On the left 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.
Treatment of mastitis and/or abscess in nonlactating women largely the same as that of lactational mastitis, generally involving antibiotics treatment, possibly surgical intervention by means of fine-needle aspiration and/or incision and drainage and/or interventions on the lactiferous ducts (for details, see also the articles on treatment of mastitis , of breast abscess and of subareolar abscess ). Additionally, an investigation for possible malignancy is needed, normally by means of mammography , and a pathological investigation such as a biopsy may be necessary to exclude malignant mastitis.  Although no causal relation with breast cancer has been established, there appears to be an increased statistical risk of breast cancer, warranting a long-term surveillance of patients diagnosed with non-puerperal mastitis.