18F-PSMA-1007 in recurrent prostate cancer, a new frontier in prostate cancer PET imaging

Abstract

Prostate cancer remains a significant cause of cancer morbidity and mortality in men worldwide accounting for the second-highest incidence of all cancers in males. A disproportionate incidence, morbidity and mortality of prostate cancer have been reported in black males than their white counterparts however very little is known of their imaging differences when presenting with biochemical recurrence. The imaging modalities employed in cancer staging (computed tomography, magnetic resonance imaging, bone scan and positron emission tomography) have been under debate due to their varying sensitivities. The prostate bed is the most common site of early recurrence of prostate cancer. The currently used PSMA ligands (68Ga-PSMA and 99mTc-PSMA) undergo early urinary clearance resulting in interfering physiological activity within and surrounding the prostate. This can result in sites of cancer recurrence being obscured. 18F-PSMA-1007 has an advantage of delayed urinary clearance thus the prostate region is reviewed without any interfering physiological activity. There however is limited data on the diagnostic performance of 18F-PSMA-1007 in early biochemical recurrence. To our knowledge we were the first to describe the differences in 68Ga-PSMA imaging findings between black and white prostate cancer patients with biochemical recurrence. We found a significant correlation between PSA values and the diagnostic performance of 68Ga-PSMA imaging in both groups. However there was no significant difference in the detection rate, distribution pattern and the median number of lesions between the two racial groups suggesting that the tumour burden and growth rate of androgen dependent prostate cancer may be similar in both races. We also found 68Ga-PSMA to be superior to bone scan in the assessment of skeletal metastases in the initial staging of high-risk prostate cancer, demonstrating a higher detection rate and specificity, indentifying marrow and lytic skeletal metastases thath had been missed by bone scan. To our knowledge we were also the first to conduct a head to head comparison of 68Ga-PSMA and 18F-PSMA-1007 in this thesis. Though limited by a small number of patients, 18F-PSMA-1007 detected more recurrence sites than 68Ga-PSMA. 18F-PSMA-1007 demonstrated a sensitivity, specificity, positive and negative predictive value of 88.9%, 100%, 100%, and 92.3% respectively while 68Ga-PSMA-11 demonstrated sensitivity, specificity, positive and negative predictive value of 44.4%, 83.3%, 80%, and 66.6% respectively. In our thesis, 18F-PSMA-1007 performed equally to other reported PSMA PET agents when compared with a similar cohort of patients with biochemical recurrence and low PSA value. PSA doubling time proved significantly related to the detection rate of 18F-PSMA-1007 whilst no significant relationship was seen with PSA velocity. We found the optimal PSA cut-off value of 1.26ng/ml to identify biochemical recurrence.