Combined Immunotherapies in Metastatic ER+ Breast Cancer

Patients randomized to arm 2 will start letrozole alone, and add palbociclib on day 21, after completion of I-SBRT. They will undergo tumor Immunogenic-SBRT(I-SBRT) days 1-12 (+/-2 days, to enable inclusion of holidays). During the week preceding day 1, they will undergo simulation and planning for radiotherapy. Each oligometastatic lesion will be treated with I-SBRT every 48 hours. Treatment may be given daily (to keep the total I-SBRT treatment time to ≤ 12 days) and lesions targeted with I-SBRT will thus be alternated each day to accommodate for the 48 hour interval between fractions

All patients start standard therapy with oral letrozole (Femara), day 1 of the study.

Patients randomized to arm 2 will start letrozole alone, and add palbociclib on day 21, after completion of I-SBRT.