Comparison of the tracer uptake into the kidneys in GA68-PSMA-PET-CT and the absorbed dose per applied GBq Lu177-PSMA-617 during a following therapy

1274 Objectives: Kidneys are considered as dose limiting organs in patient therapy with Lu-177-PSMA-617. We investigated if pretherapeutic Ga68-PSMA-PET/CT may be used as a predicting factor to dose to kidney/GBq Lu177-PSMA. The impact of tumor load on SUV-parameters were also assessed. Methods: 265 kidneys in 152 patients with mCRPC who underwent a Ga-68-PSMA-11 PET/CT-baseline scan within 2 (+-1) weeks to the start of the Lu-177-PSMA-617 were analyzed. Tumor load was classified as low (n=31), intermediate (n=47) or high (n=74), based on miTNM classification (PROMISE). The relation of Ga68-PSMA-uptake in the kidneys and tumor burden were tested. We evaluated the relation between SUV-parameters of each kidney and the absorbed dose / GBq Lu-177-617-PSMA after the first administration of Lu-177-PSMA-617 therapy to see if a correlation between the two parameters exists. The correlation was tested using nonparametric testing. Results : We compared the PET parameters with the kidney dose/ GBq Lu-177-PSMA in the following cycle. Looking at all patients, SUV parameters ranged widely (SUVmax (36+-15), SUVmean (24+-10), SULmax (26+-10) and SULmean (18+-7)). All four PET parameters showed a strong negative correlation to the tumor burden in the PET-scan (P-values: SUVmax/tb: 0,009; SUVmean/tb: 0,003; SULmax/tb: 0,005; SULmean/tb: 0,001). When comparing SUV values between the groups, we found ~10% higher tracer uptake in patients with low tumor load compared to intermediate tumor load and 30-35% lower tracer uptake in patients with higher tumor load compared to intermediate load. All four PET parameters showed a strong positive correlation to the kidney dose/ GBq Lu177-PSMA in the first PET (P-values: SUVmax/Gy/GBq: <0,001; SUVmean/Gy/GBq: <0,001; SULmax/Gy/GBq: <0,001; SULmean/Gy/GBq: <0,001). Conclusions: Tumor load correlates negatively with the SUV parameters in baseline PET/CT which in turn is associated with the absorbed dose/GBq Lu-177-PSMA. This finding encourages the consideration of higher activities of Lu177-PSMA in patients with high tumor load and low SUV-parameters in baseline PET-scan. Figure: PET images of a patient at baseline and after 3 cycles of Lu-PSMA showing an increase in the renal uptake which correlates to a higher renal absorped dose / GBq.