[F]NaF PET/CT imaging of response to single fraction SABR to bone metastases from breast cancer.

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. [F]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 [F]NaF uptake after SABR. Patients with oligometastatic breast cancer received a single fraction of 20 Gy to up to three bone metastases. [F]NaF-PET was acquired before and 12 months after SABR. Pre- and post-treatment [F]NaF-PET images were registered to the treatment planning CT. The relative change in tumour SUV and SUV was quantified. The intersection of each of the radiation therapy isodose contours with a non-tumour bone was created. The change in SUV in sub-volumes of non-tumour bone receiving doses of 0-20 Gy was quantified. In total, 14 patients, with 17 bone metastases, were available for analysis. A total of 15 metastases exhibited a reduction in SUV; the median reduction was 42% and the maximum reduction 82%. An increased absolute reduction in SUV was observed with higher pre-treatment SUV. One patient exhibited increased SUV after treatment, which was attributed to normal peri-tumoural bone regeneration in the context of a bone metastasis. There was a median reduction of 15%-34% for non-tumour bone in each dose level.