• F-Fluciclovine PET/MRI has high specificity for detection of lymph node metastasis. •F-Fluciclovine PET/MRI lacks sensitivity to replace ePLND. • F-Fluciclovine PET/MRI may be used to aid surgery and select adjuvant therapy. •F-Fluciclovine PET-positive patients have more extensive disease than PET-negative patients. • Size of metastatic lymph nodes is an important factor for detection.

The objective of this study was to evaluate the effect of including bone in DIXON-based attenuation correction for 18 F-fluciclovine Positron Emission Tomography (PET) / Magnetic Resonance Imaging (MRI) of primary and recurrent prostate cancer. Methods:18 F-fluciclovine PET data from two PET/MRI studies -one for staging of highrisk prostate cancer (28 patients) and one for diagnosis of recurrent prostate cancer (81 patients) -were reconstructed with a 4-compartment (reference) and 5-compartment attenuation map. In the latter, continuous linear attenuation coefficients for bone were included by co-registration with an atlas. The maximum and mean 50%isocontour standardized uptake values (SUV max and SUV iso , respectively) of primary, locally recurrent, and metastatic lesions were compared between the two reconstruction methods using linear mixed-effects models. In addition, mean SUVs were obtained from bone marrow in the third lumbar vertebra (L3) to investigate the effect of including bone attenuation on lesion-to-bone marrow SUV ratios (SUVRmax and SUVRiso; recurrence study only). The 5-compartment attenuation maps were visually compared to the in-phase DIXON MR images for evaluation of bone registration errors near the lesions. P-values < 0.05 were considered significant. Results: Sixty-two (62) lesions from 39 patients were evaluated. Bone registration errors were found near 19 (31%) of these lesions. In the remaining 8 primary prostate tumors, 7 locally recurrent lesions, and 28 lymph node metastases without bone registration errors, using the 5-compartment attenuation map was associated with small but significant increases in SUVmax [2.5%; 95% confidence interval (CI) 2.0%-3.0%; p<0.001] and SUViso (2.5%; 95% 3 CI 1.9%-3.0%; p<0.001), but not SUVRmax (0.2%; 95% CI -0.5%-0.9%; p=0.604) and SUVRiso (0.2%; 95% CI -0.6%-1.0%; p=0.581), in comparison to the 4-compartment attenuation map. Conclusion: The investigated method for atlas-based inclusion of bone in 18 F-fluciclovine PET/MRI attenuation correction has only a small effect on the SUVs of soft-tissue prostate cancer lesions, and no effect on their lesion-to-bone marrow SUVRs when using signal from L3 as a reference. The attenuation maps should always be checked for registration artefacts for lesions in or close to the bones.4

ABSTRACT