Monte Carlo simulations for brachytherapy
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Examensarbete för masterexamen
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Sammanfattning
Brachytherapy is an internal radiation treatment where a small, encapsulated radioactive
source is placed at different dwell positions close to or within the tumor.
The standard method for treatment planning in brachytherapy is to follow the
TG43-formalism where the full patient geometry is treated as water. This approximation
has limitations in the modelling of skin–air interfaces and tissue variations
and does not take into consideration shielding effects. To overcome the limitations
of this analytical framework, Monte Carlo (MC) simulations can be used. To reduce
the computational time, which is often a limiting factor in MC computations,
pre-generated information about the radiation can be utilized. In this work, independent
Monte Carlo simulations performed with the software egs_brachy are setup
to characterize the radiation from a high-dose-rate brachytherapy source. The output
is a phase space with information about the radiation properties. The phase
space is analyzed and resulted in a 4D-histogram which can be used as an input
to the brachy Monte Carlo dose engine prototype in the treatment planning system
RayStation®. In addition to constructing the input data, dose distributions for two
different treatment plans are scored with egs_brachy in both water and tissue. The
same treatment plans are used to compute dose with TG43 and the brachy Monte
Carlo prototype and the dose distributions are compared with the ones obtained
with egs_brachy. Local gamma tests were performed with gamma criteria (1%/1
mm) and (3%/2 mm), yielding low fail rates in comparison to both RayStation dose
engines. The favorable gamma results show that both TG43 and the brachy Monte
Carlo dose engine can reproduce the simulated dose distributions from egs_brachy
well. In conclusion, the produced phase space is suitable as input for the brachy
MC dose engine and the simulated dose distributions can be utilized in a validation
framework.
Beskrivning
Ämne/nyckelord
brachytherapy, treatment planning, dose calculation, Monte Carlo simulations, phase space