Predictive parameters of therapy response in PSMA radioligand therapy – correlation of absorbed dose and half-life with pre- and posttherapeutic SUV in ⁶⁸  Ga-PSMA-11 PET

Ziel/Aim Prediction of response is a major topic of PSMA-based radioligand therapy (RLT). The aim of this study was to evaluate the value of pretherapeutic PSMA-11 PET as a predictive marker for therapy response and dosimetry in mCRPC patients receiving ¹⁷⁷Lu-PSMA-617.

Methodik/Methods Pretherapeutic SUV_mean and SUV_max in PSMA-11 PET, as well as absorbed doses per injected activity and half-lives were calculated for 11 patients (age 68 ± 12 years) and 52 tumor lesions (38 bone, 14 lymph node). The ¹⁷⁷Lu-PSMA administered activity was 6GBq in 8 and 3.7 GBq in 3 patients. Decrease in tracer uptake was measured by subtraction of pre- and posttherapeutic SUVs in 9 patients and 41 lesions (27 bone, 14 lymph node). Absorbed dose and half-life were derived from Lu-177 SPECT scans of the Abdomen at 24, 48 and 72 hp. i., by using a mono-exponential fit model in combination with mass-scaled sphere S-values. Correlation was tested with Pearson’s correlation analysis.

Ergebnisse/Results There was a wide range in pretherapeutic SUV_mean and SUV_max in bone (24.6 ± 15.2 and 43.9 ± 27.4) and lymph node metastases (21.0 ± 11.6 and 42.4 ± 26.6). The mean absorbed dose was 5.8 ± 5.0 Gy/GBq in bone and 3.8 ± 4.1 Gy/GBq in lymph node metastases. The absorbed dose correlated strongly with pre- and posttherapeutic SUV_mean (r_pre = 0.73, r_post = 0.84) and moderately with pre – and posttherapeutic SUV_max (r_pre = 0.59, r_post = 0.53). Δ_SUVmean as well as Δ_SUVmax correlated weakly with absorbed dose (r = 0.41 and r = 0.35, respectively).

Schlussfolgerungen/Conclusions Pretherapeutic PSMA-PET SUV_mean and SUV_max correlate with absorbed dose after RLT indicating that tracer uptake in PSMA-PET can be used to tailor patient treatment. Lesions with a high absorbed dose present a decreasing SUV after RLT confirming the value of PSMA PET for response assessment.