Breast cancer commonly metastasises to the skeleton, and stereotactic ablative body radiation therapy (SABR) is an emerging treatment for oligometastatic disease. Accurately imaging bone metastases and their response to treatment is challenging. [18F]NaF-PET has a higher sensitivity and specificity than conventional bone scans for detecting breast cancer bone metastases. In this pre-defined secondary analysis of a prospective trial, we evaluated the change in [18F]NaF uptake after SABR. Patients with oligometastatic breast cancer received a single fraction of 20 Gy to up to three bone metastases. [18F]NaF-PET was acquired before and 12 months after SABR. Pre- and post-treatment [18F]NaF-PET images were registered to the treatment planning CT. The relative change in tumour SUVmax and SUVmean was quantified. The intersection of each of the radiation therapy isodose contours with a non-tumour bone was created. The change in SUVmean in sub-volumes of non-tumour bone receiving doses of 0