Initial experience with volumetric ⁶⁸Ga-PSMA I&T PET/CT for assessment of whole-body tumor burden as a quantitative imaging biomarker in patients with prostate cancer

A quantitative imaging biomarker is desirable to provide a comprehensive measure of whole-body tumor burden in patients with metastatic prostate cancer, and to standardize the evaluation of treatment-related changes. Therefore, we evaluated whether prostate-specific membrane antigen (PSMA) ligand PET/CT may be applied to provide PSMA-derived volumetric parameters for quantification of whole-body tumor burden. Methods: One hundred one patients who underwent ⁶⁸Ga-PSMA I&T PET/CT because of increasing prostate-specific antigen (PSA) levels after radical prostatectomy were included in this retrospective analysis. Tracer uptake was quantified using SUVs. Volumetric parameters, that is, PSMA-derived tumor volume (PSMA-TV) and total lesion PSMA (TL-PSMA), were calculated for each patient using a 3-dimensional segmentation and computerized volumetry technique and compared with serum PSA levels. In a group of 10 patients, volumetric parameters were applied for treatment monitoring. Results: Volumetric parameters, that is, whole-body PSMA-TV and whole-body TL-PSMA, demonstrated a statistically significant correlation with PSA levels (P , 0.0001) as a surrogate marker of tumor burden, whereas SUV_max (P = 0.22) or SUV_mean (P = 0.45) did not. Treatment response and treatment failure were paralleled by concordant changes in both whole-body PSMA-TV and whole-body TL-PSMA (P = 0.02), whereas neither the change in SUV_max (P = 1.0) nor the change in SUV_mean (P = 1.0) concordantly paralleled changes in PSA levels. Conclusion: PSMA-derived volumetric parameters provide a quantitative imaging biomarker for whole-body tumor burden, capable of standardizing quantitative changes in PET imaging of patients with metastatic prostate cancer and of facilitating therapy monitoring.