Development of a System for Microdosimetric Assessment of Clinical Proton Treatment Plans via Proton Energy Spectra

Abstract

Purpose: The biological effectiveness of proton radiation depends on variables beyond the absorbed dose. In particular, the dose deposition on a micrometer scale may be relevant for subcellular structures such as DNA. Furthermore, the microdosimetric quantity lineal energy may represent the pattern of dose deposition in particle tracks more realistically compared to the more frequently used quantity linear energy transfer. Today, lineal energy is not available in clinical treatment planning systems. In this project the aim was to develop a system for calculating the lineal energy in proton therapy and use this to map the distribution of lineal energies in the brainstem of a pediatric brain tumor patient.

Methods: Proton treatment plans for a water phantom and a patient case were simulated in the FLUKA Monte Carlo code with scoring of the proton energy spectrum on a voxel-by-voxel basis. Via the proton energy spectrum the dose-mean lineal energy (yd) was calculated with lookup tables, which were generated by interpolating between already existing lineal energy distributions from track structure simulations.

Results: Lookup tables converting between the proton energy and lineal energy was developed for energies 1-100 MeV and allowed for assessment of the lineal energy in volumes (sites) of size 10 nm – 20 μm. Entrance and peak value of yd in the spread-out Bragg peak of water phantom for 10 (~18 and ~25 keV/μm) and 100 nm site size (~6 and ~19 keV/μm) agreed with values reported in similar studies. For the patient case, the yd was found to increase from the target region to the brainstem.

Conclusions: A system for microdosimetric assessment of clinical proton treatment plans was developed enabling estimation of yd in organs at risk. Although the lookup tables must be validated before this tool can be clinically applicable, this work provides a useful framework for further microdosimetric research aiming to find new links between the physics of the proton beam and the observed biological effect.