Breast ultrasound uses high-frequency sound waves to create images of the breast’s internal structure. This non-invasive method is particularly useful for evaluating abnormalities found during a physical exam or on a screening mammogram. Ultrasound provides a detailed view of a specific area, helping to characterize whether a finding is a fluid-filled cyst or a solid mass. It also serves as a supplemental screening tool for populations with dense breast tissue, where standard mammography is less effective.
The Role of Ultrasound in Breast Cancer Screening
Ultrasound plays a distinct and complementary role to mammography in breast cancer detection. It is highly effective because it leverages the difference in how sound waves travel through various tissues, allowing it to quickly determine if a lump is a simple, fluid-filled cyst or a solid mass requiring further investigation.
This ability to differentiate mass types is particularly beneficial for individuals with dense breast tissue. On a mammogram, dense tissue and tumors both appear white, creating a “masking” effect that can obscure a cancerous lesion. In contrast, on an ultrasound, dense tissue appears bright (echogenic), while a malignant mass typically appears dark (hypoechoic), making detection easier.
Adding supplemental ultrasound to mammography for women with dense breasts significantly increases the rate of cancer detection. Studies show it can find an additional 1.9 to 4.2 cancers for every 1,000 women screened who have dense tissue and a negative mammogram result. This is often performed as a whole-breast screening ultrasound, though a targeted ultrasound may focus on a specific area identified by a patient or another imaging exam.
Recognizing Suspicious Features
When a radiologist examines a solid mass on an ultrasound, certain characteristics suggest malignancy. The shape and orientation are significant factors. Benign lesions are often oval or round and have a “wider than tall” orientation, lying parallel to the chest wall. Malignant masses tend to be irregularly shaped with a “taller than wide” orientation, suggesting aggressive growth across tissue planes.
The borders, or margins, of the mass are closely analyzed. A benign mass usually has smooth, well-circumscribed margins. In contrast, a malignant mass often presents with angular, spiculated, or ill-defined margins. These irregular borders indicate the tumor is invading or sending extensions into the adjacent breast tissue.
How sound waves interact with the tissue behind the mass provides another clue: posterior acoustic shadowing. This occurs because malignant tumors often elicit a desmoplastic reaction—a dense fibrous response that prevents sound waves from passing through, creating a dark shadow. Conversely, benign cysts typically show posterior acoustic enhancement, where sound passes easily through the fluid and appears brighter on the other side.
The internal structure and blood flow are assessed using Color Doppler technology. Malignant tumors frequently demonstrate marked hypoechogenicity, appearing very dark due to their composition. Because cancerous tumors grow rapidly, they require a rich blood supply, visible on Doppler as increased and often disorganized vascularity within or around the mass.
Confirmatory Diagnostic Steps
The presence of suspicious features on an ultrasound indicates the need for further diagnostic action, but does not confirm cancer. Radiologists use the Breast Imaging Reporting and Data System (BI-RADS) to standardize communication. This system assigns a category from 0 to 6 to the imaging results, guiding the next steps in patient care.
When an ultrasound shows highly suspicious features, the finding is typically classified as BI-RADS Category 4 (suspicious for malignancy, 2% to 95% probability) or Category 5 (highly suggestive of malignancy, >95% probability). In either case, a tissue sample is required to establish a precise diagnosis.
A biopsy is the only procedure that can definitively confirm the presence of malignant cells, involving the removal of tissue for examination by a pathologist. The most common approach is an ultrasound-guided core needle biopsy, which uses the ultrasound to precisely guide a hollow needle to extract small cylinders of tissue.
A fine-needle aspiration (FNA) may also be performed, using a very thin needle to collect cells or fluid. However, FNA yields less tissue and is primarily used to confirm if a mass is cystic or to evaluate lymph nodes. The core needle biopsy is generally preferred because it provides a sufficient sample of tissue architecture for accurate determination by the pathologist.