What condition is indicated by the halo sign on a mammogram?

The halo sign observed on a mammogram is indicative of a breast cyst. This sign typically manifests as a well-circumscribed area with a darker perimeter around a central area, which represents the cyst itself. In the case of breast cysts, the halo represents the surrounding tissue's response to the cyst, often resulting in a compression and expansion of the local breast tissue that creates this distinctive radiological feature.

Breast cysts are fluid-filled sacs and are generally benign. The presence of the halo sign aids clinicians in distinguishing cysts from solid masses, which could potentially be neoplastic or malignant. Recognizing the halo sign on imaging helps guide management decisions, as cysts may require different approaches compared to solid tumors.