SUV_95th as a Reliable Alternative to SUV_max for Determining Renal Uptake in [⁶⁸Ga] PSMA PET/CT

Widely used in clinical practice, the maximum standardized uptake value (SUV_max) 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 Ga-68 prostate-specific membrane antigen ([₆₈Ga]PSMA).

To this aim, our previously published work, proving the efficacy of mannitol, was considered as a background study. Renal SUV_max was calculated in nine patients undergoing [⁶⁸Ga]PSMA positron emission tomography (PET)/X-ray computed tomography (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 (SUV_95th), was introduced to better characterize the tracer accumulation. A comparison between SUV_max and SUV_95th was also performed. Kruskal-Wallis test was used to assess the statistical significance of the differences in SUV_95th between b-PET/CT and m-PET/CT.

SUV_95th 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 SUV_max 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 SUV_95th values were significant (p value < 0.001).

Our findings indicate that SUV_95th is a more robust index to assess high uptake level, representing a reliable alternative to SUV_max. Independently from the segmentation method, the superiority of SUV_95th and its easy computation could make its clinical impact decisive. The results obtained with SUV_95th, more representative of tracer uptake than those with SUV_max suggest, in our opinion, that mannitol infusion could be used to reduce the adsorbed dose to the kidneys during [⁶⁸Ga]PSMA PET/CT and Lu-177 or Ac-225 therapy. Our future goal will be confirming this effect in a larger cohort of patients, also verifying the role of SUV_95th in the evaluation of tumor response to therapy.