Total Bone Uptake as a Quantitative Imaging Biomarker for Prostate Cancer With Bone Metastases.

Background/Aim: Bone scintigraphy is widely used for evaluating bone metastases in prostate cancer, but conventional planar imaging is limited by its qualitative nature and poor reproducibility for longitudinal assessment. The Bone Scan Index (BSI) provides a semiquantitative measure of skeletal tumor burden, but it does not reflect tracer uptake intensity. Quantitative single-photon emission computed tomography/computed tomography (SPECT/CT) allows volumetric assessment of uptake, and total bone uptake (TBU) derived from GI-BONE (Nihon Medi-Physics Co., Ltd, Tokyo, Japan) may offer a more comprehensive biomarker of skeletal disease activity. Patients and Methods: Tc-HMDP bone SPECT/CT in routine practice between January and September 2025. Clinical data, prostate-specific antigen (PSA) levels, and quantitative indices from GI-BONE, including BSI, metabolic bone volume, and TBU, were extracted from medical records. Results: In all three cases, serial TBU measurements provided an objective representation of skeletal disease activity over time. Changes in TBU were consistent with the clinical course and treatment response, suggesting that this parameter may be useful for monitoring disease dynamics during follow-up. Conclusion: GI-BONE-derived TBU may be a practical quantitative biomarker for assessing bone metastatic burden and treatment response in prostate cancer. This case series suggests that TBU can complement conventional bone scintigraphy by providing objective and reproducible longitudinal information.