Delayed Pelvic Imaging With [68]Ga-PSMA PET/CT in a Patient With High-risk Prostate Cancer.

High-risk Prostate Cancer

PET/CT, [68]Ga - PSMA, Delay CT scan, Radiation Dose Optimization

Recent evidence suggests that both standard-time and delayed-time \[⁶⁸Ga\]Ga-PSMA PET acquisitions can reveal clinically relevant findings, and neither phase should be excluded a priori in routine practice. This study evaluates a streamlined dual-phase protocol consisting of: * A standard whole-body PET/CT acquisition performed 60 minutes after radiotracer administration. * A delayed pelvic PET-only acquisition performed 90 minutes post-injection, reconstructed using the attenuation-correction CT (CT/AC) obtained from the initial whole-body scan. Because the prostate gland and pelvic lymph nodes exhibit minimal physiological mobility, accurate PET-CT anatomical correspondence can be maintained through careful patient repositioning, without repeating the CT scan. The main advantage of this protocol is a reduction in patient radiation exposure, as the delayed phase does not require a second CT scan. It also reduces in-department time and maintains diagnostic quality of PET interpretation, provided that the PET-CT alignment remains acceptable. This method may additionally enhance workflow efficiency in the Nuclear Medicine Unit by allowing early identification-based on predefined clinical parameters-of patients most likely to benefit from delayed pelvic imaging.

This study evaluates a modified dual-phase \[⁶⁸Ga\]Ga-PSMA PET/CT protocol designed to reduce radiation exposure and streamline workflow while maintaining diagnostic quality in the assessment of prostate cancer. Conventional imaging often includes a standard whole-body PET/CT acquisition at approximately 60 minutes post-injection followed by a delayed pelvic PET/CT, which can provide additional lesion detectability but requires a second CT scan. The additional CT increases both radiation dose and examination duration. The proposed protocol replaces the delayed CT with a PET-only pelvic acquisition performed 90 minutes post-injection. Delayed PET images are reconstructed and attenuation-corrected using the CT dataset obtained during the initial whole-body acquisition. This approach is feasible because pelvic structures relevant to prostate cancer-such as the prostate gland and pelvic lymph nodes-exhibit limited physiological motion compared with intraperitoneal organs, allowing reliable PET-CT anatomical correspondence through optimized patient repositioning. The study adopts a randomized, two-arm design including patients undergoing staging for newly diagnosed prostate cancer or evaluation of biochemical recurrence. Randomization is stratified by prior radical prostatectomy to account for potential anatomical variability. The control arm follows the standard clinical workflow with both standard and delayed PET/CT acquisitions. The intervention arm undergoes standard whole-body PET/CT at 60 minutes and delayed PET-only imaging at 90 minutes. Anatomical matching between delayed PET images and the CT dataset will be independently evaluated by three Medical Imaging Technologists (TSRM1, TSRM2, TSRM3) across three blinded reading sessions. Each rater will score image alignment using a standardized 1-5 Likert scale. The primary endpoint is the quality of PET-CT matching, assessed through Fleiss' kappa statistics to quantify interrater agreement and compare matching performance between the two imaging workflows. Secondary objectives examine the potential reduction in radiation exposure, measured through CTDIvol and DLP values obtained from the CT Dose Report, and the impact on patient experience and departmental efficiency. Patient-perceived waiting time and comfort will be assessed through a short questionnaire administered after the examination, while objective in-department duration will be extracted from institutional scheduling records. If effective, the streamlined protocol may support broader adoption of CT-free delayed pelvic imaging by demonstrating that optimized technologist-led positioning can maintain diagnostic interpretability while improving patient-centered outcomes and reducing overall radiation burden.

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