Impact of definitive radiotherapy on metabolic response measured with 68Ga-PSMA-PET/CT in patients with intermediate-risk prostate cancer

Purpose To assess the early metabolic response of the primary tumor using Gallium-68 (Ga-68)-labeled-prostate-specific membrane antigen positron emission tomography (Ga-68-PSMA-PET/CT), as well as the relationship between PSMA change in the primary tumor and PSA response after definitive radiotherapy (RT), either alone or in combination with androgen deprivation therapy (ADT) in intermediate risk prostate cancer (IR-PCa) patients. Methods The clinical data of 71 IR-PCa patients treated with RT alone (36 patients, 50.7%) or RT and ADT (35 patients, 49.3%) were retrospectively analyzed. The difference between pre- and Posttreatment primary tumor PSMA expression and serum PSA values measured 4 months after completion of treatment were compared between treatment arms. Correlation between primary tumor metabolic response and serum PSA changes was analyzed. Results The median duration between pre- and Posttreatment Ga-68-PSMA-PET/CT for the entire patient population was 6.9 months (range, 5.6-8.4 months), and it was similar in both treatment arms. A decrease in primary tumor maximum standardized uptake value (SUVmax) was seen in 66 patients (93.0%), with a median value of 61.2%, which is significantly lower in patients undergoing RT alone than those undergoing RT and ADT (45.1 +/- 30.6% vs. 59.1 +/- 24.7%; p = 0.004). The complete metabolic response rate was significantly higher in patients undergoing RT and ADT than those treated with RT alone (40% vs. 0%; p < 0.001). Although moderate and positive correlation between pretreatment SUVmax and oosttreatment SUVmax was observed, there was no significant correlation between SUV change and PSA change. For patients treated with RT and ADT, posttreatment SUVmax was significantly lower and SUV change was significantly higher in patients with PSA nadir than in those without. Conclusions Our preliminary results show that RT, with or without ADT, significantly reduces primary tumor SUVmax and serum PSA levels. Nonetheless, our findings indicate that early treatment response using Ga-68-PSMA-PET/CT is not feasible for those treated with RT alone, and it may only be useful in better distinguishing patients with and without PSA nadir for those who received both RT and ADT.