Complex Sclerosing Lesion of the Breast: Understanding Your Pathology Report

by Kimberly Wood, MD MSc FRCPC
August 13, 2026

A complex sclerosing lesion (CSL) is a benign (noncancerous) growth in the breast. Under the microscope, it has a central area of scar-like tissue with ducts and glands radiating outward from it, like the spokes of a wheel. Although the name includes the word “scar,” a complex sclerosing lesion does not develop after an injury or surgery. It is an overgrowth of the normal components of the breast that happens to resemble scar tissue.

Smaller growths with these same features are usually called radial scars. The term complex sclerosing lesion is generally used when the growth is larger, typically more than 1 cm, or when it involves several areas. Both names describe the same kind of growth, and which one appears on your report depends on its size and on the pathologist’s preference. A complex sclerosing lesion cannot spread to other parts of the body, but it matters for two reasons: it can look very much like breast cancer, and it is associated with a small increase in future breast cancer risk. This article will help you understand the findings in your pathology report, what each term means, and why it matters for your care.

What are the symptoms of a complex sclerosing lesion?

Most complex sclerosing lesions cause no symptoms and are found by chance when breast imaging is performed for another reason or when tissue is removed for another condition. Rarely, one grows large enough to be felt as a lump. On mammography or ultrasound, a complex sclerosing lesion often appears as an area with strands of tissue radiating outward, a pattern that closely resembles breast cancer on imaging.

What causes a complex sclerosing lesion?

The exact cause is not known. Complex sclerosing lesions are thought to arise from an overgrowth of the normal components of the breast, particularly the ducts, glands, and supporting connective tissue. They are not caused by injury, surgery, or anything a person did.

How is the diagnosis made?

A complex sclerosing lesion is often first suspected on mammography or ultrasound, especially when it is larger than 1 cm. Because it can look very similar to breast cancer on imaging, a biopsy is usually performed to determine what is causing the abnormality. The diagnosis is confirmed when a pathologist examines the tissue under the microscope. Many complex sclerosing lesions are also found incidentally in tissue removed for another reason.

Under the microscope, a complex sclerosing lesion shows a central core of scar-like connective tissue, described as fibrosis. This tissue is often also described as elastotic or showing elastosis, meaning it contains a large amount of a stretchy material called elastic fiber. Small, irregularly shaped ducts and glands are pulled into this central core, and others radiate outward from it, producing the spoke-like pattern. Other benign changes are commonly seen in the surrounding tissue, including usual ductal hyperplasia (an increased number of normal duct-lining cells), cysts (fluid-filled spaces), sclerosing adenosis, and apocrine metaplasia (cells that take on features of sweat gland cells). On their own, these changes are noncancerous.

Because the trapped ducts and glands sit within scar-like tissue, a complex sclerosing lesion can resemble invasive ductal carcinoma under the microscope as well as on imaging. The feature that separates them is a preserved layer of myoepithelial cells surrounding the trapped glands, which is present in a complex sclerosing lesion and lost in invasive cancer. When the distinction is difficult, the pathologist performs immunohistochemistry (IHC) to highlight these cells and confirm that the growth is benign.

Is a complex sclerosing lesion linked to breast cancer?

A complex sclerosing lesion is not cancer. Studies have shown that people with one have a modestly higher lifetime risk of developing breast cancer, roughly 1.5 to 2 times that of a person without one. The overall risk remains low, and most people with a complex sclerosing lesion never develop breast cancer. The level of risk depends partly on what else is found in and around the growth:

Proliferative changes — An increase in the number of normal-looking cells in the ducts and glands. These are associated with only a small increase in risk.
Atypia — Cells that look abnormal but are not cancer, such as atypical ductal hyperplasia. When atypia is found, the increase in risk is greater and closer follow-up is usually recommended.

What happens after this diagnosis?

Because a complex sclerosing lesion can hide or sit next to a more advanced change, the main goal after diagnosis is to confirm that nothing more serious is present. The pathology findings, together with your imaging, guide what your team recommends.

Surgical excision — Removing the lesion is often recommended when it is diagnosed on a core needle biopsy, particularly if atypia is present or the imaging raises concern. Examining the whole area sometimes reveals atypical ductal hyperplasia, ductal carcinoma in situ, or a small cancer that the needle did not capture.
Observation — When the lesion is small, found incidentally, contains no atypia, and the biopsy result matches the imaging, close follow-up with examination and imaging may be recommended instead of surgery.
Routine screening — Your doctor will discuss the right breast screening schedule based on your overall risk factors.

Questions to ask your doctor

Was the growth in my breast diagnosed as a complex sclerosing lesion or a radial scar?
Did the biopsy show any atypia or other changes?
Did my biopsy result match my imaging findings?
Do you recommend surgical removal, or can this be safely monitored with imaging?
If it was removed, was anything additional found once the whole area was examined?
How much does this finding increase my risk of breast cancer?
How often should I have follow-up mammograms or other screening?
What symptoms should prompt me to come back for re-evaluation?