Carbon-ion therapy for breast cancer has produced strong long-term results as a way to treat early breast cancer without surgery. In the world's first prospective trial, run by QST Hospital (National Institutes for Quantum Science and Technology, Chiba, Japan), 12 patients with stage I breast cancer who received carbon-ion beams instead of surgery had a 5-year overall survival of 100% and a 5-year local control rate of 92%. The results were published in IJROBP in February 2026. In carbon-ion therapy in Japan, breast cancer is still at the clinical-trial stage, and enrollment is currently limited to Japanese patients. This article explains the new results, the eligibility criteria of the two ongoing trials, the options overseas patients can explore today, and the published costs. To learn which options fit your diagnosis, please use our consultation on eligibility and cost.
What was newly reported about carbon-ion therapy for breast cancer?
The standard treatment for early breast cancer is breast-conserving surgery followed by radiotherapy. It works well, but many patients find anesthesia and surgery difficult because of age or other illnesses, and many worry about scars and changes in breast shape. Since April 2013, QST Hospital has run a clinical trial that treats the tumor with carbon-ion beams without surgery.
Carbon-ion beams stop at a set depth in the body and release most of their energy there (the Bragg peak). This lets doctors concentrate a high dose on a small tumor while reducing the dose to the heart and lungs. Carbon ions are known to kill cancer cells two to three times more effectively than conventional X-rays at the same dose. See What is carbon-ion therapy?
The new report covers the 5-year results of the phase II part of this trial. QST announced the results in January 2026, and the paper appeared in the International Journal of Radiation Oncology, Biology, Physics (IJROBP) in February 2026. The French National Cancer Institute (INCa) research bulletin also highlighted it. It is the first prospective long-term report of early breast cancer treated with carbon-ion therapy alone.
What were the 5-year results without surgery?
The phase II part enrolled 12 patients aged 60 or older with low-risk stage I breast cancer. None had surgery. Each received 15 Gy (RBE) once a day for 4 days, a total of 60 Gy (RBE). Median follow-up was 73 months, more than six years.
| Item | Result | Meaning |
|---|---|---|
| Complete response (tumor disappeared on imaging) | All 12 (100%) | Median 12 months to disappear (4–36 months) |
| 5-year local control | 92% | No regrowth at the treated site |
| 5-year disease-free survival | 92% | Living without recurrence |
| 5-year overall survival | 100% | All participants alive |
| Recurrence | 1 patient | High Ki-67 index; treated with surgery |
The tumor shrinks slowly after treatment; it took a median of one year to disappear completely on imaging. A tumor still visible right after treatment does not mean failure, so scheduled follow-up scans are important.
The one patient with recurrence continued treatment with surgery. The researchers suggest that selecting patients more precisely with markers such as Ki-67 could improve results further.
Side effects and breast appearance
No serious side effects were reported.
- During treatment (acute): mild dermatitis (grade 1) in 6 patients; none of grade 2 or higher.
- After treatment (late): grade 1 rib fracture in 2 and grade 1 mastitis-related pain in 3; all improved with conservative care.
- MRI findings: symptom-free inflammation of the chest muscle seen only on imaging in 7.
- Breast appearance: rated excellent in all patients except the one who had surgery for recurrence.
Keeping the breast shape without a surgical scar is a major advantage. General side effects and their management are covered in Carbon-ion therapy side effects.
Which carbon-ion trials are ongoing?
Building on these results, QST Hospital is running two trials registered in Japan's jRCT system.
| Trial | Eligibility | Treatment | Status |
|---|---|---|---|
| Phase I/II with standard adjuvant therapy (jRCTs032180154) | Women 20 or older, single tumor 2 cm or less on MRI, no lymph-node or distant metastasis | Carbon ions without surgery, then guideline adjuvant therapy | Recruiting, target 30 |
| Single-fraction phase I/II (jRCTs032200066) | Women 50 or older, 2 cm or less, stage 0–I, luminal A-like if invasive | One carbon-ion session | Recruiting, target 45 |
The single-fraction trial aims to finish treatment in one day, which would greatly reduce the burden of visits. QST plans larger trials to confirm the benefit. QST statistics show 52 breast cancer patients treated with carbon ions from June 1994 to March 2024. See also Early breast cancer carbon-ion trial criteria.
