Modeling cell survival after irradiation with Ultrasoft X rays using the giant loop binary lesion model

Area of Science:

• Radiation biology
• Radiobiology
• Cellular and molecular biology

Background:

• Ultrasoft X-rays exhibit higher relative biological effectiveness (RBE) than high-energy photons.
• Understanding the mechanisms behind this increased RBE is crucial for radiation therapy and safety.
• DNA double-strand breaks (DSBs) are critical lesions underlying cell killing by ionizing radiation.

Purpose of the Study:

• To apply the Giant LOop Binary LEsion (GLOBLE) model to explain cell killing by ultrasoft X-rays.
• To investigate the role of DNA double-strand break (DSB) clustering and dose distribution in ultrasoft X-ray-induced RBE.
• To compare RBE mechanisms of ultrasoft X-rays with high-LET ion beams.

Main Methods:

• Application of the Giant LOop Binary LEsion (GLOBLE) model, which analyzes DSB clustering at the megabase pair chromatin level.
• Distinguishing between isolated DSBs (iDSBs) and clustered DSBs (cDSBs) based on dose and DSB yield.
• Analysis of dose attenuation effects within cell layers for low-energy photons.
• Comparison of RBE mechanisms with high-LET ion irradiation.

Main Results:

• The increased yield of DSBs with decreasing photon energy sufficiently explains the elevated RBE of ultrasoft X-rays, without requiring increased lethality per DSB.
• The GLOBLE model demonstrates that DSB clustering is a key factor in ultrasoft X-ray RBE.
• Dose attenuation in cell layers leads to inhomogeneous dose distributions, further increasing biological effectiveness and impacting the dose-response curve's beta-term.

Conclusions:

• The GLOBLE model successfully explains the increased RBE of ultrasoft X-rays primarily through an increased yield of DSBs and their clustering.
• Localized energy deposition and subsequent DSB clustering, rather than increased individual DSB lethality, are key to ultrasoft X-ray effectiveness.
• Inhomogenous dose distributions due to attenuation significantly enhance the biological impact of ultrasoft X-rays, particularly affecting the dose-response curve.