Quantitative Analysis of Serial Positron Emission Tomography Imaging in Men with Metastatic Castration-resistant Prostate Cancer Treated with Enzalutamide.

BACKGROUND: The emergence of positron emission tomography (PET) in prostate cancer is impacting clinical practice, but little is known about PET imaging as a tool to determine treatment failure in metastatic castration-resistant prostate cancer (mCRPC). OBJECTIVE: To evaluate PET imaging dynamics in mCRPC patients on enzalutamide with stable computed tomography (CT) and technetium-99m (Tc99) bone scans. DESIGN, SETTING, AND PARTICIPANTS: All patients were on treatment with enzalutamide for first-line mCRPC in a clinical trial at the National Cancer Institute (Bethesda, MD, USA). A volunteer sample had serial 18F-sodium fluoride (NaF) PET in parallel with CT and Tc99. Regions of interest (ROIs) on NaF were analyzed quantitatively for response. INTERVENTION: Patients were randomized to enzalutamide with/without a cancer immunotherapy, Prostvac. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: A post hoc, descriptive analysis was performed comparing the changes seen on CT and Tc99 as per RECIST 1.1 with NaF PET scans including the use of a quantitative analysis. RESULTS AND LIMITATIONS: Eighteen mCRPC patients had 67 NaF scans. A total of 233 ROIs resolved after treatment, 52 (22%) of which eventually retuned while on therapy. In all, 394 new ROIs were seen, but 112(28%) resolved subsequently. Of 18 patients, 14 had new ROIs that ultimately resolved after appearing. Many patients experienced progression in a minority of lesions, and one patient with radiation intervention to oligoprogression had a remarkable response. This study is limited by its small number of patients and post hoc nature. CONCLUSIONS: These data highlight the dynamic nature of NaF PET in mCRPC patients treated with enzalutamide, where not all new findings were ultimately related to disease progression. This analysis also provides a potential strategy to identify and intervene in oligoprogression in prostate cancer. PATIENT SUMMARY: In this small analysis of patients with prostate cancer on enzalutamide, changes on 18F-sodium fluoride positron emission tomography (PET) imaging were not always associated with treatment failure. Caution may be indicated when using PET imaging to determine whether new therapy is needed.