A breast ultrasound is a non-invasive medical imaging technique that uses high-frequency sound waves to create detailed pictures of the tissues inside the breast. Receiving an abnormal result can cause anxiety, leading many people to search for information about the likelihood of cancer. This article provides context and specific statistics regarding the probability of malignancy when an abnormality is detected.
The Primary Function of Breast Ultrasound
A breast ultrasound is primarily a diagnostic tool used to clarify findings identified on a mammogram or during a physical examination. It is often the preferred initial imaging choice for women under 30 or those who are pregnant, as it avoids radiation exposure. It is also useful for individuals with dense breast tissue, where masses can be obscured on a standard mammogram.
The most important function of the ultrasound is its ability to differentiate between a simple, fluid-filled cyst and a solid mass. Simple cysts are very common, appear anechoic (dark with no internal echoes), and are nearly always benign, requiring no further action.
If the ultrasound reveals a solid mass, further evaluation is necessary, as this structure could represent a benign tumor, like a fibroadenoma, or a malignant lesion. The characteristics of the solid mass, such as its shape and margins, help the radiologist determine the next steps. This distinction between fluid and solid significantly reduces the number of unnecessary biopsies.
The Statistical Reality of Malignancy Following Ultrasound
The percentage of breast abnormalities found on ultrasound that ultimately turn out to be cancerous is low. When an abnormality requires a biopsy, the malignancy rate is typically in the range of 2% to 7% of all findings. This confirms that the vast majority of lumps or masses seen are non-cancerous, such as cysts or fibroadenomas.
This low percentage is due to the ultrasound’s effectiveness in characterizing masses; 80% to 90% of all solid masses that are ultimately biopsied are found to be benign. The malignancy rate may be slightly higher if the scan was performed due to a suspicious finding or a palpable lump, compared to a routine screening ultrasound.
Studies on supplemental screening ultrasound in women with dense breasts show that the test finds cancer in approximately two to three women per 1,000 screened. While this highlights the tool’s ability to detect additional cancers missed by mammography, the low detection rate means many benign findings must be checked, resulting in false positives.
Standardized Classification of Ultrasound Findings (BI-RADS)
To standardize the interpretation of breast imaging results and provide a consistent risk assessment, radiologists use the Breast Imaging Reporting and Data System (BI-RADS). This system assigns a category score from 0 to 6, with higher numbers indicating an increased probability of malignancy. The BI-RADS score guides the recommendation for follow-up care.
The categories are defined by the estimated risk of malignancy:
- BI-RADS 1: Negative result with no suspicious findings (0% risk).
- BI-RADS 2: Benign finding noted, such as a simple cyst or fibroadenoma (0% risk).
- BI-RADS 3: Probably benign finding, requiring short-interval follow-up (less than 2% risk).
- BI-RADS 4: Suspicious abnormality that warrants a biopsy. This is subdivided into 4A (2% to 10% risk), 4B (10% to 50% risk), and 4C (50% to 95% risk).
- BI-RADS 5: Highly suggestive of malignancy (greater than 95% risk), virtually always requiring a biopsy.
Required Follow-Up After a Suspicious Ultrasound Result
The specific BI-RADS score determines the required management plan for the patient. For lesions classified as BI-RADS 3, the standard recommendation is a short-interval follow-up imaging schedule, typically involving a repeat ultrasound at six, 12, and 24 months. This monitoring ensures the finding remains stable over time, confirming its benign nature.
If the finding remains stable for two or three years, the BI-RADS category is downgraded to a 2, indicating a confirmed benign result. If the lesion grows or develops more suspicious features, it is immediately upgraded to a BI-RADS 4, and a biopsy is recommended.
For results categorized as BI-RADS 4 or 5, the next step is a tissue biopsy to obtain a definitive diagnosis. This procedure typically involves a core needle biopsy guided by ultrasound. The sample is sent for pathological analysis, providing the final confirmation of whether the abnormality is benign or malignant and guiding subsequent treatment decisions.