Which statement best describes a well-circumscribed mass on mammography?

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On mammography, the edges of a lesion tell you a lot about its behavior. A well-circumscribed, smooth margin usually means the lesion has grown by expansion and is pushing aside rather than invading surrounding tissue. That pattern is most commonly seen with benign processes, such as fibroadenomas or simple cysts. Because these lesions tend to stay well defined as they enlarge, they hardly distort the surrounding architecture.

In contrast, malignant tumors often disrupt the boundary and show irregular, spiculated, or ill-defined margins, reflecting invasion into adjacent tissue, which is a hallmark of cancer. Calcifications accompanying a mass can also influence interpretation, but the margin appearance itself is a strong clue: well-defined borders favor benignity, while spiculated or hazy edges raise concern for malignancy.

Keep in mind there are exceptions, so clinical context matters. Age, density of the lesion, growth over time, and stability on prior images all help refine the assessment. If a well-circumscribed mass remains stable over time and lacks suspicious calcifications or symptoms, it’s usually managed with routine imaging follow-up. If there are any worrisome features or changes, ultrasound or biopsy may be indicated for a definitive diagnosis.