Is the dose-averaged LET a reliable predictor for the relative biological effectiveness?

Area of Science:

• Radiation oncology
• Medical physics
• Radiobiology

Background:

• Dose-averaged linear energy transfer (LET_D) is a common metric for assessing radiation field biological effectiveness.
• Relative biological effectiveness (RBE) is often correlated with LET_D in mixed radiation fields.
• Accurate RBE prediction is crucial for effective radiation therapy planning.

Purpose of the Study:

• To evaluate the suitability of LET_D as a predictor for RBE in mixed proton and heavy ion radiation fields.
• To identify potential limitations of using LET_D for RBE assessment in complex radiation scenarios.

Main Methods:

• Utilized TRiP98 treatment planning software and the Local Effect Model to predict RBE.
• Investigated RBE predictions for protons, helium, and carbon ions in monoenergetic beams, pristine Bragg peaks, and spread-out Bragg peaks (SOBP).
• Compared RBE values from mixed fields (pristine and SOBPs) with those from monoenergetic beams under track segment conditions.

Main Results:

• Pristine Bragg peaks showed minimal RBE deviations (0-5%) compared to monoenergetic beams, except in the distal fall-off.
• Spread-out Bragg peaks (SOBPs) exhibited significant RBE deviations, up to 25% for lighter ions and more for carbon ions.
• Deviations in SOBPs are attributed to broader energy spectra and the non-linear RBE-LET relationship.

Conclusions:

• LET_D is a reliable RBE predictor only for narrow LET distributions, not for broad ones like in SOBPs or overlapping fields.
• The non-linear nature of the RBE(LET) relationship fundamentally limits LET_D's predictive power, especially for heavy ions.
• Accurate RBE assessment in mixed fields, particularly with carbon ions, necessitates understanding the detailed LET distribution.