Can overseas patients be treated? What does it cost?
This is important to know. According to QST Hospital's English pages, the two early breast cancer trials currently enroll only Japanese patients, and the overseas-patient page lists breast cancer as a condition not accepted. For now, overseas patients cannot receive carbon-ion therapy in Japan for a primary early breast tumor.
There are still options. QST Hospital's indications list lung and liver metastases (3 or fewer) and lymph-node metastasis confined to one region. If such oligometastases develop after breast cancer treatment, a consultation is possible; QST physicians make the final decision after reviewing the records. See Carbon-ion therapy for oligometastases.
| Item | Amount (QST overseas-patient page) | Note |
|---|---|---|
| Carbon-ion therapy fee | 5.786 million yen (tax included) | Self-pay, patients without Japanese public insurance |
| Consultations, tests, admission | 33 yen per medical-fee point (tax included) | Based on care actually received |
| Medical coordinator fee | Not included above | Separate |
Figures are from QST Hospital's overseas-patient page viewed on 26 October 2026; the page shows no reference date. Accommodation and flights are extra. See Carbon-ion therapy cost for a hospital comparison.
What can be discussed depends on when you were diagnosed, your previous treatment and where any metastases are. Send us your records and we will help prepare a consultation with a Japanese hospital — Contact us · Tel +82-2-517-5117
Documents to prepare
- Pathology report: histology, hormone receptors (ER, PR), HER2, Ki-67
- Imaging CD and reports: breast MRI, chest and abdominal CT, PET-CT, bone scan
- Treatment records: surgery, chemotherapy and radiotherapy dates, prior radiation fields
- Referral letter: current condition and your doctor's opinion
- Medication list: hormone therapy, targeted drugs, anticoagulants
Japanese carbon-ion centers are listed in Japanese institutions.
FAQ
Can breast cancer be treated with carbon-ion therapy?
QST Hospital runs early breast cancer trials, and 12 patients treated without surgery had 100% 5-year overall survival. Enrollment is currently limited to Japanese patients.
Can carbon ions alone replace surgery?
In the phase II trial, the tumor disappeared on imaging in all 12 patients and 5-year local control was 92%. The one patient with recurrence was treated with surgery.
How long does treatment take?
The reported trial used one session a day for 4 days. A single-session trial is now under way.
If breast cancer has spread to the lung or liver, is carbon-ion therapy in Japan possible?
QST Hospital lists lung or liver metastases (3 or fewer) and one-region lymph-node metastasis, so a consultation is possible. QST physicians decide.
What is the cost for overseas patients?
The QST carbon-ion fee is 5.786 million yen (tax included, viewed 26 October 2026); consultations and tests are 33 yen per point.
Related articles
Wondering if carbon ion therapy is possible for you, and what it costs?
We help you prepare consultations with Japanese institutions and provide medical interpreting.
Free consultationCall +82-2-517-5117Sources
- Okonogi N et al. Carbon-ion RT as nonsurgical treatment for early breast cancer: 5-year phase 2 results, IJROBP 2026 (QST repository)
- DOI 10.1016/j.ijrobp.2025.10.020
- French National Cancer Institute (INCa) bulletin on the 5-year results
- QST Hospital — breast cancer carbon-ion clinical studies
- QST Hospital — overseas patients and fees (viewed 2026-10-26)
- jRCT — Phase I/II carbon-ion RT with standard adjuvant therapy for early breast cancer (jRCTs032180154)
- jRCT — Single-fraction carbon-ion RT for early breast cancer, phase I/II (jRCTs032200066)
- QST Hospital — patient numbers by site (1994–2024)
- MONOist — report on QST breast cancer phase II results (Japanese)
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.