Enhanced RBE of Particle Radiation Depends on Beam Size in the Micrometer Range

Area of Science:

• Radiation Biology
• Radiobiology
• Biophysics

Background:

• High-linear energy transfer (LET) radiation exhibits higher relative biological effectiveness (RBE) than low-LET radiation due to distinct cellular interaction and dose distribution.
• High-LET particles deposit high doses locally, concentrating DNA damage, while low-LET particles require multiple hits for homogeneous damage distribution.

Purpose of the Study:

• To investigate how RBE depends on spatial dose deposition by examining different focused proton beam spot sizes.
• To assess the induction of chromosome aberrations and clonogenic cell survival under varying proton beam spot sizes.

Main Methods:

• Human-hamster hybrid (AL) and Chinese hamster ovary (CHO-K1) cells were irradiated with 20 MeV protons at a mean dose of 1.7 Gy.
• Focused proton beams with spot sizes ranging from 0.8 µm to 2.8 µm were used in a matrix pattern.
• Relative biological effectiveness (RBE) was determined by comparing dose-response curves with X-ray reference radiation.

Main Results:

• The RBE for dicentric chromosomes and cell inactivation increased with smaller beam spot sizes (0.8-1.0 µm) compared to homogeneous proton radiation.
• Increasing spot size gradually decreased RBE, with no significant difference from homogeneous radiation observed at 2.7-2.8 µm.
• The RBE remained lower than that of high-LET single ion hits, with the difference attributed to nanometer-scale energy deposition.

Conclusions:

• Varying the spot size of low-LET radiation gradually modifies RBE, highlighting the role of micrometer-scale energy concentrations.
• A significant fraction of enhanced RBE stems from inhomogeneous energy deposition on the micrometer scale.
• The study quantifies the relationship between energy concentration and RBE, comparing results with PARTRAC and LEM models.