Response Assessment and Prediction of Progression-Free Survival by 68Ga-PSMA-11 PET/CT Based on Tumor-to-Liver Ratio (TLR) in Patients with mCRPC Undergoing 177Lu-PSMA-617 Radioligand Therapy.

At present, little is known about the molecular imaging-based response assessment of prostate-specific membrane antigen (PSMA)-targeted radioligand therapy with 177Lutetium (177Lu-PSMA-617 RLT) in metastatic castration-resistant prostate cancer (mCRPC). Our study evaluated the response to RLT using both molecular imaging and biochemical response assessments, and their potential prediction of progression-free survival (PFS). Fifty-one consecutive patients given two cycles of RLT at 6-week intervals were analyzed retrospectively. 68Ga-PSMA-11 PET/CT was obtained about 2 weeks prior to the first and 4-6 weeks after the second cycle. Molecular imaging-based response using SUVpeak and tumor-to-liver ratio (TLR) was determined by modified PERCIST criteria. ∆TLR and ∆SUV were significantly correlated with ∆PSA (p < 0.001, each). After a median follow-up of 49 months, the median PFS (95% CI) was 8.0 (5.9-10.1) months. In univariate analysis, responders showing partial remission (PRPSA and PRTLR) had significantly (p < 0.001, each) longer PFS (median: 10.5 and 9.3 months) than non-responders showing either stable or progressive disease (median: 4.0 and 3.5 months). Response assessment using SUVpeak failed to predict survival. In multivariable analysis, response assessment using TLR was independently associated with PFS (p < 0.001), as was good performance status (p = 0.002). Molecular imaging-based response assessment with 68Ga-PSMA-11 PET/CT using normalization of the total lesion PSMA over healthy liver tissue uptake (TLR) could be an appropriate biomarker to monitor RLT in mCRPC patients and to predict progression-free survival (PFS) of this treatment modality.

Performance Evaluation of a Newly Developed MR-Compatible Mobile PET Scanner with Two Detector Layouts.

PURPOSE: A mobile positron emission tomography (PET) scanner called flexible PET (fxPET), designed to fit existing magnetic resonance imaging (MRI) or computed tomography (CT) system, has been developed. The purpose of this study was to evaluate the image quality, lesion detection rate, and quantitative values of fxPET compared with conventional bismuth germanium oxide (BGO)-based PET/CT without time-of-flight capability. PROCEDURES: Fifty-nine patients underwent whole-body (WB) PET/CT scans approximately 1 h after injection of 2-deoxy-2-[18F]fluoro-D-glucose, followed by the fxPET scans with detectors located above and below the patients (layout A) and with detectors closer to the patients (layout B). Two readers assessed the image quality using a 4-point grade for each layout and reached a consensus. We evaluated the differences and/or correlations between fxPET and WB PET/CT, including the lesion detection rates, the standardized uptake value (SUV), the metabolic tumor volume (MTV), the total lesion glycolysis (TLG), the tumor-to-normal liver ratio (TLR), and the background liver signal-to-noise ratio (SNR). RESULTS: The image quality of layout B was better than layout A (p < 0.0001). Of 184 lesions, the detection rate of layout B was significantly higher than WB PET/CT (p = 0.041), while the detection rate of layout A was comparable to WB PET/CT. The SUVmax/mean/peak were larger, and the MTVs were smaller in fxPET than WB PET/CT, especially in the lesions smaller than 2 cm (p < 0.01). The SUVmax/mean/peak, the MTVs and the TLGs of fxPET had significant positive correlations with WB PET/CT (p < 0.0001). The TLRs were significantly larger (p < 0.0001), but the background SNRs were significantly lower in fxPET than WB PET/CT (p < 0.05). CONCLUSIONS: The fxPET system yielded reasonable image quality and quantitative accuracy. Bringing the detectors closer to the patient yielded improved results.