Abstract Number: 1 68Ga-PSMA PET/CT in staging and restaging of Prostate Cancer Patients: comparative study with 18F- Choline PET/CT

Number: 1 68Ga-PSMA PET/CT in staging and restaging of Prostate Cancer Patients: comparative study with 18FCholine PET/CT W. Langsteger1, A. Rezaee1, W. Loidl2, H.s Geinitz3, F. Fitz1, M. Steinmair1, G. Broinger4, L. Pallwien-Prettner5, M. Beheshti1. 1 PET-CT Center Linz, Department of Nuclear Medicine & Endocrinology, 2 Prostate Cancer Center Linz, Department of Urology,3 Department of Radiation Oncology,4 Department of RadiologyST VINCENT’S HOSPITAL, LINZ, AUSTRIA Aim: 11Cand 18F-choline PET/CT have been established as a promising modality in the assessment of prostate cancer patients1. However, it suffers to detect small malignant lesions. 68Ga-PSMA (PSMA) PET/CT showed promising in the detection of small lesions with a high tumor to background contrast2. This study was designed for comparison of detection rate between PSMA and 18F-fluoromethylcholine (FCH) PET/CT scan in pre or post-op prostate cancer patients. Methods: In this prospective study 15 consecutive prostate cancer patients (mean age 67.9, range 57-83) underwent both PSMA and FCH PET/CT with a maximum interval of 4 weeks without any treatment in between. The imaging modalities were performed in 9 patients (mean age: 70.3; range: 63 83) in preand 6 Patients (mean age: 64.2; range: 57 71) in post-operative setting. Prostate cancer patients with histopathologic verification or biochemical recurrence were included in this study. Patients with systemic therapy and known second cancer were excluded. Pathologic findings in each imaging modalities have to be clarified histopathologically or by conventional imaging modalities and/or clinical follow-up. Results: Staging: The mean of PSA was 35.1 ng/ml (range: 3.44 81.17 ng/ml). Pathologically increased tracer uptake was detected on both imaging modalities in the prostate gland in all patients [mean size on PSMA PET/CT: 20 mm (range: 12-43); FCH PET/CT: 23.6 mm (range: 12-34) & mean SUVmax on PSMA PET/CT: 16.2 (range: 6.128.1); FCH PET/CT: 7.4 (range: 3.6-15.9)]. Overall, a total number of 15 and 14 positive lymph nodes were detected on PSMA PET/CT and FCH PET/CT images, respectively. Malignant lymph nodes showed significantly higher uptake on PSMAcomparing FCH PET/CT [mean SUVmax on PSMA PET/CT: 12.8 (range: 2.8-34); FCH PET/CT: 6.6 (range: 2.3-11.9)]. However, there was no appreciable difference in the tracer intensity of the detected bony lesions in each modality. Restaging: The mean of PSA was 2.3 ng/ml (range: 0.48 5.35). Local recurrence with pathologically increased tracer uptake was detected in one patient on both imaging modalities, However, it shows markedly higher uptake on PSMA PET/CT (SUVmax: PSMA: 12.4; FCH: 4.9). In addition, PSMA PET/CT was able to detect higher number of metastatic lymph nodes compared with FCH PET/CT (14 vs. 10) with significantly better tumor to background ratio (SUVmax 8.7 on PSMA vs 4.45 on FCH). One false positive bone lesion was detected on PSMA PET/CT study. Although the MRI was negative, follow-up imaging was planned within 4-6 months. Also, two false positive bone lesions were detected on FCH PET/CT images in bilateral femurs. Moreover, PSMA PET/CT was false negative in one bony lesion detected by FCH PET/CT. Conclusion: PSMA PET/CT reveals a more promising role for staging and re-staging of prostate cancer patients even with low PSA level. Markedly higher tumor to background contrast is seen on PSMA PET/CT which allows higher detection rate especially in the small lesions. However, the value of this modality in the assessment of bone metastases should be further evaluated in future studies. References: 1. Mohsen Beheshti et al. Impact of 18F-Choline PET/CT in Prostate Cancer Patients with Biochemical Recurrence: Influence of Androgen Deprivation Therapy and Correlation with PSA Kinetics. J Nucl Med 2013; 54:1–8 2. Joshua Morigi et al. Prospective Comparison of 18F-Fluoromethylcholine Versus 68Ga-PSMA PET/CT in Prostate Cancer Patients Who Have Rising PSA After Curative Treatment and Are Being Considered for Targeted Therapy. J Nucl Med 2015; 56:1185–1190