Degradation of proton depth dose distributions attributable to microstructures in lung‐equivalent material

Purpose: The purpose of the work reported here was to investigate the influence of sub‐millimeter size heterogeneities on the degradation of the distal edges of proton beams and to validate Monte Carlo (MC) methods’ ability to correctly predict such degradation. Methods: A custom‐designed high‐resol...

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Bibliographic Details
Published in:Medical physics (Lancaster) 2015-11, Vol.42 (11), p.6425-6432
Main Authors: Titt, Uwe, Sell, Martin, Unkelbach, Jan, Bangert, Mark, Mirkovic, Dragan, Oelfke, Uwe, Mohan, Radhe
Format: Article
Language:English
Subjects:
60 APPLIED LIFE SCIENCES
Absorption, Radiation - physiology
ANIMAL TISSUES
Biological material, e.g. blood, urine
Haemocytometers
biological tissues
Biomimetic Materials - chemistry
BRAGG CURVE
Computed tomography
Computer Simulation
Computerised tomographs
computerised tomography
COMPUTERIZED SIMULATION
COMPUTERIZED TOMOGRAPHY
convolution
DEPTH DOSE DISTRIBUTIONS
dose degradation
dosimetry
Dosimetry/exposure assessment
Humans
Ion beams
lung
Lung - physiology
lung tissue
LUNGS
measurements
Medical imaging
Medical treatment planning
MICROSTRUCTURE
Models, Biological
Modulators
Monte Carlo
MONTE CARLO METHOD
Monte Carlo methods
PHANTOMS
PROTON BEAMS
proton therapy
Proton Therapy - methods
Protons
RADIATION DOSES
radiation therapy
Radiation Therapy Physics
Radiotherapy Dosage
Scattering, Radiation
Scintigraphy
SPATIAL RESOLUTION
Therapeutic applications, including brachytherapy
Tissues
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Summary:Purpose: The purpose of the work reported here was to investigate the influence of sub‐millimeter size heterogeneities on the degradation of the distal edges of proton beams and to validate Monte Carlo (MC) methods’ ability to correctly predict such degradation. Methods: A custom‐designed high‐resolution plastic phantom approximating highly heterogeneous, lung‐like structures was employed in measurements and in Monte Carlo simulations to evaluate the degradation of proton Bragg curves penetrating heterogeneous media. Results: Significant differences in distal falloff widths and in peak dose values were observed in the measured and the Monte Carlo simulated curves compared to pristine proton Bragg curves. Furthermore, differences between simulations of beams penetrating CT images of the phantom did not agree well with the corresponding experimental differences. The distal falloff widths in CT image‐based geometries were underestimated by up to 0.2 cm in water (corresponding to 0.8–1.4 cm in lung tissue), and the peak dose values of pristine proton beams were overestimated by as much as ˜35% compared to measured curves or depth‐dose curves simulated on the basis of true geometry. The authors demonstrate that these discrepancies were caused by the limited spatial resolution of CT images that served as a basis for dose calculations and lead to underestimation of the impact of the fine structure of tissue heterogeneities. A convolution model was successfully applied to mitigate the underestimation. Conclusions: The results of this study justify further development of models to better represent heterogeneity effects in soft‐tissue geometries, such as lung, and to correct systematic underestimation of the degradation of the distal edge of proton doses.
ISSN:0094-2405
2473-4209
DOI:10.1118/1.4932625