Download Clinical validation of a proton ocular Treatment Planning System

Clinical validation of a proton ocular Treatment Planning System
with Geant4 Monte Carlo simulations and measurements
G.A.P. Cirone, G. Cuttone, F. Di Rosa, V. Salamone
Laboratori Nazionali del Sud – Istituto Nazionale di Fisica Nucleare, Catania (I)
Azienda Policlinico Universitario, Univbersità di Catania, Catania (I)
Abstract
Protons, thanks to their physical features, offer the dosimetric advantage of a highly conformal high
dose region with the possibility of covering the tumour volume with elevated accuracy, protecting
the surrounding healthy tissue.
These advantages are due to absence of exit dose and to improved geometric control of both lateral
and distal fall-off in a single treatment field.
Proton therapy is particularly indicated to those tumours placed close to serially organized tissues
where a small local overdose can cause fatal complication, such as tumours close to the optic nerve.
High protons ballistics precision has to match an elevated accuracy in the patient positioning and in
the dose calculation distributions performed by the Treatment Planning System (TPS). So, the
ability to deliver the dose precisely results in the need for accurate treatment planning. A realistic
prediction of the distribution of dose deposited in the irradiated tissue and in a relatively short
calculation times are the major criteria of treatment planning algorithms.
One of the major application of proton therapy is the treatment of ocular melanoma. For this
disease, numerous dose calculation algorithms for treatment planning with protons take as a basis
an analytical description of the proton energy deposition: among these EYEPLAN today the most
widely used ocular proton treatment planning system.
EYEPLAN characteristics can apply to treatment planning programs in general. These include the
three-dimensional definition of the tumour volume and of normal structures, the possibility of
delivering the treatment beam from any direction in space, the provision of arbitrary viewpoints
including a “beam’s eye” point of view, the specification of the necessary beam quality (range and
modulation), the calculation and display of isodose contours on any plane in space and on relevant
surfaces and the stimulation of the desired alignment film with which patient alignment can be
verified.
It is generally accepted that analytical models result in dose distributions that are an approximation
of the true dose distribution and that the true dose distribution can only be obtained from Monte
Carlo simulations. Therefore, to verify the correct prediction of the TPS dose distributions, a
validation process has been performed using the Monte Carlo through the GEANT4 toolkit.
GEANT4 is not a stand-alone executable but an object-oriented toolkit of libraries based on the
programming language C++. All aspects of a simulation process are included in the toolkit and can
be organized in different functions for efficient use with the C++ class structure.
The entire proton beam line at Laboratori Nazionali del Sud INFN in Catania has been simulated,
from the double foils scattering system up to the isocenter. Finally the simple geometric eye
reconstruction in EYEPLAN has been reproduced in GEANT4. Moreover we used special eyephantoms to reproduce experimentally two different clinical configuration planned by EYEPLAN.
The first step of our work was the validation of the Monte Carlo application versus experimental
data: dose distributions on different planes, planned by TPS, are compared to measurements and the
maximum differences between absorbed dose profiles resulted 5% and 0.6 mm.
Secondarily the agreement between the TPS and Monte Carlo data has been verified its principally
differences being influenced by geometric complexity of the eye structure. The isodoses
distributions, calculated on different planes, have been compared versus the Monte Carlo ones with
various methods such as composite analysis and gamma index. The results of the comparisons will
be deeply discussed.
The results also show how Monte Carlo approach is still time consuming for routine treatment
planning but it provide the most accurate method to verify the dose distribution planned by TPS.