PSMA-PET Imaging and Unspecific Bone Uptake (UBUs)Diagnostic Challenge and Role of the BUMP Score in Urologic Practice.

Chih-Yi Wu, BS¹, Chih-Wei Wu, MD² (co-first author), Allen W. Chiu MD, PhD ², Yen-Kung Chen, MD, PhD³, Yu-Cheng Lu MD ², Cheng-Hui Lee, MS³

1. Primo Biotechnology Co., Ltd, Taipei, Taiwan.
2. Division of Urology, Department of Surgery, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan
3. Department of Nuclear Medicine and PET Center, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan

The study has been selected for poster presentation at the 2025 Urological Association of Asia (UAA) Annual Meeting.

Abstract:

Treating all unspecific bone uptakes (UBUs) on PSMA PET-CT as mere imaging artifacts may mislead clinicians, especially in oligometastatic prostate cancer (omPCa) into withholding curative therapies and opting for palliative strategies. Conversely, mislabeling benign UBUs as metastases may cause overtreatment and unnecessary systemic interventions. These misinterpretations contribute to increased imaging workload and clinical uncertainty.[1][2][3]

UBUs occur across all PSMA tracers. [¹⁸F]PSMA-1007 has the highest reported UBU rate (11.6–71.7%), followed by [⁶⁸Ga]PSMA-11 (0–23.9%) and [¹⁸F]rhPSMA-7 (up to 11.1%). [¹⁸F]DCFPyL also shows intermediate frequency (~19.8%). Unlike other tracers, [¹⁸F]PSMA-1007 undergoes hepatobiliary clearance, bypassing bladder interference and offering superior pelvic tumor visualization. However, it remains particularly prone to UBU-related diagnostic ambiguity.[1][4]

To address this, we applied our implementation of the BUMP score (Bone Uptake Metastatic Probability), a probability model incorporating SUVmax, HUmean, and ADT status to enhance diagnostic clarity for [¹⁸F]PSMA-1007 PET. The model supports clinicians in determining whether focal bone uptake warrants concern, particularly when evaluating candidates for metastasis-directed therapy (MDT).[5]

We validated our calculator on four patients undergoing [¹⁸F]PSMA-1007 PET-CT using RadeluminTW® injection (259 MBq) on a SIEMENS AI PET Biograph Vision system. One patient showed no UBUs. Across the remaining three patients, 15 UBUs were evaluated. Three lesions (20%) exceeded the BUMP malignancy threshold (>0.25), yet only one (6.7%) was ultimately classified for follow-up due to proximity to a lung PSMA-avid lesion. This finding strongly matched expert nuclear medicine interpretation and confirms the model’s ability to exclude benign solitary UBUs while flagging clinically relevant sites.

Bauckneht et al. (2024) showed that 41.9% of 448 foci were malignant, while Grünig et al. (2021) reported 14% malignancy in 65 UBUs. These studies highlight the need for contextual UBU risk assessment rather than reflexive assumptions. The BUMP score provides this context by integrating metabolic, anatomical, and treatment status markers. With external validation (AUC 0.92), high specificity (88%), and utility even for non-sclerotic lesions, the BUMP score reclaims diagnostic value from the challenge of UBUs.[1][2][5]

In conclusion, [¹⁸F]PSMA-1007 offers high-resolution imaging and reduced bladder interference. When interpreted with our BUMP score calculator, its susceptibility to UBUs becomes an asset—supporting triage, reducing misclassification, and optimizing MDT eligibility. We recommend routine incorporation of BUMP scoring into clinical workflows to enhance confidence in equivocal skeletal findings.

References:

1. 1. 1. Grünig H, Maurer A, Thali Y, et al. 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. Eur J Nucl Med Mol Imaging. 2021;48(12):4483–4494. https://doi.org/10.1007/s00259-021-05350-x

1. 1. 2. Ost P, Reynders D, Decaestecker K, et al. Surveillance or metastasis-directed therapy for oligometastatic prostate cancer recurrence: a prospective, randomized, multicenter phase II trial. Lancet Oncol. 2018;19(5):674–684. https://doi.org/10.1016/S1470-2045(18)30166-5

1. 1. 3. Palma DA, Olson R, Harrow S, et al. Stereotactic ablative radiotherapy for the comprehensive treatment of oligometastatic cancers (SABR-COMET): results of a randomised trial. J Clin Oncol. 2020;38(25):2830–2838. https://doi.org/10.1200/JCO.20.00818

1. 1. 4. Bauckneht M, Rizzo A, Morbelli S, et al. The homunculus of unspecific bone uptakes associated with PSMA-targeted tracers: a systematic review-based definition. Eur J Nucl Med Mol Imaging. 2024;51(3):730–742. https://doi.org/10.1007/s00259-023-06578-5

1. 1. 5. Bauckneht M, D’Amico F, Albano D, et al. Composite Prediction Score to Interpret Bone Focal Uptake in Hormone-Sensitive Prostate Cancer Patients Imaged with [18F]PSMA-1007 PET/CT. J Nucl Med. 2024;65(1):55–61. https://doi.org/10.2967/jnumed.122.265492