Abstract
1467
Introduction: Prostate cancer is the most common solid malignancy in men and the third leading cause of cancer related death in the United States. Prostate-specific membrane antigen (PSMA) is a type II transmembrane protein that acts as a glutamate carboxypeptidase enzyme. Its' high expression in prostate cancer cells makes it a useful target for diagnostic and therapeutic applications in nuclear medicine. In patients with newly diagnosed prostate cancer Ga68 PSMA PET-CT can be used for localization and staging, can guide biopsy and help in defining a local therapeutic strategy. Results of recent studies have suggested that at staging this diagnostic modality should be reserved for patients with intermediate to high grade disease as defined by NCCN guidelines. The aim of the study was to find common clinical characteristics for patients with negative Ga68-PSMA staging studies in comparison to those with positive studies. Materials and methods: Ga68-PSMA studies performed for staging of newly diagnosed prostate cancer in two academic centers over a period of 36 months were retrospectively reviewed. Clinical, laboratory and imaging data including demographic data, Gleason score at diagnosis, PSA level and NCCN risk group, where available, were analyzed. Characteristics of patients with negative PET/CT studies were compared to those of patients with positive results. Differences between groups were analyzed using student t-test (for PSA levels and Gleason scores) and Chi square test (for NCCN risk groups) with p<0.05 considered to indicate statistically significant difference.
Results: Two hundred and seventy studies were performed for staging prostate cancer. There were 227 positive studies (84%) indicating active prostate cancer and 44 negative studies (16%). The mean age of patients with positive studies (PSMA+) was 70.3 years (range 50-87y) and of those with negative studies (PSMA-) 69.5 years (42-87y), p not significant. The mean Gleason score at diagnosis was 7.8 (range 6-10) in PSMA+ patients and 7.3 (6-10) in the PSMA- group (p<0.05). The average PSA level at the time of the study was 30.5 ng/mL (0.08-533 ng/mL) in PSMA+ and 13.5 ng/mL (0.06-69 ng/mL) in PSMA- patients (p<0.05). NCCN risk group was available in 226 patients including 184 with PSMA+ and 42 with PSMA- studies. The PSMA+ group included 125 high risk (68%), 57 intermediate risk (31%) and two low risk patients. In the PSMA- group 16 patients were categorized as high risk (38%), 25 as intermediate risk (60%) and one patient as low risk (p<0.05).
Conclusions: Ga68-PSMA PET/CT was negative in 16% of patients referred for staging primary prostate cancer. Negative studies were associated with significantly lower Gleason scores and PSA values as well as significantly higher proportion of intermediate NCCN risk group.