinic promptly if you notice any of the following, even if a cyst was found before:

• A new lump in the breast or armpit, or a known lump that grows or changes.
• Skin dimpling or puckering, a nipple that has newly turned inward, or bloodstained nipple discharge.
• A swollen area that is hot, red and painful, especially with a high temperature or flu-like symptoms. This can be a sign of infection and needs prompt medical review.
• A cyst that stays painful or keeps growing after it has been drained.

Some cysts are felt as a smooth, movable lump, which may become tender before a period. Others are too small to feel and are found by chance on a screening mammogram or a scan done for another reason. Very small cysts, sometimes called microcysts, are common in younger women who still have periods.

At a breast clinic, assessment may include an examination, a mammogram and an ultrasound. Breast Cancer Now notes that women under 40 are more likely to have an ultrasound, because dense breast tissue can make mammograms less clear. Ultrasound is the test that shows whether a lump is solid or filled with fluid, which is why it is central to describing a cyst. If the lump is easy to feel, drawing off fluid with a fine needle can also confirm that it is a cyst.

Ultrasound uses sound waves to show whether a lump is solid or filled with fluid. Radiologists then describe what the fluid-filled area looks like. The three common terms are:

• Simple cyst: entirely filled with fluid, with a smooth thin wall and no solid area. Cleveland Clinic says about 90% of cysts are simple. Simple cysts do not increase breast cancer risk and do not turn into cancer.
• Complicated cyst: still fluid-filled, but the fluid contains echoes or debris, or the edges are less clear. The American Cancer Society says cancer is unlikely, and a follow-up scan or fine needle aspiration may be suggested. Cleveland Clinic gives a cancer risk of less than 2%.
• Complex cyst: has both fluid and solid parts, or a thick wall. This type carries a higher chance of cancer, and the American Cancer Society says a core needle biopsy is usually needed to find out what it is.

Usually not. Breast Cancer Now says most cysts need no treatment or routine follow-up, and many go away on their own, often after menopause. Some people continue to have cysts for many years, and new ones can appear.

If a simple cyst is large, uncomfortable or painful, a clinician may drain it. A cyst that is not causing symptoms is often left alone. A simple cyst seen on ultrasound is often reported as benign, which is the meaning of BI-RADS category 2 on breast imaging reports.

Aspiration means drawing fluid out of the cyst with a fine needle, often guided by ultrasound. The cyst usually collapses once the fluid is removed. It can be used to confirm that a lump is a cyst, to relieve pain, or to check fluid from a complicated cyst.

The fluid can range from clear to very dark. Breast Cancer Now says blood-stained fluid is usually sent to a laboratory, and fluid may also be tested if infection is suspected. The area can feel bruised and tender for a few days.

Draining a cyst does not stop it coming back. Cysts can refill, or new ones can form, and treatment is usually the same each time. The American Cancer Society notes that surgery may be considered for cysts that keep returning and causing symptoms.

Breast Cancer Now and the American Cancer Society both state that a simple breast cyst does not increase the risk of breast cancer. A benign cyst does not change into a tumour.

Complicated and complex cysts are handled differently because their appearance is less certain, not because every one is cancer. Sources such as Cleveland Clinic give higher percentages for complex cysts, but these are population figures. Your own result depends on the scan, any biopsy and your clinician's assessment.

Whatever the cyst type, staying aware of how your breasts normally look and feel, and reporting new changes, still matters. Routine screening continues as advised for your age and history.

Bring the written report to the appointment. These questions can help you leave with a clear plan:

• Is my cyst described as simple, complicated or complex, and what BI-RADS category was given?
• Does the cyst match the lump I can feel, or was it found somewhere else?
• Is any follow-up scan, aspiration or biopsy planned, and when?
• If fluid was drained, was it sent for testing, and how will I get the result?
• What changes should make me contact the clinic before my next routine appointment?
• Does this change my routine screening schedule?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a similar report wording moved through follow-up scans, aspiration or biopsy can help you notice questions you had not thought of.

We do not diagnose, and another person's case cannot predict yours. Use what you read to prepare questions, then bring them to the team that has your images and history.