Abstract
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Objectives: We investigated the whole-body distribution and the radiation dosimetry of 18F-PSMA-7, a novel 18F-labeled PSMA-ligand for the PET/CT imaging of prostate cancer.
Methods: Ten patients with prostate cancer and biochemical recurrence or radiologic evidence of metastatic diseases were examined with 329 - 384 MBq (mean 359 ± 17 MBq) 18F-PSMA-7. Eight sequential positron emission tomography scans were acquired from 30 minutes to 3 hours after injection with IRB approval. Kidneys, liver, spleen and salivary glands were segmented into volumes of interest using the QDOSE dosimetry software suite (ABX-CRO, Germany). Absorbed and effective dose were calculated using the ICRP endorsed IDAC 1.0 package. The absorbed dose of the salivary glands was determined using the spherical model of OLINDA 1.1. PSMA-positive lesions were evaluated separately. Quantitative assessment of the uptake in suspicious lesions was performed by analysis of maximum (max) and peak SUV values. The gluteus maximus muscle (SUVmean) served as a reference region for the calculation of the tumor-to-background ratios (TBR’s).
Results: Physiologic radiotracer accumulation was observed in the salivary and lacrimal glands, liver, spleen and intestines, in a pattern resembling the distribution known from other PSMA-tracers with excretion via biliary and urinary pathways. The effective dose from 18F-PSMA-7 for the whole body was calculated with 9.75E-03 mGy/MBq. The highest radiation dose was observed in the kidneys (1.74E-01 mGy/MBq), followed by liver (7.11E-02 mGy/MBq), salivary glands (4.82E-02 mGy/MBq) and spleen (1.44E-02 mGy/MBq). No adverse effects to tracer injection were observed. Six out of ten patients were scored as PSMA-positive. A total of nine suspicious lesions were analyzed which included two bone lesions, five lymph nodes and two local lesions within the prostate fossa. The values for the SUVmax (mean 7.59 ± 3.92) and SUVpeak (mean 3.85 ± 2.35) in the PSMA-positive lesions increased until 60 min p.i. (SUVmax 12.81 ± 7.69, SUVpeak 5.75 ± 3.22) and remained at this intensity in the subsequent PET/CT scans. Due to the decreasing activity within the background, the highest TBR’s were seen in the latest PET/CT scans (3 h p.i.). Compared to early PET/CT scans TBR’s (1 h p.i. mean 10.32 ± 6.86, 3 h p.i. mean 33.63 ± 15.81) were increased in mean by a factor of 3. Conclusion: Data from this initial study demonstrated that PET imaging with 18F-PSMA-7 is safe and feasible in the clinical setting. Physiologic accumulation of the radiotracer corresponded to the known distribution of the PSMA-expressing organs. The radiation dose from 18F-PSMA-7 was similar to that from other 18F-labeled PSMA-ligands. The highest TBR was found three hours after injection. The high uptake in suspicious lesions in terms of absolute SUVmax and relative TBR values indicates potentially high sensitivity of the tracer for detection of prostate cancer manifestations. Research support: none
