SUV95th as a reliable alternative to SUVmax for determining renal uptake in 68Ga-PSMA PET/CT

Purpose: Widely used in clinical practice, the maximum standardized uptake value (SUVmax) is a statistical index highly prone to physical and biological variations, which can lead to unpredictable errors. This study has a methodological aim: to identify a more robust SUV-based index representing the tracer accumulation. In particular, the new metric was tested to confirm the potential of mannitol to reduce renal uptake [68Ga] prostate-specific membrane antigen (68Ga-PSMA). Procedures: To this aim, our previously published work, proving the efficacy of mannitol, was considered as a background study. Renal SUVmax was calculated in nine patients undergoing 68Ga-PSMA positron emission tomography/computed tomography (68Ga-PSMA-PET/CT) at baseline (b-PET/CT) and at follow-up after intravenous infusion of 500 ml of 10% mannitol (m-PET/CT). SUV values of kidney volumes were extracted by a new 3D segmentation method. A new parameter, the median computed on the upper 10% of the SUV distribution (SUV95th), was introduced to better characterize the tracer accumulation. A comparison between SUVmax and SUV95th was also performed. Kruskal-Wallis test was used to assess the statistical significance of the differences in SUV95th between b-PET/CT and m-PET/CT. Results: SUV95th not only confirmed the efficacy of mannitol as demonstrated in the previous study but improved the separability of b-PET/CT and m-PET/CT examinations, overturning SUVmax findings in two cases. The outcomes of the Kruskal-Wallis test computed for each kidney proved that differences between b-PET/CT and m-PET/CT SUV95th values were significant (p-value <0.001). Conclusions: Our findings indicate that SUV95th is a more robust index to assess high uptake level, representing a reliable alternative to SUVmax. Independently from the segmentation method, the superiority of SUV95th and its easy computation could make its clinical impact decisive. The results obtained with SUV95th, more representative of tracer uptake than those with SUVmax suggest, in our opinion, that mannitol infusion could be used to reduce the adsorbed dose to the kidneys during 68Ga-PSMA-PET/CT and 177Lu or 225Ac therapy. Our future goal will be confirming this effect in a larger cohort of patients, also verifying the role of SUV95th in the evaluation of tumour response to therapy.