68GaPSMA-11 PET/CT to monitor treatment response in patients with metastatic prostate cancer: The concordance between biochemical response and PSMA response.

Abstract

49 Background: The aim of this study was to assess whether PSMA PET is associated with flares, with discordant results in regard to biochemical response, and with positivity rates at low PSA values while monitoring metastatic prostate cancer (mPCa) receiving systemic treatment. Methods: A total of 96 patients with PSMA PET-detected mPCa who underwent at least one follow-up scan after receiving systemic treatment were included in the study. PSA levels at baseline PSMA PET-CT (bPSMA) and follow-up PSMA PET scans (fPSMA) were recorded. Any significant PSMA uptake at prostate and/or metastatic lesions were considered PSMA positivity. PSMA response were decided according to PSMA PET Progression (PPP) criteria . Biochemical progression was defined as ≥25% increase in PSA (1 ng/dL was the minimal starting value). PSMA PET and PSA response were dichotomized into progressive disease (PD) vs non-PD. Discordant responses among different metastases and prostate were defined as the condition when some metastatic lesions (or prostate) are responding (CR or PR) to systemic treatment while others had progressed. Results: A total of 346 serial PSMA PET/CT (96 bPSMA and 249 fPSMA) scans were evaluated. The median time between consecutive PSMA PET scans was 6.3 months (IQR: 4.7 – 10.6) Overall PSMA PET positivity rate of fPSMA scans was 88.4% (220/249). PSMA positivity rates according PSA levels were summarized. PPP was detected in 93 cases (37.8%) and biochemical progression was observed in 73 cases (30.2%). PSA and PSMA responses were highly concordant (Cohen’s K=0.623, p<0.001). PSA-PSMA discordant responses were detected in 39 scans (17%). The possible causes could be identified in 34 of 39 scans (87%) while no obvious reason for discordance could be shown in the remaining 5. Conclusions: PSMA PET/CT showed very high detection rates of malignant lesions even in very low PSA values when monitoring treatment response in patients receiving systemic treatment for mPCa. PSA and PSMA responses were highly concordant. In cases with discordance, the reason seems to be the discordant behaviour of intraprostatic and metastatic lesions and not necessarily PSMA flares. PSMA PET is a promising biomarker to assess treatment response in patients with mPCa. [Table: see text]