Because of its significantly higher speed, FLASH proton therapy can be administered in less time and fewer sessions, making it superior to conventional radiotherapy, but exposure to any form of radiation, no matter how short, comes with a risk for side effects.

In a recent preclinical study, scientists at Cincinnati Children’s/University of Cincinnati Medical Center Proton Therapy Center evaluated these effects on the skin, which is often left red, darker, and itchy in conventional radiotherapy. This can lead to blisters, uncomfortable peeling, and sores.

They found that administering multiple radiation beams between intervals reduced the skin-saving effects of preclinical models in FLASH proton therapy, putting patients at more risk for these side effects, versus a single, uninterrupted beam.

The problem, the researchers say, is that multiple beam treatments are often needed to reduce exposure of normal tissue to radiation and to ensure the entire cancerous area has been identified, targeted, and treated. Also, clinicians stop treatment every few minutes to reposition patients and equipment.

“This study suggests the number of beams and the spatial arrangement of those beams are important parameters for FLASH studies. Furthermore, the effect of multibeam delivery is likely different for different organs of interest,” said Mathieu Sertorio, corresponding author on the paper and a member of the basic science research program within the Center, in a statement.

Using a Varian ProBeam system, Sertorio and his colleagues irradiated the hind leg of a mouse at conventional (1 Gy/s) and FLASH (100 Gy/s) average field dose rates via pencil beam scanning over seven weeks, applying treatment in one, two or three equal beams with intervals of two minutes.

They then studied the skin tissue over 16 weeks, finding that two beams separated by an interval reduced the skin-saving effect, and three further decreased it.

"This study is important because it informs the clinical translation of FLASH radiotherapy, with respect to beam delivery modalities, treatment planning, and radiation prescriptions. We need to identify where FLASH works and where FLASH doesn't work to move forward,” said co-author Anthony Mascia, of Cincinnati Children’s, in a statement.

Varian, a Siemens Healthineers company, funded the research and provided research grants for it.

The findings were published in International Journal of Radiation Oncology, Biology, Physics.

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