ESR1 and aneurysmal bone cyst: While the RAD1901-005 phase 1 trial was ongoing, the current phase 1b study (RAD1901-106) was initiated to determine the effect of elacestrant treatment on the availability of ER in lesions from patients with ABC using FES-PET with low-dose computed tomography (FES-PET/CT) imaging as a measure of ER downregulation.