Researchers at the University of Osaka have identified key conditions under which ultrahigh dose rate (uHDR) carbon ion radiation can reduce damage to healthy cells, a finding that may someday expand the clinical use of carbon ion therapy with fewer side effects.

The study, published in Anticancer Research, marks the first documented observation of the so-called FLASH effect with carbon ion beams under normoxic (normal oxygen) conditions. The FLASH effect, previously demonstrated with X-rays and proton beams, refers to a phenomenon where radiation delivered at dose rates exceeding 40 Gy/s selectively spares normal tissue while still effectively targeting tumors.

Using a compact synchrotron system at the Osaka Heavy Ion Therapy Center, researchers exposed two types of normal human cells and one tumor cell line to carbon ion beams under controlled conditions. Variables included dose rate, oxygen concentration, and linear energy transfer (LET). The cell-sparing effect emerged most clearly at a LET of 50 keV/μm, a range commonly seen at tumor margins during carbon therapy.
Notably, DNA damage markers were reduced in normal cells exposed to uHDR compared to those receiving conventional dose rates, suggesting a distinct biological mechanism underpinning the observed sparing.

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

The findings could help refine treatment protocols by adjusting LET and oxygen parameters to optimize patient outcomes. Further studies are planned to examine immune system responses and the potential for tumor-specific applications.

The full article, “The Appropriate Conditions for the Cell Sparing (FLASH) Effect Exist in Ultra-high Dose Rate Carbon Ion Irradiation,” is available at Anticancer Research.

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