This prospective observational study will evaluate whether the blood biomarker TRACP-5b can help diagnose and monitor patients with giant cell tumor of bone (GCTB). GCTB is a rare but locally aggressive bone tumor that can return even after surgical treatment. Currently, patients require long-term follow-up based mainly on imaging examinations such as CT or MRI, and suspected recurrence often requires invasive biopsy procedures.

TRACP-5b is an enzyme associated with osteoclast activity that can be measured from a blood sample. Previous studies suggest that TRACP-5b levels may reflect disease activity and risk of recurrence in patients with GCTB. In this study, patients with newly diagnosed GCTB and patients already undergoing long-term follow-up after treatment will undergo regular blood sampling for TRACP-5b measurement during routine outpatient visits. Laboratory results will be compared with the patient's clinical condition, imaging findings, and histological results when available.

The goal of the study is to determine whether TRACP-5b could become a reliable and minimally invasive tool for earlier detection of disease recurrence and improved long-term monitoring of patients with giant cell tumor of bone.

Giant cell tumor of bone (GCTB) is a rare, locally aggressive primary bone tumor characterized by osteoclast-like multinucleated giant cells and neoplastic stromal cells. The disease most commonly affects young adults and typically arises in the epiphyseal region of long bones. Although GCTB is classified as an intermediate tumor with limited metastatic potential, it is associated with substantial local bone destruction and a clinically significant risk of local recurrence even after adequate surgical treatment. Standard management consists primarily of surgical treatment, including intralesional curettage with local adjuvant treatment or en bloc resection in selected cases. Despite advances in surgical techniques and the use of systemic therapies such as denosumab in selected patients, long-term surveillance remains essential due to the possibility of recurrence.

Current follow-up strategies are based mainly on periodic imaging examinations, including radiography, computed tomography (CT), and magnetic resonance imaging (MRI). However, imaging-based surveillance may not always detect recurrence at its earliest stage, and suspicious findings frequently require histopathological confirmation through biopsy procedures. At present, there is no routinely established serum biomarker for monitoring disease activity or predicting recurrence in patients with GCTB.

TRACP-5b (tartrate-resistant acid phosphatase isoform 5b) is an enzyme secreted predominantly by activated osteoclasts and reflects osteoclastic bone resorption activity. Since giant cell tumor of bone is characterized by pronounced osteoclast-like giant cell activity, TRACP-5b has emerged as a potential disease-specific biomarker in this patient population. Previous clinical studies have demonstrated significantly elevated serum TRACP-5b levels in patients with active GCTB compared with healthy controls, and several reports suggest that changes in serum TRACP-5b levels may correlate with tumor burden, treatment response, and recurrence risk.

The primary objective of this prospective observational clinical study is to evaluate the diagnostic and predictive value of serum TRACP-5b in patients with histologically confirmed giant cell tumor of bone. The study aims to determine whether serial measurement of TRACP-5b can reliably reflect disease activity and assist in the detection of local recurrence during routine clinical follow-up.

Secondary objectives include evaluation of the temporal dynamics of TRACP-5b during treatment and long-term surveillance, assessment of correlations between TRACP-5b levels and imaging findings, and analysis of the relationship between TRACP-5b levels and histopathological or clinical indicators of disease activity. The study will additionally explore whether implementation of TRACP-5b monitoring could reduce the need for repeated imaging examinations or invasive biopsy procedures in selected clinical situations.

The study is designed as a prospective observational single-center clinical research study conducted at the First Department of Orthopaedic Surgery, St. Anne's University Hospital Brno and Faculty of Medicine, Masaryk University. Approximately 70 patients are expected to be enrolled during the study period. The study population will include both newly diagnosed patients with histologically confirmed giant cell tumor of bone and patients undergoing long-term post-treatment surveillance.

Blood samples for serum TRACP-5b analysis will be collected during routine outpatient visits performed as part of standard clinical care. Approximately 2 mL of venous blood will be obtained at each collection timepoint. In newly diagnosed patients, sampling will be performed at the time of diagnosis and subsequently during scheduled follow-up visits throughout treatment and surveillance. In patients already undergoing long-term follow-up after prior treatment, blood sampling will be performed during routine surveillance visits at approximately 6-month intervals. Samples will be processed and stored at the Department of Clinical Biochemistry of St. Anne's University Hospital Brno and subsequently analyzed using a validated immunochemical assay at the Department of Clinical Biochemistry, University Hospital Ostrava.

TRACP-5b laboratory results will be compared with the patient's clinical condition, including symptoms and physical examination findings, as well as with standard imaging examinations performed during routine oncologic follow-up, including radiography, CT, or MRI where clinically indicated. Available histopathological findings obtained from diagnostic or follow-up biopsy procedures will also be evaluated in relation to TRACP-5b levels when applicable.

The primary outcome measure will be the ability of serum TRACP-5b to detect active disease and local recurrence of giant cell tumor of bone. Secondary outcome measures will include longitudinal changes in TRACP-5b levels during treatment and surveillance, correlations between TRACP-5b and radiologic findings, and evaluation of the feasibility of incorporating TRACP-5b testing into routine clinical management of GCTB patients.

Inclusion Criteria:

• Histologically confirmed Giant Cell Tumor of Bone
• Newly diagnosed patients or patients undergoing long-term follow-up after previous treatment
• Age ≥18 years
• Ability to understand the study procedures and provide written informed consent

Exclusion Criteria:

• Concurrent diseases significantly affecting bone metabolism, including metabolic bone disorders
• Inability or unwillingness to provide informed consent
• Patients unable to comply with routine follow-up procedures
• Active conditions that may substantially influence serum TRACP-5b levels independently of Giant Cell Tumor of Bone evaluation

The study population consists of adult patients with histologically confirmed Giant Cell Tumor of Bone treated or followed at the First Department of Orthopaedic Surgery, St. Anne's University Hospital Brno. The study will include both newly diagnosed patients and patients undergoing long-term surveillance after previous treatment for giant cell tumor of bone. Approximately 70 participants are expected to be enrolled during the study period.