Focal unspecific bone uptake on [18F]-PSMA-1007 PET: a multicenter retrospective evaluation of the distribution, frequency, and quantitative parameters of a potential pitfall in prostate cancer imaging

Hannes Grünig,Klaus Strobel,Alexander Maurer,Zsofia Kovacs,Yannick Thali,Joachim Müller,Irene A Burger

doi:10.1007/s00259-021-05424-x

Abstract

PurposeImproved logistics and availability led to a rapid increase in the use of [18F]-PSMA-1007 for prostate cancer PET imaging. Initial data suggests increased uptake in benign lesions compared to [68 Ga]-PSMA-11, and clinical observations found increased unspecific bone uptake (UBU). We therefore investigate the frequency and characteristics of UBU in [18F]-PSMA-1007 PET.MethodsWe retrospectively analyzed [18F]-PSMA-1007 PET scans from four centers for the presence of UBU, defined as a focal mild-to-moderate uptake (SUVmax < 10.0) not obviously related to a benign or malignant cause. If present, up to three leading UBUs were quantified (SUVmax), localized, and correlated to clinical parameters, such as age, PSA, injected dose, Gleason score, tumor size (T1–T4), and type of PET scanner (analog vs. digital). Additionally, clinical and imaging follow-up results and therapeutic impact were evaluated.ResultsUBUs were identified in 179 out of 348 patients (51.4%). The most frequent localizations were ribs (57.5%) and pelvis (24.8%). The frequency of UBUs was not associated with PSA, Gleason score, tumor size, age, or the injected [18F]-PSMA-1007 dose. UBUs were significantly more frequent in images obtained with digital PET/CT scans (n = 74, 82%) than analog PET/CT scans (n = 221, 40.3%) (p = .0001) but not in digital PET/MR (n = 53, 51%) (p = .1599). In 80 out of 179 patients (44.7%), the interpretation of UBUs was critical for therapeutic management and therefore considered clinically relevant. For 65 UBUs, follow-ups were available: three biopsies, three radiotherapies with PSA follow-up, and 59 cases with imaging. After follow-up, UBUs were still considered unclear in 28 of 65 patients (43%), benign in 28 (43%), and malignant in nine (14%) patients.ConclusionUBUs occur in two-thirds of patients imaged with [18F]-PSMA-1007 PET/CT and are significantly more frequent on digital PET scanners than analog scanners. UBUs should be interpreted carefully to avoid over-staging.

Highlights

Positron emission tomography (PET), combined with either computer tomography (CT) or magnetic resonance imaging (MRI), utilizing radiotracers that bind to prostate-specific membrane antigen (PSMA) is an excellent diagnostic tool for prostate cancer imaging
The [18F]-labeled DCFPyL showed promising results with high image quality and good lesion detection [17], and similar results were observed with a new class of radiohybrid tracers that can be labeled with [18F] or metals such as [68 Ga], respectively [18]
Age was not associated with frequency of unspecific bone uptake (UBU) (U = 13,014.00, Z = 2.25, p = 0.02); the p-value was < 0.05, the 95% CI of median difference was not significant (Fig. 4a)

Summary

Introduction

Positron emission tomography (PET), combined with either computer tomography (CT) or magnetic resonance imaging (MRI), utilizing radiotracers that bind to prostate-specific membrane antigen (PSMA) is an excellent diagnostic tool for prostate cancer imaging. PSMA-PET/CT has shown better detection efficacy in early BCR than MRI, CT, conventional imaging [3,4,5,6], or choline-labeled PET ligands [7], and showed substantial impact on management [8]. Several PSMA ligands are available, radiolabeled with currently primary two different positron-emitting isotopes: gallium-68 [68 Ga] and fluorine-18 [18F].The most commonly used PSMA agent in Europe was initially [68 Ga]PSMA-11, with well-established application for cancer localization in early BCR, with high detection rates, and an impact on management following 60% of scans [15]. The lower positron emission energy of [18F]-PSMA ligands than [68 Ga]-PSMA ligands (0.6 MeV vs 2.3 MeV) leads to a higher image resolution in comparative phantom studies [19]. One of the candidate ligands already implemented in clinical routines in several hospitals in Switzerland is [18F]-PSMA-1007, which benefits from low background activity in the urinary tract [20], which is an important advantage in suspected local recurrence [5, 21]

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Results

Conclusion