Purpose: To investigate the value of 68Ga-HBED-CC PSMA (68Ga-PSMA) PET/CT for response assessment in metastatic castration-sensitive and castration-resistant prostate cancer (mCSPC and mCRPC) during docetaxel chemotherapy.
Methods: 68Ga-PSMA PET/CT was performed in seven mCSPC patients before and after six cycles of upfront docetaxel chemotherapy and in 16 mCRPC patients before and after three cycles of palliative docetaxel chemotherapy. Radiographic treatment response was evaluated separately on the 68Ga-PSMA PET and CT datasets. Changes in 68Ga-PSMA uptake (SUVmean) were assessed on a per-patient and a per-lesion basis using the PERCIST scoring system with slight modification. Treatment response was defined as absence of any PSMA uptake in all target lesions on posttreatment PET (complete response, CR) or a decrease in summed SUVmean of ≥30% (partial response, PR). The appearance of a new PET-positive lesion or an increase in summed SUVmean of ≥30% (progressive disease, PD) indicated nonresponse. A moderate change in summed SUVmean (between -30% and +30%) without a change in the number of target lesions was defined as stable disease (SD). For treatment response assessment on CT, RECIST1.1 criteria were used. Radiographic responses on 68Ga-PSMA PET [RR(PET)] and on CT [RR(CT)] were compared and correlated with biochemical response (BR). A decrease in serum PSA level of ≥50% was defined as biochemical PR.
Results: Biochemical PR was found in six of seven patients with mCSPC (86%, 95% confidence interval 42% to 99.6%). The concordance rate was higher between BR and RR(PET) than between BR and RR(CT) (6/7 vs. 3/6 patients. 68Ga-PSMA PET and CT were concordant in only three patients (50%, 12% to 88%). In mCRPC patients, biochemical PR was found in six of 16 patients (38%, 15% to 65%). Outcome prediction was concordant between BR and RR(PET) in nine of 16 patients (56%), and between BR and RR(CT) in only four of 12 patients (33%) with target lesions on CT. 68Ga-PSMA PET and CT results corresponded in seven of 12 patients (58%, 28% to 85%).
Conclusion: Our preliminary results suggest that 68Ga-PSMA PET might be a promising method for treatment response assessment in mCSPC and mCRPC. The data indicate that for different metastatic sites, the performance of 68Ga-PSMA PET in response assessment might be superior to that of the conventional CT approach and could help differentiate between progressive disease and treatment response. Because of the limited number of patients, the differences revealed in our study were not statistically significant. Thus larger and prospective studies are clearly needed and warranted to confirm the value of 68Ga-PSMA PET as an imaging biomarker for response assessment.
Keywords: 68Ga-PSMA-HBED-CC; Castration-resistant prostate cancer; Castration-sensitive prostate cancer; Prostate-specific membrane antigen; Therapy response.