Impact of reduced scan extent on tumor detection in 68Ga-PSMA PET/CT for biochemical recurrence of prostate cancer

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Abstract
Background: PSMA PET/CT plays an important role in biochemically recurrent prostate cancer (BCR-PCa). In general, PET/CT is performed as a limited or true whole-body examination. The majority of PET/CT scanners operate by measuring several bed positions or continuous motion with a limited axial field of view (FOV). Shortening the scan area thus results in a time saving, with unknown information loss regarding the detection of tumor manifestations. Methodology: This retrospective single-center study included 200 patients with BCR-PCa after initial curative therapy who underwent a 68 Ga-PSMA-PET/CT scan from vertex to toe (true whole-body). Using retrospective subdivision into hypothetical partial body measurement protocols, the undetected tumor manifestations were examined. Classic PET/CT protocols were used: vertex to mid-thigh, skull base to mid-thigh, as well as CT-equivalent examination areas: chest/abdomen/pelvis (CAP), abdomen/pelvis (AP) and pelvis. Results: Of 200 patients with a median PSA of 1.1 ng/mL, 128 (64%) showed at least one structural correlate of BCR-PCa in the true whole-body scan. The overall detection rate did not change for limited classic PET-protocols, or CAP measurements. CAP scans would not have detected all tumor manifestations in 16 patients (8%) and not all tumor-affected organ systems in six patients (3%), AP scans in 41 patients (21%) and 18 patients (9%), pelvic scans in 61 patients (31%) and 37 patients (19%), respectively. PSA may contribute to the prospective restriction of the scan area. Patients with PSA ≤ 0.75 ng/mL showed no diagnostic gain from whole-body imaging. This means that in 45% of patients in our cohort, a limitation to CAP scan would have been possible without losing information. Conclusions: For a resource-saving use of routine PSMA-PET/CT, a scan from the vertex to the mid-thigh seems sufficient for detecting relevant metastases. For patients with low PSA levels, a more focused protocol that includes only the CAP region can be considered.

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