The differential diagnosis for a breast lesion in a man is always gynecomastia. 

The mammographic and ultrasound characteristics of male carcinoma breast are exactly the same as those of the cancer in  a female breast. There is no imaging difference between genders. This lesion is mammographically dense, poorly defined and in a female it would always be suggestive of a malignancy.

In practice, the mammogram images of gynecomastia show the mass-like density abutting the posterior margin of the nipple; note this lesion is a little away from the nipple.

The ultrasound images are always helpful (see above). Always include ultrasound in the workup of the male breast as with females. The typical triangular density of gynecomastia is a dead giveaway on ultrasound. The apex of the triangular breast tissue abuts the posterior nipple.