A breast ultrasound uses high-frequency sound waves to create real-time pictures of the tissue inside the breast. It is commonly used after an abnormality is detected on a screening mammogram or when a patient presents with a palpable lump. The primary goal is to determine the nature of a mass, distinguishing between a fluid-filled sac (a cyst, which is almost always benign) and a solid mass, which may require further investigation. By visualizing the internal architecture of a mass, the ultrasound helps a radiologist assess the likelihood that the abnormality is malignant.
Identifying Suspicious Structural Features
The visual appearance of a mass provides the first clues suggesting a malignant process. Unlike benign masses, which tend to have smooth and well-defined contours, malignant tumors typically present with angular, irregular, or spiculated margins. These jagged borders reflect the invasive nature of cancer cells as they push into and distort the surrounding breast tissue. The presence of these spiky extensions is a strong indicator of malignancy.
The shape and orientation of the mass also offer important diagnostic details. A malignant lesion often exhibits a “taller-than-wide” orientation, meaning its vertical dimension is greater than its horizontal dimension. This vertical growth pattern suggests that the mass is aggressively growing across natural tissue planes. Conversely, benign lesions usually lie parallel to the skin, appearing wider than they are tall.
Malignant masses are typically hypoechoic, meaning they appear darker on the ultrasound image compared to the surrounding fatty or glandular tissue. The mass may also exhibit heterogeneous internal echoes, indicating a mixed tissue composition, potentially due to areas of necrosis or fibrosis within the tumor. Another significant sign is posterior acoustic shadowing, where the tissue immediately behind the mass appears abnormally dark. This suggests the lesion is dense and has absorbed or scattered the sound waves, often resulting from a dense fibrous reaction frequently associated with invasive breast cancer.
Assessing Blood Flow and Tissue Stiffness
Beyond structural appearance, advanced ultrasound techniques examine the functional properties of the suspicious tissue, such as its blood supply and mechanical stiffness. Color Doppler ultrasound assesses the vascularity, or blood flow, within and around a mass. Malignant tumors require a high volume of nutrients to support their rapid and uncontrolled growth.
The presence of increased vascularity within the mass, rather than just at its periphery, is a strong indicator of malignancy. Doppler analysis reveals a chaotic and branching pattern of vessels within the lesion, reflecting the disorganized nature of the new blood vessel growth that feeds the tumor.
Another technique, elastography, measures the mechanical stiffness of the breast tissue. Cancerous tissue is typically much harder and less compressible than normal breast tissue or benign masses. During elastography, mild pressure is applied, and the ultrasound measures how the tissue deforms. A malignant lesion will show minimal deformation, appearing significantly stiffer than the surrounding soft tissue. Integrating the findings from Doppler and elastography with the structural features improves the accuracy of differentiating between a benign and a malignant mass, helping to avoid unnecessary biopsies.
The BI-RADS Classification for Malignancy Risk
Radiologists use the Breast Imaging Reporting and Data System (BI-RADS) to communicate findings and manage patient care consistently. This standardized system assigns a number from 0 to 6 to every finding, indicating the level of suspicion for cancer. Findings suggesting a low probability of cancer, such as BI-RADS 2 or BI-RADS 3, are typically managed with routine or short-interval follow-up imaging.
The categories relevant to a potential diagnosis of malignant breast cancer are BI-RADS 4 and 5. A BI-RADS 4 designation signifies a Suspicious Abnormality, where features warrant a tissue sample. The probability of malignancy ranges widely (2% to 95%), leading to subcategories: 4A (low suspicion, 2-10%), 4B (moderate suspicion, 10-50%), and 4C (high suspicion, 50-95%).
A BI-RADS 5 assessment is reserved for findings Highly Suggestive of Malignancy. The lesion exhibits most classic malignant features, such as spiculated margins and a taller-than-wide orientation. In this category, the likelihood of cancer is greater than 95%, and biopsy is strongly recommended to obtain a definitive diagnosis.
Guiding the Next Step: Ultrasound-Assisted Biopsy
Once a suspicious lesion is identified and assigned a BI-RADS score of 4 or 5, ultrasound shifts from a diagnostic tool to a guidance system. The definitive diagnosis requires a biopsy, which is the removal of a tissue sample for examination under a microscope. Ultrasound guidance is frequently the preferred method because it provides real-time visualization of the needle’s path.
The radiologist uses the live ultrasound images to precisely direct a thin, hollow needle into the suspicious mass, ensuring the sample is taken from the most representative part of the lesion. After the tissue is sampled, a tiny marker is often placed at the biopsy site to mark the location for future reference, such as surgical planning.