FLASH radiotherapy delivers the whole radiation dose in less than one second. It is a research-stage treatment now being studied in human clinical trials in the United States. In the FAST-02 trial, published in Radiotherapy and Oncology in June 2026, 6 of the 8 patients with three-month data (75%) reported complete relief of pain from bone metastases in the chest, and no serious side effects were reported. This article explains how FLASH works, the results of the first two trials, who took part, how it differs from carbon ion therapy, and which particle therapy and costs are available in Japan today. To find out which particle therapy may suit your diagnosis, please use our particle therapy eligibility and cost consultation.
What is FLASH radiotherapy?
Conventional radiotherapy delivers dose at about 0.03 gray (Gy) per second. Each session takes several minutes, and treatment is usually spread over many days. FLASH radiotherapy, also called ultra-high-dose-rate radiotherapy, delivers more than 40 Gy per second, so the same dose is given within one second — over 1,000 times faster.
Researchers are interested in FLASH because of normal-tissue protection. When radiation is given in an extremely short burst, it has been observed to keep its effect on the tumor while causing less damage to surrounding healthy tissue. This is called the FLASH effect. In 2018, Lausanne University Hospital (CHUV) in Switzerland treated one patient with skin lymphoma using electron FLASH and showed that the treatment was feasible and safe. Formal clinical trials using proton FLASH then began in Cincinnati, USA.
Protons release their energy at a set depth and then stop (the Bragg peak), which already makes proton therapy precise. FLASH adds a new tool: speed. The basics of particle therapy are explained on What Is Carbon Ion Therapy?
FAST-01: the first trial in humans
FAST-01 (NCT04592887), the world's first proton FLASH trial, was run by Cincinnati Children's Hospital and the University of Cincinnati proton therapy center. Ten patients with painful bone metastases in the arms or legs (12 sites) received a single 8 Gy dose. The delivered dose rate was 51 to 61 Gy per second. Results were published in JAMA Oncology in October 2022.
| Item | FAST-01 result |
|---|---|
| Patients | 10 patients with limb bone metastases (12 sites) |
| Treatment | Proton FLASH, 8 Gy once |
| Complete pain relief | 6 of 12 sites (50%) |
| Partial pain relief | 2 of 12 sites (16.7%) |
| Overall pain response | 8 of 12 sites (67%); 7 of 10 patients |
| Serious side effects | None (mostly grade 1 skin color change or itching) |
| Time on treatment couch | 18.9 minutes per patient on average |
The investigators concluded that proton FLASH could be delivered smoothly in a real clinic, with pain control and side effects comparable to conventional radiotherapy. There were no treatment delays or device problems, and a follow-up trial in another part of the body began soon after.
What did FAST-02 show for chest bone metastases?
FAST-02 (NCT05524064) was the first trial of FLASH inside the chest, close to the heart and lungs. Ten adults with painful spread of lung, prostate or kidney cancer to the ribs, sternum, shoulder blades or collarbones took part. Each received a single 8 Gy dose at more than 40 Gy per second, finished within one second. Results were presented at the ESTRO congress in May 2026 and published online in Radiotherapy and Oncology on June 24, 2026.
| Item | FAST-01 (limbs) | FAST-02 (chest) |
|---|---|---|
| Patients | 10 (12 sites) | 10 |
| Sites treated | Arm and leg bones | Ribs, sternum, shoulder blades, collarbones |
| Dose | 8 Gy once | 8 Gy once |
| Complete pain relief | 50% (by site) | 75% (3 months, 6 of 8) |
| Partial pain relief | 16.7% | 25% (2 of 8) |
| Heart or lung side effects | Not applicable | None reported |
| Serious side effects | None | None |
All 8 patients with three-month data had their pain resolve or decrease. No patient had the cancer grow back or spread at the treated site during follow-up. The most common side effect was mild skin change, which resolved over time. The University of Cincinnati team said FLASH controlled pain effectively with minimal side effects even near critical organs. Varian, the equipment maker, said larger multi-center trials across cancer types are the next step.
Bone metastasis pain greatly affects quality of life. If one short session can ease that pain, the burden of repeated hospital visits falls sharply. Particle therapy for a small number of metastases is also covered in Carbon ion therapy for oligometastases.
Who took part in the trials?
The main conditions in the FAST-02 protocol (published in Radiation Oncology in 2024) were:
- Age 18 or over, with 1 to 3 painful bone metastases in the ribs, clavicles, scapulae or sternum
- Life expectancy of more than 6 months, and a FLASH plan meeting safety limits
- Not eligible with spinal metastases, pathologic fractures or metal implants in the field
- Not eligible after previous symptomatic radiation pneumonitis
- Systemic therapy affecting radiation response paused 2 weeks before and 1 week after treatment
Patients kept a daily pain diary from day 2 to day 11 and completed pain questionnaires and side-effect checks at set points from 1 to 12 months. This careful record made it possible to measure the effect of a very short treatment accurately.
