Assessment of benign bone marrow uptake with ¹⁸F-PSMA-1007 in prostate cancer using PET/MRI

Objectives: The primary objective was to evaluate the phenomenon of benign bone marrow uptake which has been described with ¹⁸F-PSMA-1007 using PET/MRI. Secondary objectives were to review the initial experience and technical feasibility of staging and restaging of prostate cancer using 18F-PSMA-1007 PET/MRI and to assess if a pelvic section of Diffusion Weighted Imaging provided any additional lesion detection over the standard PET/MRI dataset.

Methods: Thirty-four consecutive patients were scanned on a Siemens mMR Biograph PET/MR following a single IV bolus of ¹⁸F-PSMA-1007 injection 3MBq/kg at 3minutes/bed position from vertex to mid thighs. Two point Dixon, T1 VIBE & T2 HASTE whole body sequences were acquired. Additional diffusion weighted images of the pelvis were acquired at b0, 400 and 800. Images were reviewed by a Nuclear Medicine Radiologist, using Osirix software, with access to clinical history but blinded to clinical reports. Sites of tracer avid disease were recorded in the following categories: Prostate/Prostate bed, seminal vesicles, pelvic lymph nodes, extra-pelvic lymph nodes, visceral/ other metastases and finally bone lesions. Focal bone marrow uptake with suspicious MRI features was interpreted as metastasis, and uptake without suspicious MRI features considered as benign. Image quality was graded (poor, acceptable, good) and presence of any artefacts documented on PET or MRI (minor, moderate, severe). A DWI series was the final sequence to be evaluated and specifically assessed for the presence of disease not already demonstrated on the PET data.

Results: Of the 34 patients evaluated, 11/12 studies were positive for disease in those performed for primary staging (Group A), 8/22 were positive for disease patients with biochemical recurrence post prostatectomy (Group B). In total, 7/34 patients had benign focal marrow uptake without anatomic correlate, 3/12 in Group A and 4/22 in Group B. Distributions of disease were as follows: Group A; 11/12 prostate, 3/12 seminal vesicles, 3/12 pelvic lymph nodes, 2/12 extrapelvic nodes, 1/12 bone metastases. Group B; 4/22 prostate bed, 4/22 pelvic nodes, 4/22 extrapelvic nodes, 2/22 bone metastases. Good diagnostic image quality was achieved in all patients. Artefact related to respiratory motion was present in 6 patients, minor in 4, moderate in 1 and severe in 1. Halo artefact around the kidneys +/- liver/spleen was present in 10 patients (all minor). No patient had halo artefact around the urinary bladder. In no case did DWI of the pelvis identify additional disease.

Conclusions: Benign focal bone uptake with ¹⁸F-PSMA 1007 was demonstrated in a significant proportion (approximately one fifth). This finding can produce diagnostic challenges on PET/CT but the MRI sequences acquired as part of PET/MR could enable reliable distinction between benign from malignant bone marrow uptake. PET/MRI with ¹⁸F-PSMA 1007 was performed successfully, all studies of good diagnostic quality. Minor halo artefact around the kidneys +/- liver/spleen was demonstrable in under a third of patients and did not significantly impede study interpretation. Diffusion weighted imaging of the pelvis did not reveal any additional sites of disease and may not be necessary in an optimised protocol.