Abstract
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Objectives: Prostate cancer (PCa) is the most common male cancer and bone is the most frequent metastatic site. Ga-68-PSMA and sodium fluoride-18 (NaF) are widely used radiopharmaceuticals for assessing PCa and associated bone metastases. In advanced PCa these tracers often demonstrate different skeletal distribution due to mechanism of uptake - enzymatic activity on cancer cell membrane and bone mineralization. Here we aim to characterize this difference in a more detailed manner.
Methods: Our cohort consisted of 14 patients with advanced disease (>5 lesions) who had had routine PET/CT both with GaPSMA and NaF within one week. Bone regions in CT were used to co-register the two PET/CT scans. Whole skeleton VOI was defined on CT of PET with HU>150, similarly sclerotic/dense bone was defined as HU>600. Additional VOIs were defined on PET uptake with different pathologic threshold values on both GaPSMA (SUV>3.0) and NaF (SUV>10). PET based GaPSMA and NaF VOIs that overlapped with the CT based skeletal and sclerotic VOIs were separately generated and analyzed. We analyzed the pathologic bone volumes in each technique (CT, HU>600), NaF (SUV>10) and GaPSMA (SUV >3.0). For comparison we had 12 patients diagnosed with prostate cancer who did not have skeletal disease.
Results: The skeletal VOI volumes varied from 5.91 L to 11.4 L whereas sclerotic bone volumes were from 1.10 L to 3.21 L, in patients with skeletal disease, and in patients with no metastases from 6.36 L to 11.6 L and from 1.27 L to 2.72 L, respectively. In analogue to TLG (total lesion glycolysis), we also analyzed total enzyme activity (total enzyme activity) for PSMA (TEA) and total accelerated osteoblastic activity (total bone demineralization) activity for NaF (TBA). The TEA varied from 0.001 to 5.68 in patients with skeletal metastases and was 0.000-0.194 in a PCa control patients with no skeletal metastases. The TBA varied from 0.981 to 20.0 in patients with skeletal metastases and was 0.000-2.00 in a PCa control patients (p<0.03). The pathologic PSMA volume represented 0.44%-132% of the sclerotic bone volume in pts with metastases, and only 0.55-2.33% in patients with no metastases. The pathologic NaF volume was 0.27- 67.9 % of sclerotic bone in patients with metastases and only 0.00-6.49% (p<0.03).
Conclusion: Our results suggest that CT cannot be used for assessment of the extent of active metastatic skeletal disease in PCa. It is also obvious that NaF and PSMA give different information about activity in the skeletal disease, but the active sites differ less from each other than sclerotic bone regions (CT) from the PET activity. Research Support: None