[18F]Sodium-fluoride PET/MRI monitoring of hormonal therapy response in breast cancer bone metastases – Proof of concept

AbstractRECIST 1.1 tumour size measurements on CT/MRI are the mainstay of cancer therapy monitoring. However, bone metastases are consistently difficult to evaluate for hormonal therapy response often escaping CT detection. This study aimed to assess dynamic and static [18F]sodium fluoride-([18F]NaF)-PET/MRI by combining SUV and net influx rate (Ki) from PET with the apparent diffusion coefficient (ADC), proton density fat fraction (PDFF) and effective transverse relaxation rate (R2*) from MRI for monitoring hormonal therapy effect on bone metastases. In this prospective study, three breast cancer patients underwent a 60-minute dynamic whole-body [18F]NaF-PET/MRI before and after hormonal therapy. In PET images, pelvic and spine metastases (approx. n = 10/patient) with high/intermediate uptake were delineated by applying an adaptive threshold algorithm to provide SUVmeanand SUVmax. Pharmacokinetic modeling was performed and Kiwas calculated using a two-tissue reversible model. VOI measurements of ADC, PDFF and R2*utilized the OLEA medical software. The changes between baseline and follow-up data were calculated, statistically analysed and utilized linear regression. [18F]NaF-PET/MRI provided a powerful method for monitoring hormonal therapy response in breast cancer bone metastases as reflected by decreases in SUV and Ki. MRI parameters showed changes consistent with therapy response, although only R2*reached statistical significance.