How does FLASH differ from carbon ion therapy?
Both use particle beams, but with different aims. FLASH is about fast delivery to protect normal tissue. Carbon ion therapy uses heavy carbon ions for a strong biological effect. Carbon ion therapy has been used in Japan for more than 30 years and is an approved treatment, covered by Japanese public insurance for some cancers.
| Item | Proton FLASH | Carbon ion therapy |
|---|---|---|
| Particle | Protons (hydrogen nuclei) | Carbon ions |
| Key strength | Delivery within one second, expected to spare normal tissue | Strong cell-killing power, effective even for radiation-resistant cancers |
| Current status | Clinical trials (under a US FDA investigational device exemption) | Approved in Japan (insurance, Advanced Medical Care, self-pay) |
| Studied so far | Pain relief for bone metastases | Prostate, pancreatic, liver and lung cancer, bone and soft tissue sarcoma and more |
| Number of sessions | 1 | Usually 1 to about 16 (by cancer type) |
| How to receive it | Only patients who meet trial criteria | Japanese carbon ion centers accept international patients |
The basic differences between protons and carbon ions are compared in Carbon ion vs proton therapy.
Treatments available now and their cost
FLASH is currently available only to patients enrolled in US trials, so there is no published fee. Carbon ion therapy in Japan, by contrast, is open to international patients now. QST Hospital in Chiba lists its carbon ion radiotherapy fee for patients without Japanese insurance as 5.786 million yen including tax (English guide, updated May 27, 2026). Hospitalization, consultations and tests are charged separately at 33 yen per medical fee point, and coordinator fees are also separate. Costs at other hospitals are compared in Carbon ion therapy cost in Japan.
| Cost item | Amount | Source and date |
|---|---|---|
| Proton FLASH | No published fee (clinical trial stage) | University of Cincinnati and Cincinnati Children's, July 2026 |
| QST Hospital carbon ion fee (international patients) | 5.786 million yen (tax included) | QST Hospital English guide, updated May 27, 2026 |
| Hospitalization, consultations, tests | 33 yen per medical fee point | Same guide |
If you are looking into particle therapy for bone metastasis pain or a cancer that is hard to operate on, send your medical reports and imaging CD for a carbon ion therapy eligibility consultation in Japan. Phone +82-2-517-5117.
Preparing the following speeds up review by Japanese hospitals:
- CT, MRI or PET imaging CD from the last 3 months, with reports
- Pathology (biopsy) report and diagnosis letter
- Records of past surgery, chemotherapy and radiotherapy
- Current pain level and list of pain medicines
Japanese carbon ion centers are listed on Japanese Institutions and in QST Hospital guide for international patients.
Frequently asked questions
Can international patients receive FLASH radiotherapy?
At present it is a research-stage treatment for patients who meet the criteria of trials in Cincinnati, USA. The researchers and equipment maker have said larger multi-center trials are the next step, so access is expected to widen.
How many FLASH sessions are needed?
Both FAST-01 and FAST-02 gave a single 8 Gy dose, with the beam on for less than one second. This is very different from conventional radiotherapy given over many days.
Should I choose FLASH or carbon ion therapy?
FLASH is available only through trials, while carbon ion therapy is approved in Japan and open to international patients. Japanese doctors decide eligibility based on cancer type, location and previous treatment.
Is there particle therapy available now for bone metastasis pain?
Japanese carbon ion centers treat some bone and soft tissue tumors and a small number of metastases. Eligibility depends on the number and location of metastases, so sending imaging for review is the fastest way to find out.
Related articles
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- FAST-02 clinical trial shows pain relief after treating bone metastases in chest — Cincinnati Children's (2026-07-15)
- University of Cincinnati advances FLASH radiation therapy with new clinical trial (2026-07)
- Proton FLASH Radiotherapy Trial Demonstrates Pain Reduction in Thoracic Bone Metastases — Applied Radiation Oncology
- Mascia AE et al. Proton FLASH Radiotherapy for the Treatment of Symptomatic Bone Metastases: The FAST-01 Nonrandomized Trial. JAMA Oncol 2023;9(1):62-69
- Daugherty EC et al. FLASH radiotherapy for symptomatic bone metastases in the thorax (FAST-02): protocol. Radiat Oncol 2024;19:34
- FLASH Therapy — ISREC Foundation (CHUV first patient, 2018)
- QST Hospital — For patients from abroad (fee, updated 2026-05-27)
General information based on public sources; not a substitute for diagnosis or treatment. Eligibility is decided by the treating physicians. Written by the FTH editorial team.