Diagnostic Value of Routine Delayed Pelvic Imaging for Patients with Biochemical Recurrence of Prostate Cancer Undergoing <sup>68</sup>Ga-PSMA PET/CT.

Abstract

The detection of biochemical recurrence (BCR) of prostate cancer remains challenging, as small-volume disease may fall below the sensitivity of standard imaging. Prostate-specific membrane antigen (PSMA) PET/CT is currently the most sensitive imaging modality for tumor detection at low prostate-specific antigen (PSA) levels, but recurrence continues to be difficult to detect. We evaluated whether routine delayed pelvic imaging at 90 min postinjection improves diagnosis of sites of recurrence in patients referred for BCR. Methods: We retrospectively analyzed 201 patients with prostate cancer undergoing dual-phase 68Ga-PSMA PET/CT for BCR of prostate cancer (median PSA, 0.52 ng/mL). Imaging of vertex to midthighs was acquired at approximately 60 min and delayed pelvic imaging at approximately 90 min postinjection. Two nuclear medicine physicians independently assessed whether delayed imaging improved diagnostic certainty or revealed additional 68Ga-PSMA-avid lesions. Results: Delayed pelvic imaging improved diagnostic certainty for 68 patients (34%) and revealed additional 68Ga-PSMA-avid lesions for 23 patients (11%). Of patients with additional lesions detected on delayed imaging, roughly one third-representing 3% of the total cohort-subsequently received focal radiation dose escalation during treatment planning. Improvements were primarily influenced by increased tracer uptake confirming subtle lesions, reduced bladder or ureteral interference, and differentiation of physiologic from pathologic uptake. All measurable lesions within the field of view of the delayed pelvic image were quantified. The median SUVmax for malignant lesions increased from 4.3 to 5.0, whereas benign lesions remained stable (median SUVmax of 2.7 on normal imaging and 2.6 on delayed imaging). Conclusion: Routine delayed pelvic imaging at 90 min postinjection enhanced lesion detection and diagnostic confidence in BCR of prostate cancer, particularly at low (<1 ng/mL) PSA levels. The addition of this additional acquisition into standard PSMA PET/CT protocols can improve staging accuracy and clinical decision-making.