New five-year results from Japan’s National Institutes for Quantum Science and Technology (QST) indicate that carbon-ion radiotherapy may offer a viable nonsurgical alternative for some women with early-stage breast cancer, delivering durable tumor control with minimal side effects while preserving breast appearance.
The findings come from a single-center, prospective phase II study published online on October 24, 2025, in the International Journal of Radiation Oncology • Biology • Physics. Researchers reported that carefully selected patients who were unable or unwilling to undergo surgery achieved strong oncologic outcomes without compromising quality of life.
At five years, local control and disease-free survival rates both reached 92%, while overall survival was 100%. No patients experienced grade 2 or higher toxicities. Most reported only mild, temporary skin reactions, and cosmetic outcomes were rated “excellent” in all but one case. That patient later required mastectomy following a local recurrence.
“Carbon-ion radiotherapy combines high biological effectiveness with highly precise dose delivery,” said Noriyuki Okonogi, MD, PhD, of QST Hospital. “In selected patients with small, estrogen receptor–positive, HER2-negative tumors, we observed complete responses in all cases, with sustained tumor control and minimal treatment-related burden.”
The study enrolled 12 women aged 60 or older with stage I invasive ductal carcinoma. All tumors were 2 cm or smaller. Patients received a total dose of 60 Gy (RBE) of carbon-ion radiotherapy delivered in four fractions over one week, followed by standard adjuvant aromatase inhibitor therapy for at least five years.
Follow-up involved serial MRI scans until complete response, typically observed at around 12 months, and semiannual imaging thereafter. One in-field recurrence occurred in a tumor with a high Ki-67 index, highlighting the potential role of biological markers in refining patient selection.
“The sharp dose conformity enabled by the Bragg peak allows us to spare healthy tissue while delivering ablative radiation to the tumor,” said Kumiko Karasawa, MD, PhD, of QST Hospital. “The absence of clinically significant toxicity, together with preservation of breast appearance, is central to patients’ well-being and daily quality of life.”
Kazutoshi Murata, MD, PhD, section manager of the Urinary and Reproductive Oncology Section at QST Hospital, emphasized the broader implications of the findings.
“Our results suggest that carbon-ion radiotherapy could offer a new treatment pathway for patients who prioritize both oncologic safety and quality of life,” Murata said. “As technology advances, we aim to expand access to this approach and reduce the physical and emotional burden associated with surgery.”
The trial underscores how the distinctive physical and biological properties of particle therapy—such as high linear energy transfer and tightly confined dose deposition—can translate into clinical benefits. Compared with photon-based ablative regimens, which have been associated with higher rates of moderate skin toxicity, carbon-ion therapy in this cohort achieved comparable tumor control with fewer adverse effects.
The authors call for larger, multi-institutional controlled trials to validate the results and to further refine biomarkers, including Ki-67, to optimize patient selection.
Looking ahead, the QST team anticipates broader clinical availability of carbon-ion radiotherapy for nonsurgical breast cancer candidates within the next five years through expanded trials and dedicated treatment pathways. Over the next decade, they suggest, the approach could be incorporated into standard treatment algorithms for well-defined patient subgroups—reducing the need for surgery while preserving body image and maintaining oncologic safety.
About the National Institutes for Quantum Science and Technology (QST), Japan
Established in April 2016, the National Institutes for Quantum Science and Technology (QST) was formed through the merger of the National Institute of Radiological Sciences (NIRS) and select operations of the Japan Atomic Energy Agency (JAEA). QST is dedicated to advancing quantum science and technology, building world-leading research and development platforms, and generating academic, social, and economic impact through innovation.
