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

Abstract

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. One hundred one patients who underwentGa-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. Volumetric parameters, that is, whole-body PSMA-TV and whole-body TL-PSMA, demonstrated a statistically significant correlation with PSA levels ( < 0.0001) as a surrogate marker of tumor burden, whereas SUV ( = 0.22) or SUV ( = 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 ( = 0.02), whereas neither the change in SUV ( = 1.0) nor the change in SUV ( = 1.0) concordantly paralleled changes in PSA levels. 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.

qPSMA: Semiautomatic Software for Whole-Body Tumor Burden Assessment in Prostate Cancer Using 68Ga-PSMA11 PET/CT

“…Therefore, a semiautomatic method would pave the way toward a whole-body tumor load quantification in prostate cancer patients. In the present study, the mean analyzed PSMA-TV and PSMA-TL were 823 mL and 7,273, respectively, indicating that our patient cohort was much more advanced than previous reports in the literature using manual segmentation (24,25). Furthermore, this factor could also explain why we obtained stronger correlations between serum PSA and PSMA ligand PET-derived parameters than those previously described in 2 reports (24,25).…”

“…In the present study, the mean analyzed PSMA-TV and PSMA-TL were 823 mL and 7,273, respectively, indicating that our patient cohort was much more advanced than previous reports in the literature using manual segmentation (24,25). Furthermore, this factor could also explain why we obtained stronger correlations between serum PSA and PSMA ligand PET-derived parameters than those previously described in 2 reports (24,25). However, those 2 reports analyzed patients with biochemical recurrence, and our study investigated patients with metastatic castration-resistant prostate cancer.…”

“…A first approach in whole-body tumor burden assessment using 68 Ga-PSMA11 PET/CT was introduced by Schmuck et al (24) using an isocontour SUV threshold method. Because the process was time-consuming, only patients with a low tumor load (,10 lesions) were manually analyzed.…”

“…No segmentation method has been yet established for PET as the gold standard. Fixed threshold and isocontour relative threshold have been largely used in PET quantification (22)(23)(24). However, it has been shown that despite different results using various segmentation methods or partial-volume effect correction, no significant impact on the predictive or prognostic power of PET-derived parameters could be found (28,29).…”

Prostate Cancer Molecular Imaging Standardized Evaluation (PROMISE): Proposed miTNM Classification for the Interpretation of PSMA-Ligand PET/CT

“…Consequently, direct assessment of tumor volume, instead of assessment of lesion diameter sums, could be done. For PSMA-ligand PET, initial attempts have been made by introducing PSMAderived tumor volume, total lesion PSMA, or bone PET indices (59,61). Further advances in the field of software-assisted tumor delineation will help to automatically delineate-separately for bone and soft tissue-total tumor volume, total tumor target expression, or a combination of these.…”