This woman should undergo a diagnostic core needle biopsy of the palpable breast mass to obtain a tissue diagnosis. Although mammography is indicated with a newly detected breast lump to assist in evaluation, it has false-negative rates of between 10% and 20% even if a standardized reporting system, such as the Breast Imaging Reporting and Data System (BI-RADS), suggests a negative finding as in this patient with a BI-RADS category of 1, indicating that no radiographic masses, architectural disturbances, or suspicious calcifications are present. Therefore, biopsy is required for a palpable breast mass even if a mammogram is negative or nondiagnostic, as in this patient. Fine needle aspiration cytology is also an option, but evidence indicates slightly better sensitivity and specificity with larger bore core biopsy needles.

Breast MRI may further define the characteristics of the mass but would not provide adequate information to obviate the need for a definitive tissue diagnosis.

Conservative management, with re-evaluation at a follow-up visit in 4 weeks or resuming regular mammographic screening, is not appropriate in a postmenopausal woman with a palpable breast mass, which requires further, immediate evaluation.