World-first discovery of FLASH effect conditions using compact carbon ion synchrotron opens door to safer cancer therapy

Osaka, Japan―A research team from The University of Osaka has made a significant breakthrough in cancer radiotherapy by identifying conditions under which carbon ion beams—delivered at ultra-high dose rates (uHDR)—can protect normal cells. This phenomenon, known as the “FLASH effect,” could revolutionize cancer treatment by reducing side effects and improving patients’ quality of life.

Radiation therapy is a standard cancer treatment, but it often damages not only tumors but also surrounding healthy tissues. The FLASH effect, discovered in 2014, shows that delivering radiation at dose rates exceeding 40 Gy/s can spare normal tissue while maintaining tumor control. Although FLASH effects have been reported with X-rays, electron and proton beams, evidence for similar effects using carbon ion beams—known for their high precision and strong biological impact—was lacking. Due to the complexity of creating a controlled uHDR environment for carbon ions, little was known about the specific conditions needed for this effect to occur.

Using a specially modified synchrotron-based system at the Osaka Heavy Ion Therapy Center, the researchers irradiated three types of human cells, two normal and one tumor, under varying conditions of oxygen concentration and linear energy transfer (LET). They found that even under normoxic (normal oxygen) conditions, uHDR carbon ion beams produced a significant “cell-sparing” effect—an increase in the survival of normal cells compared to conventional dose rates.

This sparing effect was more prominent when using higher LET (50 keV/μm), a value typical near tumor sites in carbon therapy. Moreover, markers of DNA damage were notably lower in cells exposed to FLASH-like irradiation, suggesting a fundamental biological mechanism distinct from conventional radiation damage.

“This is the first time we have observed the cell-sparing effect with carbon ions under normoxic conditions,” said Kazumasa Minami, the lead author, “it was a challenging experiment, but the results open new possibilities for safer radiotherapy.” says corresponding author, Masashi Yagi.

This is the first study to demonstrate the FLASH effect under normal oxygen conditions using carbon ions, opening new possibilities for applying this technique in clinical settings. By fine-tuning irradiation parameters, such as dose, LET, and oxygen levels, researchers hope to deliver powerful cancer treatments with fewer side effects. The team envisions future studies expanding into immune response effects and tumor-specific applications.

###

The article, “The Appropriate Conditions for the Cell Sparing (FLASH) Effect Exist in Ultra‐high Dose Rate Carbon Ion Irradiation,” was published in Anticancer Research at DOI: https://doi.org/10.21873/anticanres.17483

About The University of Osaka

The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world, being named Japan's most innovative university in 2015 (Reuters 2015 Top 100) and one of the most innovative institutions in the world in 2017 (Innovative Universities and the Nature Index Innovation 2017). Now, Osaka University is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.

Website: https://resou.osaka-u.ac.jp/en