KEY POINTS
- FAST-02 was a prospective, single-arm clinical trial evaluating proton FLASH radiotherapy in adults with painful, non-vertebral thoracic bone metastases. Of 12 enrolled patients, 10 received protocol treatment to one rib or scapular lesion.
- Patients received 8 Gy in one fraction using a single 250 MeV transmission proton field at a prescribed dose rate above 40 Gy/s. All treatments met the protocol criterion requiring at least 90% of voxels receiving more than 7.2 Gy to exceed 48 Gy/s.
- At a median follow-up of 189 days, no grade 2 or higher acute or late adverse events were considered at least possibly related to FLASH treatment. Five related events occurred in four patients, all grade 1, including skin changes and one transient asymptomatic focal pneumonitis.
- At the latest available pain assessment, 6 patients had a complete response, 3 had a partial response, and 1 had stable pain. Three patients experienced a pain flare during the first 11 days.
- All patients completed treatment without dose-delivery malfunction. Mean time on the treatment table was 14.6 minutes, and all target and organ-at-risk planning goals were met.
CLINICAL TAKEAWAY
FAST-02 shows that single-fraction proton FLASH can be delivered near thoracic organs at risk with acceptable early toxicity, conventional treatment-room workflow, and encouraging pain control. However, the trial included only 10 treated patients, lacked a comparator, and predominantly involved rib lesions, so it establishes feasibility rather than comparative clinical benefit.
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