Predictive value of interim ¹¹C-choline PET/CT for response to radium-223 in metastatic castration-resistant prostate cancer

Radium-223 (Ra-223) has been approved for the treatment of metastatic castration-resistant prostate cancer (mCRPC) with predominant osseous metastases. PSA flare phenomenon with Ra-223 limits its use as a prognostic tool to predict treatment response. Currently, there is a lack of diagnostic tools to predict treatment response to Ra-223. Therefore, we sought to investigate the role of mid treatment C-11 choline PET/CT scan in predicting overall response to Ra-223. In a single institute retrospective study, we identified 32 patients who were treated with a full course of 4–6 cycles of Ra-223 and were evaluated with PSA, and C-11 choline PET scan before treatment, at mid-treatment point, and after complete course of Ra-223 between 2013 and 2018. Ra-223 was used as a salvage therapy for their predominant bone disease after failing chemotherapy and 2nd generation hormone therapy. Blind repeat radiographic evaluation of patients’ scans has been done by a radiologist specialized in nuclear medicine. Favorable response was defined by achieving partial response or stable disease on imaging, while unfavorable response was defined by showing progressive disease on imaging. Mean age (± SD) at starting Radium223 was 67.6 (± 7.1) years, median (IQR) primary Gleason score was 9 (8–9), and median (IQR) pretreatment PSA was 13.5 (5.4–39.6) ng/ml. 78% of the patients (n = 25) completed 6 cycles of Ra-223, and 15% of the patients (n = 5) completed 5 cycles while 6% (n = 2) received only 4 cycles. At mid treatment scanning, 25% of the patient (n = 8) showed favorable response (Group A), while 75% of the patients (n = 24) showed unfavorable response (B). After complete course of treatment, 50% of patients (n = 4) in Group (A) continued to show favorable response, versus only 8.3% of patients (n = 2) in Group (B) showed favorable response (p-value = 0.023). Mid treatment evaluation with C-11 choline PET/CT scan can predict overall response to Radium 223. Patients with progressive disease at mid- treatment evaluation are very likely not going to respond to their treatment. Further studies and clinical trials are warranted.