Inclusion Criteria:

• * Patient age at least 18 years
• * 1 to 3 painful bone metastasis(-es) requiring treatment, in the ribs, clavicles, scapulae, or sternum (if more than one metastasis can be treated within the same treatment field, it will be counted as one metastatic site for the purpose of trial treatment).
• * Bone metastases that can be treated within a field size up to 7.5 cm x 30 cm without overlap of radiation fields
• * Life expectancy \>6 months (in the judgement of the investigator)
• * Patients who are able to comply with the protocol
• * Provision of signed and dated informed consent form
• * Clinically acceptable treatment plan

Exclusion Criteria:

• * More than 3 painful bone metastases of the thorax requiring palliative radiotherapy (if more than one metastasis can be treated within the same treatment field, it will be counted as one metastatic site for the purpose of trial treatment).
• * Overlap of FLASH radiation fields with any previous or planned radiation fields to the same site
• * Patients with pathologic bone fractures in the treatment field
• * Patients with metal implants in the treatment field
• * Patients with symptomatic pneumonitis at the time of screening, or a history of symptomatic radiation pneumonitis
• * Patients with known contraindications to thoracic radiation
• * Patients who received or will receive cytotoxic chemotherapy and/or any prescribed systemic therapy known to impact tissue response to radiation, within 2 weeks prior or 1 week following their planned radiation treatment
• * Prior local therapy modality to the treatment site(s) within 2 weeks of study enrollment that, in the judgement of the investigator, might compromise interpretation of pain response
• * Patients with persistent toxicity \> Grade 1 from prior systemic therapy within the proposed treatment field (with the exception of alopecia and peripheral neuropathy)
• * Patients with pacemakers or other implanted devices at risk of malfunction during radiotherapy
• * Patients with any other medical condition or laboratory value that would, at the discretion of the investigator, preclude the patient from participation in this clinical investigation
• * Patients at known risk of enhanced normal tissue sensitivity to radiotherapy due to inherited predisposition or documented comorbidity that might lead to hypersensitivity to ionizing radiation
• * Patients enrolled in any other clinical studies that the investigator believes to be in conflict with this clinical investigation.
• * Patients who are pregnant or nursing