Comparison of Ga 68 –PSMA- 11 and F18-PSMA-1007 regarding Prostate Cancer Molecular Imaging Standardized Evaluation (PROMISE) in prostate cancer patients.

Introduction: PSMA-ligand PET imaging is a well establish study for evaluation of prostate cancer in the last decade. Due to its high sensitivity and specificity, it is used in staging of moderate to high risk prostate cancer, in cases of biochemical failure and also for response assessment.

There are many PSMA tracers in use, some of them are bound to Gallium-68 (Ga68) and some to Fluorine-18 (F18) with some variations of their bio distribution.

The prostate cancer molecular imaging standardized evaluation (PROMISE) was developed to use as a framework for standardized reporting of PSMA PET/CT scans. According to the PROMISE guidelines a score is given to each suspected lesion in reference to the physiological uptake in the blood pool, liver (spleen is replaced for markers that are secreted primarily form the bile system) and parotid gland- PROMISE score ranging from 0 to 3.

Aim: To compare the bio distribution of Ga68 –PSMA- 11 and F18-PSMA-1007 with emphasize on the uptake in the blood pool, liver, spleen and parotid gland and to assess the influence of the different bio distribution to the PROMISE score in prostate cancer patients.

Methods: A retrospective study assessing consecutive patients who underwent PET/CT PSMA for prostate cancer staging, biochemical failure or treatment response assessment during 2021 in our institution.

PET/CT imaging were preformed according to our institutional protocol. Patient were scheduled for a PSMA scan, were randomly injected one of the two isotopes depend on availability at the day of the scan.

For each patient, uptake in reference organs (blood pool, liver or spleen, parotid gland) was recorded as well as SUVmax of pathological lesion in the prostate or prostate bed, lymph nodes, bone lesions and others.

A comparison of the uptake in the reference organ and the pathological lesions was done between the two markers. Each lesion was assigned a PROMISE score and those scores where compared in relation to the two PSMA markers.

Results: Our cohort was composed of 152 patients: 59 underwent F18-PSMA-1007 scan and 92 underwent Ga68 –PSMA- 11. The two groups' characteristics had similar age and PSA levels (age p-value- 0.5003; PSA p- value- 0.7707).

We found significantly higher uptake in the blood pool of F18-PSMA-1007 scans compared to Ga68 –PSMA- 11 scans (p- value <0.001) as well as higher uptake in the spleen on the F18-PSMA-1007 scans compared to liver uptake on the Ga68 –PSMA- 11 scans (p- value <0.001). There was no significant difference in the parotid gland uptake between the two PSMA markers (p –value 0.17) and no difference in the pathological lesions uptake (p- value 1.99)

When comparing the distribution of the PROMISE scores given to the lesions we found a significant difference between score 1 and 2, with more lesions given a score of 1 on the scans done with F18-PSMA-1007 and on the Ga68 –PSMA- 11 scans there were more lesions with score 2 (p- value 0.04).

Conclusions: Differences in the bio distribution between F18-PSMA-1007 and Ga68 –PSMA- 11 mainly in the blood pool and liver/ spleen has a significant impact on PROMISE score. Therefore, raising the question whether the PROMISE score as suggested can be used for evaluating PET/CT scans performed with F18-PSMA-1007, or a modification to the scoring should be considered.