Gynecomastia and invasive ductal cancer are the most common lesions in the male breast, but there are other rarer benign and malignant lesions.
Gynecomastia and carcinoma can usually be differentiated, but biopsy is sometimes necessary to separate them.All lesions eccentric to the nipple need biopsy unless they are characteristically benign, i.e.contain fat or typical lymph node.
On this diagram a list of characteristics of gynecomastia versus carcinoma.Notice that there are many similarities.
Both gynecomastia and carcinoma occur mostly at the age of 60 and can be soft, mobile, subareolar and unilateral.
So that does not help.
$$Carcinoma is usually eccentric, while gynecomastia is never eccentric.
$$Gynecomastia has to have extensions into the surroundig fat.
$$Carcinoma sometimes may have spiculations, that can look the same.
Actually we call it extension into the fat, if we think it is gynecomastia and spiculation, if we think it is a carcinoma.
To differentiate between true gynecomastia and pseudogynecomastia