In a 60-year-old woman with a history of estrogen replacement therapy for 20 years following a hysterectomy, a mammogram reveals a 1.5 cm mass in the upper outer quadrant. The surgeon does not detect any masses on physical examination, and assessments of axillary, supraclavicular, and cervical areas are negative. How would you interpret these findings, and what further diagnostic steps or considerations would you recommend in this scenario?