Exploring New Multimodal Quantitative Imaging Indices for the Assessment of Osseous Tumor Burden in Prostate Cancer Using ⁶⁸Ga-PSMA PET/CT.

Abstract

PET combined with CT and prostate-specific membrane antigen (PSMA) ligands has gained significant interest for staging prostate cancer (PC). In this study, we propose 2 multimodal quantitative indices as imaging biomarkers for the assessment of osseous tumor burden using ⁶⁸Ga-PSMA PET/CT and present preliminary clinical data. <b>Methods:</b> We defined 2 bone PET indices (BPIs) that incorporate anatomic information from CT and functional information from ⁶⁸Ga-PSMA PET: BPI_VOL is the percentage of bone volume affected by tumor and BPI_SUV additionally considers the level of PSMA expression. We describe a semiautomatic computation method based on segmentation of bones in CT and of lesions in PET. Data from 45 patients with castration-resistant PC and bone metastases during ²²³Ra-dichloride were retrospectively analyzed. We evaluated the computational stability and reproducibility of the proposed indices and explored their relation to the prostate-specific antigen blood value, the bone scan index (BSI), and disease classification using PERCIST. <b>Results:</b> On the technical side, BPI_VOL and BPI_SUV showed an interobserver maximum difference of 3.5%, and their computation took only a few minutes. On the clinical side, BPI_VOL and BPI_SUV showed significant correlations with BSI (<i>r</i> = 0.76 and 0.74, respectively, <i>P</i> < 0.001) and prostate-specific antigen values (<i>r</i> = 0.57 and 0.54, respectively, <i>P</i> < 0.01). When the proposed indices were compared against expert rating using PERCIST, BPI_VOL and BPI_SUV showed better agreement than BSI, indicating their potential for objective response evaluation. <b>Conclusion:</b> We propose the evaluation of BPI_VOL and BPI_SUV as imaging biomarkers for ⁶⁸Ga-PSMA PET/CT in a prospective study exploring their potential for outcome prediction in patients with bone metastases from PC.