The most common reason for imaging the male breast is finding an asymmetric thickening or mass clinically that almost always turns out to be asymmetric benign gynecomastia. Most male breast cancers are eccentrically located, away from the subareolar region. In contrast, gynecomastia is typically central and subareolar in location. The normal male breast contains ducts similar to those found in prepubertal girls but, even when there's gynecomastia, fibroadenomas don't occur since lobule formation is rare. It's also unusual to find cysts in men, but they can develop ductal carcinomas. Breast carcinoma in males is extremely uncommon, though, making up fewer than 1% of breast cancer cases diagnosed each year. Although male breast cancer can occur at almost any age, the vast majority develop in men older than 50 years. If a man under the age of 50 presents with bilateral or unilateral diffuse breast enlargement, or a mass that's palpable, nonindurated, central and subareolar, mammography usually isn't indicated. Exceptions may include a history of previous irradiation as a child, suspicious nipple discharge or the presence of Klinefelter's syndrome. In these men, as well as those over 50, mammography is necessary. On a mammogram, male breast carcinoma looks like that seen in women; lesions may be lobulated, spiculated, or ill-defined, and can be associated with microcalcifications. Gynecomastia, on the other hand, resembles normal active female breast tissue, with patchy areas of dense fibroglandular tissue, interspersed among islands of fatty tissue. In that case, no further imaging is required. If the mammogram reveals a suspicious focal mass, a breast ultrasound would help to confirm it; if the mass turns out to be solid, ultrasound-guided core biopsy can be done for tissue diagnosis. MM Reference: Radiology 1994;191:651-6.