Optimal 68Ga-PSMA PET/CT imaging in assessment of prostate cancer: feasibility of only delayed acquisition using a total-body PET/CT scanner?

Introduction: While dual-time point ⁶⁸Ga-PSMA PET/CT could increase the lesion detection rate in PCa patients, it remains difficult to perform due to busy clinical service, the low count statistics and more radiation in delayed imaging. To prove that when performed on a total-body PET/CT, only one-time delayed acquisition of ⁶⁸Ga-PSMA is feasible and provides increased lesion detection rate compared to standard acquisition. 

Methods: A retrospective clinical study between December 2020 and July 2021 was performed in 56 patients with PCa who had undergone ⁶⁸Ga-PSMA-11 total-body PET/CT. All patients received a standard acquisition at 1 h p.i. and a late acquisition at 3 h p.i. . All images were obtained in a single bed position for 5 min on the total-body PET/CT. The image quality was qualitatively evaluated using a 5-point Likert scale. Lesions uptakes (SUV_max) and target-to-liver ratio (TLR) were compared using a Wilcoxon matched-pairs signed rank test, background uptakes (SUV_mean) was compared using an Independent t test. And all lesions were further analyzed based on uptake values, sizes, types and sites.

Results: A total of 56 patients (mean age, 68&nbsp;±&nbsp;7 [standard deviation]) were enrolled in this study. The image quality was qualitatively assessed as modestly lower in the delayed imaging (3.4&nbsp;±&nbsp;0.5 at 3 h p.i.&nbsp;vs 4.8&nbsp;±&nbsp;0.2 at 1 h p.i.). The delayed images showed significantly increased PSMA-avid lesion&nbsp;uptake values (SUV_max)&nbsp;(median, 11.0 [range, 2.3 ~&nbsp;193.6] vs&nbsp;7.0&nbsp;[2.0&nbsp;~&nbsp;124.3], p&nbsp;<&nbsp;0.001) and elevated TLR (median, 3.3&nbsp;[range, 0.5&nbsp;~&nbsp;62.2]&nbsp;vs&nbsp;1.7 [0.3 ~ 30.7], p&nbsp;<&nbsp;0.001) compared to the standard images.&nbsp;The delayed images&nbsp;also presented reduced&nbsp;uptake values in most of the background regions (SUV_mean). Delayed images provided additional lesion detection in 14 of 56 patients (25%), impacting management plans in 8&nbsp;of the 14 patients. Late acquisitions identified 9.7% additional PSMA-avid&nbsp;lesions&nbsp;(22/226) in the pelvic and extra-pelvic regions, 13&nbsp;of these 22 additional lesions were small lymph nodes (D&nbsp;<&nbsp;10mm) with significantly higher&nbsp;SUV_max&nbsp;and TLR.

Conclusions: Compared with the standard imaging, delayed imaging with ⁶⁸Ga-PSMA-11 total-body PET/CT offered higher lesion uptake, reduced background uptake and improved lesion contrast. The additional findings in the delayed imaging provided a valuable information in the assessment of prostate cancer patients. The proposed protocol with only one-time acquisition of 5 min performed at 3 h p.i. is feasible in the clinical practice when performed on a total-body PET/CT system with an AFOV of 194 cm.

Acknowledgement:

This work was supported in part by the National Key Research&nbsp;and Development Program of China (Grant No. 2020YFA0909000), National Natural Science Foundation of China (Grant No.82102089), Construction Project of Shanghai Key&nbsp;Laboratory of Molecular Imaging (Grant No.&nbsp;18DZ2260400), and&nbsp;Shanghai Municipal Education Commission (Class II Plateau&nbsp;Disciplinary Construction Program of Medical Technology of SUMHS,&nbsp;2018&ndash;2020)

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