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The role of the gallium-68 (⁶⁸Ga)-labeled prostate-specific membrane antigen (PSMA) ligand in treatment response evaluation after definitive radiotherapy (RT) has not been evaluated in prostate cancer (PC) patients. The aim of this study is to evaluate standardized uptake value (SUV) changes after definitive RT and to assess the factors affecting SUV changes in primary tumors (SUV_p) and metastatic lymph nodes (SUV_ln) and PSA response rates. 

The clinical data of 108 PC patients receiving neoadjuvant ADT and definitive intensity-modulated RT between March 2015 and May 2020 were retrospectively analyzed. All patients underwent ⁶⁸Ga-PSMA-PET/CT before the start of ADT for initial staging and RT planning and after the completion of RT. Patients receiving ADT or chemotherapy before ⁶⁸Ga-PSMA-PET/CT, patients undergoing radical prostatectomy, and patients with clinical and radiological evidence of distant metastasis were excluded from the study. Differences in SUV_p, SUV_ln, and PSA before and after RT were evaluated. Receiver operating characteristic curves were generated for primary tumor pre- and posttreatment SUV response to determine the cutoff for predicting recurrence.

The median serum PSA level at a median of 3.4 months (range: 2.7–4.9 months) after ADT initiation was 0.3 ng/mL (range: 0–53.2 ng/mL), and the median PSA response rate was 98.8% (range: 1.4–100%). A PSA response at a median of 4.1 months (range: 2.8–4.8 months) after RT was observed in all patients, with a median decrease of 99.9% (range: 51.1–100%) and a median PSA level of 0 ng/mL (range: 0–14.2 ng/mL) (Fig 1). Posttreatment ⁶⁸Ga-PSMA-PET/CT imaging was performed at a median of 9.7 months after ADT initiation. Decreases of a median of 100% in SUV_p and a median of 100% in SUV_ln were seen in 101 and 46 patients, respectively. The response rates in serum PSA, SUV_p, and SUV_ln did not differ significantly according to the Gleason score, PSA groups, and clinical T and N stages. A decrease in SUV_ln was observed in 46 patients, with a median of 100% (range: 45.2–100%). One patient showed no change in SUV_ln, and one showed a 53% increase. Bone metastasis was observed on the posttreatment ⁶⁸Ga-PSMA-PET/CT of two patients. According to the PERCIST, 55 (52.4%) patients had CR, 36 (34.3%) had PR, 9 (8.6%) had SD, and 5 (4.7%) had PD. Disease progression was observed in 20% of the patients. The progression rate was significantly higher in patients with a SUV_p response rate of <61.7% than in those with a ≥61.7% response rate (46.6% vs. 9.2%; p<0.001). Patients with higher SUV response rates also had significantly better PSA responses after definitive RT and ADT than patients with lower SUV responses (99.7 ± 0.8% vs. 95.6 ± 10.5%; p = 0.001).

The findings suggest that ⁶⁸Ga-PSMA-PET/CT could be used as a quantitative imaging modality for therapy monitoring after neoadjuvant ADT and RT in PC patients. The findings should be validated by prospective studies.