This is a prospective, randomized, single-center clinical trial evaluating whether early prophylactic radiotherapy to high-risk asymptomatic or minimally symptomatic bone metastases reduces the 1-year rate of skeletal-related events (SREs) in patients with metastatic solid cancers. Patients will be randomized in a 1:1 ratio to receive either standard of care systemic therapy or observation, or prophylactic radiotherapy in addition to standard of care. The primary endpoint is the 1-year rate of SREs. Secondary endpoints include adverse events, quality of life, economic burden, pain-free survival, and overall survival.

Bone metastases are a common complication in patients with metastatic solid cancers and are associated with significant morbidity, including skeletal-related events (SREs) such as pathological fractures, spinal cord compression, and severe pain requiring intervention. While palliative radiotherapy is an established treatment for symptomatic bone metastases, the role of prophylactic radiotherapy in asymptomatic or minimally symptomatic high-risk lesions remains insufficiently defined.

This prospective, randomized, single-center phase II clinical trial evaluates the effectiveness of early prophylactic radiotherapy in reducing the incidence of SREs. Eligible patients with metastatic solid cancers and up to five high-risk asymptomatic or minimally symptomatic bone metastases will be randomized in a 1:1 ratio into two groups. The control group will receive standard of care systemic therapy or observation according to tumor type and clinical guidelines. The intervention group will receive prophylactic external-beam radiotherapy (25 Gy in 5 fractions) to selected high-risk bone metastases in addition to standard of care.

Patients will be followed from baseline up to 12 months, with assessments conducted at predefined intervals. Study outcomes include the incidence of SREs, adverse events (graded according to CTCAE), patient-reported outcomes using validated questionnaires (EORTC QLQ-C30, EQ-5D-5L, BPI), economic burden, pain-free survival, and overall survival.

This study aims to determine whether early radiotherapy intervention can prevent skeletal complications, reduce healthcare burden, and improve patient outcomes in metastatic bone disease.

Inclusion Criteria:

• Age ≥18 years
• Histologically confirmed solid malignancy
• Metastatic disease confirmed by imaging (CT, MRI, or PET-CT)
• Presence of up to five high-risk bone metastases
• High-risk bone metastases defined according to protocol criteria
• Asymptomatic or minimally symptomatic bone metastases (not requiring opioid analgesics)
• ECOG performance status 0-2
• Ability to provide written informed consent

Exclusion Criteria:

• Prior radiotherapy to the index bone metastases preventing safe treatment planning
• Symptomatic bone metastases requiring immediate palliative radiotherapy
• Evidence of spinal cord compression
• Bone metastases causing significant pain requiring opioid analgesics
• Oligometastatic disease suitable for radical treatment
• Leptomeningeal disease
• ECOG performance status ≥3
• Life expectancy less than 3 months
• Any condition that, in the investigator's opinion, would interfere with study participation or assessment

This is an open-label study due to the nature of the intervention (radiotherapy), which does not allow blinding of participants or investigators.