A 53-year-old woman presents to the clinic for evaluation of a new left breast lump that she discovered 1 week ago. The nodule is not painful; she feels well and has no other symptoms. On physical examination the lump is palpable, and there are no axillary lymph nodes. The mammogram confirms the presence of a 1-cm breast mass which on biopsy is positive for invasive breast cancer. Which of the following is the most appropriate local therapy for her tumor?