Ga-PSMA PET outperforms planar BS for the detection of affected bone regions as well as determination of overall bone involvement in PC patients. Our results indicate that BS in patients who have received PSMA PET for staging only rarely offers additional information; however, prospective studies, including a standardized integrated x-ray computed tomography (SPECT/CT) protocol, should be performed in order to confirm the presented results.

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 Ga-PSMA PET/CT and present preliminary clinical data. We defined 2 bone PET indices (BPIs) that incorporate anatomic information from CT and functional information from Ga-PSMA PET: BPI is the percentage of bone volume affected by tumor and BPI 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 Ra-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. On the technical side, BPI and BPI showed an interobserver maximum difference of 3.5%, and their computation took only a few minutes. On the clinical side, BPI and BPI showed significant correlations with BSI ( = 0.76 and 0.74, respectively, < 0.001) and prostate-specific antigen values ( = 0.57 and 0.54, respectively, < 0.01). When the proposed indices were compared against expert rating using PERCIST, BPI and BPI showed better agreement than BSI, indicating their potential for objective response evaluation. We propose the evaluation of BPI and BPI as imaging biomarkers for Ga-PSMA PET/CT in a prospective study exploring their potential for outcome prediction in patients with bone metastases from PC.

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