An 80-year-old woman is hospitalized after a mechanical fall. She has a history of stage I estrogen receptor–positive and progesterone receptor–positive left breast cancer diagnosed 13 years ago; HER2 testing was not done at that time. She was treated with breast-conserving surgery, primary breast radiation, and adjuvant tamoxifen for 5 years. She is not having any current bone pain or headaches.
On physical examination, vital signs are normal. A large palpable lesion is present over the left frontal skull. There is no lymphadenopathy. Examination of the left breast shows a healed incision with no masses. There are no right breast masses. The remainder of the examination is unremarkable.
Serum alkaline phosphatase level is elevated at 264 U/L (normal 36-92 U/L) and serum CA 15-3 level is 100.2 U/mL (normal <30 U/mL). Remaining laboratory studies, including serum calcium level, are normal.
CT scan of the head done in the emergency room shows a 3-cm lytic lesion in the left frontal skull. MRI of the brain confirms the presence of a large frontal skull lesion but shows no brain metastases. Bone and CT scans show lesions in the spine, skull, sternum, and bilateral ilium bones consistent with metastases. No visceral disease is present.
Biopsy of a lytic lesion in the right ilium shows metastatic adenocarcinoma consistent with primary breast cancer (estrogen receptor positive, progesterone receptor positive, and HER2 negative).
Which of the following is the most appropriate treatment?