Aim: Conventional imaging modalities(CIM) have limited sensitivity for detection of prostate cancer(Pca),however,Ga-68PSMA-HBED-CC(PSMA)PET/CT is increasingly being used for identification of loco-regional/systemic recurrences. The aim of this study was to 1)investigate the diagnostic yield of ⁶⁸Ga-PSMA-PET-CT at different trigger prostate specific antigen(tPSA) levels,2)correlate tPSA with ⁶⁸Ga-PSMA-PET-CT semi-quantitative indices and3)determine the clinical influence of this modality in patients with recurrent Pca.

Patients and Methods:One hundred twenty patients who were referred to ⁶⁸Ga-PSMA PET-CT for biochemical recurrence from PCa after initial therapies were included.The lesion detection rates and semi-quantitative PET parameters(Total tumor volume(TTV), total tumor index(TTI)(SUV meanxtumor volume)) were correlated with primary Gleason score(GS),androgen deprivation therapy(ADT),tPSA level and PSA kinetics.The clinical impact of ⁶⁸Ga-PSMA-PET-CT compared to CIM was also assesed with an impact scale(0:No contribution,1:Detected more lesions than CIM but no change in therapy,2:Minor change in the therapy plan/Lu-177-PSMA was indicated,3:Major change in the treatment plan and modality).

Results:⁶⁸Ga-PSMA PET-CT detected recurrence in 91(75.8.%) patients.Majority of the recurrences were in the lymph nodes followed by bone metastases(51.4%and 23.1 respectively).Higher detection rates were found in patients who had higher GSs(p:0,031) or received ADT(p<0,0001).The detection rates were increased with higher tPSA levels;46,9%, 80% and 86,8% for tPSA levels of <1, 1-1,99 and >2 ng/mL respectively(p<0,0001).Higher tPSA velocity is correlated with positive PET-CT scans 73.6%, 90,9% and 100% detection rate in <1, 1-4,99 and ≥5 ng/mL/months respectivelyp:0,018).PSA doubling time showed negative correlation with detection rate;66.7%, 81.8% and 91.8% in >6, 4.1–6, and ,≤4 months respectively(p:0,004). TPSA levels showed good correlation with PET parameters,TTV and TI(Spearman corrrelation coefficient:0,638 and 0,596p<0,0001).⁶⁸Ga-PSMA PET-CT was superior to CIM in 65% of patients as defined by impact scale

Conclusion: ⁶⁸Ga-PSMA PET/CT can identify the presence and extent of recurrent PCa which in many cases can substantially change subsequent treatment.Our initial experience revealed a good association between the semi-quantitative PET/CT parameters and tPSA which significantly influences the detection rate.