| Summary: | Background: Inconclusive bone scans are a challenge but there is no consensus about follow-up imaging. We evaluated the use of <sup>68</sup>gallium-labelled prostate-specific membrane antigen (<sup>68</sup>Ga-PSMA) PET/CT if <sup>18</sup>F-sodium fluoride (<sup>18</sup>F-NaF) PET/CT was inconclusive. Methods: This retrospective study included patients with no previously known bone metastases who had one or more equivocal bone lesions on <sup>18</sup>F-NaF PET/CT and underwent additional <sup>68</sup>Ga-PSMA PET/CT. The bone lesions were deemed as true metastases or not based on follow-up by surveying supplemental imaging modalities and hospital records. A subgroup of patients with “most valid follow-up” was created, which included patients with unmeasurable PSA after prostatectomy or subsequent imaging (additional <sup>18</sup>F-NaF PET/CT, <sup>68</sup>Ga-PSMA PET/CT, CT, or MRI). Results: Of the 2918 patients referred for <sup>18</sup>F-NaF PET/CT from the department of urology in the inclusion period, 51 (1.7%) were inconclusive regarding bone metastases and underwent additional <sup>68</sup>Ga-PSMA PET/CT. Thirteen of these patients (25%) were ultimately diagnosed with bone metastases. Patient-based sensitivity, specificity, and accuracy of additional <sup>68</sup>Ga-PSMA PET/CT were 100%, 95%, and 96%, respectively. In patients with “most valid follow-up”, the same parameters were 100%, 93%, and 94%, respectively. Conclusion: <sup>68</sup>Ga-PSMA PET/CT is an excellent complementary modality in when <sup>18</sup>F-NaF PET/CT is equivocal.
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