A finding of “circumscribed mass” in a radiology report is a common occurrence that can cause immediate concern for a patient. This term is part of a standardized language used by radiologists to describe the appearance of a lump or growth seen on an imaging scan. A circumscribed mass is a finding with distinct, smooth edges, which is a specific visual characteristic used to help determine the nature of the mass. Understanding this terminology clarifies the radiologist’s observation and reduces the anxiety that comes with seeing the word “mass.”
What Circumscribed Means on Medical Imaging
The term “circumscribed” describes the margin, or border, of a finding on a medical image, such as a mammogram, ultrasound, or MRI. A circumscribed mass has well-defined, sharp borders with an abrupt transition between the mass and the surrounding healthy tissue. This appearance suggests the mass has grown by pushing the adjacent tissue aside rather than infiltrating it. On mammography, a mass qualifies as circumscribed if at least 75% of its margin is sharply defined.
The shape of a circumscribed mass is typically round or oval, or it may have a gently lobular contour. This visual description is significantly different from suspicious masses, which are often described as “irregular” in shape. Radiologists rely on this clear border to differentiate the finding from those with microlobulated, indistinct, or spiculated margins, which appear fuzzy, ill-defined, or have lines radiating out.
The Clinical Significance of a Clear Border
The presence of a well-circumscribed margin is generally considered a favorable sign, as it correlates strongly with a benign, non-cancerous finding. Benign lesions, such as cysts, lipomas, and fibroadenomas, are the most common causes of circumscribed masses. The sharp border suggests a slow growth pattern that has not aggressively invaded the surrounding tissue, which is typical of non-cancerous growths.
However, a clear border does not guarantee a benign diagnosis, and circumscribed masses are not always harmless. A small percentage of circumscribed masses are ultimately found to be malignant. Certain cancers, such as medullary carcinoma, mucinous carcinoma, or encapsulated papillary carcinoma, can sometimes present with a smooth, well-defined border, mimicking a benign lesion.
The Breast Imaging-Reporting and Data System (BI-RADS) scale uses the circumscribed margin to categorize the finding and guide management. A mass that is completely circumscribed, oval, and low-density may be classified as BI-RADS Category 2 (benign) or Category 3 (probably benign). A Category 3 finding often suggests a follow-up imaging schedule rather than an immediate biopsy. This standardized classification underscores that the clear border is a strong indicator of low suspicion, but a comprehensive assessment of all features is required.
Next Steps and Diagnostic Procedures
Once a circumscribed mass is identified, the next steps characterize the finding to confirm its benign nature. The first additional procedure is a targeted ultrasound, which uses sound waves to create a detailed image of the mass. Ultrasound is effective at determining whether the mass is a simple fluid-filled cyst, which requires no further intervention, or a solid mass.
If the mass is solid, a diagnostic mammogram may be performed, often with spot compression to ensure the margins are completely circumscribed. For a solid, non-palpable mass, the radiologist may recommend a short-interval follow-up, such as a re-scan in six months, to confirm the mass is stable and not growing. If the mass remains unchanged after a follow-up period, typically one to two years, it is then considered definitively benign.
A biopsy may be necessary even for a circumscribed mass if it is new, growing, or presents with suspicious features, such as increased density or an irregular shape. This procedure involves using a needle guided by imaging to extract a tissue sample, which is then analyzed by a pathologist. The final decision to biopsy is based on the radiologist’s assessment of all imaging features combined with the patient’s history and overall risk profile.