Breast ultrasound uses high-frequency sound waves to create detailed pictures of breast tissues. This non-invasive procedure helps healthcare providers assess various breast conditions by distinguishing between different tissue types, such as fluid-filled cysts and solid masses. Unlike X-rays, ultrasound does not use radiation, making it a safe option. It provides a real-time view, allowing radiologists to observe blood flow and tissue characteristics as they scan.
Why a Breast Ultrasound Is Performed
A breast ultrasound is often performed to investigate specific concerns or to provide additional information when other imaging methods are inconclusive. One common reason is to examine a palpable lump or other breast change that an individual or their doctor has noticed. The ultrasound can help determine if the lump is a simple fluid-filled cyst, a solid but benign mass, or a potentially suspicious solid mass.
Another frequent scenario for an ultrasound is to further evaluate an abnormality detected on a mammogram. Mammograms are a primary screening tool, but some findings may require a closer look to determine their nature. Ultrasound is particularly effective at distinguishing between solid masses and fluid-filled cysts, which mammography cannot always do definitively. For individuals with dense breast tissue, where mammograms can be less effective at detecting abnormalities due to the similar appearance of dense tissue and masses, ultrasound serves as a valuable supplementary screening tool.
Differentiating Benign and Malignant Features
Radiologists analyze visual characteristics on an ultrasound image to differentiate between benign and malignant breast lesions. The shape and margins of a mass provide clues. Benign lesions, such as simple cysts or fibroadenomas, often appear with smooth, well-defined, round, or oval shapes. In contrast, suspicious lesions often exhibit irregular shapes with spiculated (spiky) or angular margins, indicating potential invasion into surrounding tissue.
Echogenicity, which describes how much sound is reflected by tissue, is another feature. Simple cysts appear anechoic, meaning they are black on the ultrasound image because sound waves pass through the fluid without reflecting back. Solid masses, both benign and malignant, appear hypoechoic, meaning they are darker gray than the surrounding fatty tissue.
The orientation of a mass relative to the skin surface also offers diagnostic information. Benign masses often have a “wider-than-tall” orientation, running parallel to the skin. Conversely, masses that are “taller-than-wide” are more suspicious for malignancy, suggesting growth across tissue planes.
Posterior acoustic features reveal how sound waves behave after passing through the lesion. Cysts often cause “posterior enhancement,” appearing as a brighter area behind the lesion due to sound waves passing through fluid unimpeded. Malignant tumors, however, can cause “posterior shadowing,” appearing as a darker area behind the mass because the dense tumor tissue absorbs or scatters the sound waves.
Understanding the BI-RADS Score
After analyzing the ultrasound images, radiologists use a standardized system called the Breast Imaging Reporting and Data System (BI-RADS) to categorize their findings. This system provides a numerical score, ranging from 0 to 6, to communicate the level of suspicion for malignancy and recommend appropriate follow-up actions. It helps standardize reporting across different imaging centers.
A BI-RADS Category 0 indicates an incomplete assessment, meaning additional imaging is needed for a definitive score. Categories 1 and 2 are considered negative or benign findings, respectively, indicating no suspicious abnormalities or findings that are not cancerous, such as simple cysts. For example, Category 1 means the breast tissue appears normal and symmetrical.
A Category 3 suggests a probably benign finding, with a very low probability of malignancy, less than 2%. For these cases, a short-term follow-up ultrasound, in about six months, is recommended to monitor for any changes. Categories 4 and 5 indicate suspicious or highly suggestive findings for malignancy, respectively, and a biopsy is recommended to obtain a tissue sample for definitive diagnosis. Category 4 can be further subdivided into 4A (low suspicion, 2-10% chance of malignancy), 4B (moderate suspicion, 10-50%), and 4C (high suspicion, 50-94%). Finally, Category 6 is assigned when a biopsy has already confirmed the presence of malignancy.
Potential Next Steps After an Ultrasound
The BI-RADS score directly guides the subsequent actions recommended by healthcare providers. If an ultrasound results in a BI-RADS Category 1 or 2, indicating negative or benign findings, the individual returns to routine screening schedules, such as annual mammograms, based on their age and risk factors. No immediate further action is required.
For a BI-RADS Category 3 finding, which is considered probably benign, a short-term follow-up ultrasound is recommended. This involves repeating the ultrasound in six months to ensure the lesion remains stable and does not show any suspicious changes over time. If the lesion remains unchanged after a series of follow-up scans, it may then be downgraded to a BI-RADS Category 2.
When an ultrasound yields a BI-RADS Category 4 or 5, indicating suspicious or highly suggestive findings for malignancy, a biopsy is the next step. During a biopsy, a small tissue sample is removed from the suspicious area, guided by ultrasound, and then examined under a microscope by a pathologist. This procedure provides a definitive diagnosis, determining whether the mass is cancerous or benign.