Low LET protons focused to submicrometer shows enhanced radiobiological effectiveness

Area of Science:

• Radiation Biology
• Nuclear Physics
• Medical Physics

Background:

• Radiobiological effectiveness (RBE) is crucial for radiation therapy planning.
• Understanding RBE enhancement through focused radiation is key to optimizing treatment.
• Current models like LEM III may not fully capture RBE variations with focused beams.

Purpose of the Study:

• To investigate enhanced RBE values by focusing low linear energy transfer (LET) radiation.
• To compare RBE of focused proton beams with homogeneous irradiation in human-hamster hybrid cells.
• To evaluate the impact of microdose distribution on RBE and compare with existing models.

Main Methods:

• Utilized a 20 MeV proton beam focused to submicrometer diameter using the superconducting nanoprobe at the Munich tandem accelerator.
• Irradiated human-hamster hybrid (A(L)) cells with focused and homogeneous proton beams at an average dose of 1.7 Gy.
• Quantified RBE by measuring micronuclei (RBE(MN)) and dicentrics (RBE(D)), normalized to X-ray irradiation.

Main Results:

• Focused proton irradiation yielded significantly higher RBE values (RBE(MN) = 1.48 ± 0.07, RBE(D) = 1.92 ± 0.15) compared to homogeneous irradiation (RBE(MN) = 1.28 ± 0.07, RBE(D) = 1.41 ± 0.14).
• The energy deposited by 117 focused protons per point matched that of a (12)C ion, with observed RBE enhancements being approximately half of those for (12)C ions.
• Measured RBE values deviated from predictions of the Local Effect Model (LEM III).

Conclusions:

• Focusing low LET radiation can significantly enhance RBE by altering microdose distribution and promoting intertrack interactions.
• These findings suggest a potential for improved tumor targeting and efficacy in radiation therapy.
• The study highlights the need for refined models to accurately predict RBE for focused particle beams.