Evaluation of a simplified method for dosimetry in PSMA-Radioligand therapy

Ziel/Aim In PSMA RLT, the achievement of sufficient radiation doses is essential for a favorable therapy response. Full dosimetry is very time-consuming. In this project a simplified method for dosimetry in PSMA radioligand therapy was evaluated. The patients were divided into responders and non-responders according to their biochemical response and image response in PSMA PET/CT. The mean dose of the single hottest tumor lesions in posttherapeutic SPECT were compared between the groups.

Methodik/Methods 31 patients, who received 2 cycles of Lu-177-PSMA-617 radioligand therapy, were included. Dose was calculated using Lu-177 SPECT scans 24, 48 and 72 hours p. i. Tumor lesions were segmented using an isocontour of 40 % of maximum intensity. Doses were calculated with a mono-exponential fit model in combination with mass-scaled sphere S-values. For image response assessment modified PERCIST criteria were applied to baseline and follow-up Ga-68-PSMA-PET/CT. Biochemical response was defined as PSA decline ≥ 50 %. Initial median PSA level was 13 (range: 0.88-9237). Data was analyzed using Mann-Whitney test and GraphPad Prism, Version 8.4.3.

Ergebnisse/Results In PERCIST analysis, image responders (11 patients with partial metabolic response) showed a significant higher mean dose of the tumor lesions than non-responders (20 patients labelled with stable or progressive disease; 14.0 ± 14.3 Gy/GBq vs. 5.8 ± 4.9 Gy/GBq; p = 0.03). Regarding biochemical response median PSA decline was 46.8 %. There was no significant difference of the dose of the hottest lesion between biochemical responders (n = 18; 10.0. ± 12.2 Gy/GBq) and non-responders (n = 13; 6.9 ± 5.8 Gy/GBq; p = 0.70).

Schlussfolgerungen/Conclusions Responders according to modified PERCIST evaluation show a significant higher mean dose in the hottest tumor lesions. Nevertheless, there was no difference regarding biochemical response. This data has to be interpreted carefully as PERCIST evaluation works similarly as our suggested simplified method focussing on the hottest tumor lesions. In order to make a valid statement, the suggested method should be validated with more patients and further therapy response assessment, for example evaluation of whole tumor volume.

Publication History

Article published online:
08 April 2021