Highlights
- •Monte Carlo simulations performed on wedged ocular proton fields.
- •Compared with radiochromic film measurements for a range of geometries.
- •Good agreement between simulation and experimental measurement observed.
- •Significant in-field scattering and penumbral broadening can occur.
- •Monte Carlo provides an accurate tool for determining the dose effects.
Abstract
A physical wedge may be partially introduced into a proton beam when treating ocular
tumours in order to improve dose conformity to the distal border of the tumour and
spare the optic nerve. Two unwanted effects of this are observed: a predictable broadening
of the beam penumbra on the wedged side of the field and, less predictably, an increase
in dose within the field along a relatively narrow volume beneath the edge (toe) of
the wedge, as a result of small-angle proton scatter. Monte Carlo simulations using
MCNPX and direct measurements with radiochromic (GAFCHROMIC® EBT2) film were performed to quantify these effects for aluminium wedges in a 60 MeV
proton beam as a function of wedge angle and position of the wedge relative to the
patient. For extreme wedge angles (60° in eye tissue) and large wedge-to-patient distances
(70 mm in this context), the 90–10% beam penumbra increased from 1.9 mm to 9.1 mm.
In-field dose increases from small-angle proton scatter were found to contribute up
to 21% additional dose, persisting along almost the full depth of the spread-out-Bragg
peak. Profile broadening and in-field dose enhancement are both minimised by placing
the wedge as close as possible to the patient. Use of lower atomic number wedge materials
such as PMMA reduce the magnitude of both effects as a result of a reduced mean scattering
angle per unit energy loss; however, their larger physical size and greater variation
in density are undesirable.
Keywords
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Article info
Publication history
Published online: March 14, 2016
Accepted:
January 2,
2016
Received in revised form:
December 4,
2015
Received:
August 10,
2015
Identification
Copyright
© 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
