Transcript Highligh in Physics 2005

Congresso del Dipartimento di Fisica
Highlights in Physics 2005
11–14 October 2005, Dipartimento di Fisica, Università di Milano
A method for 3D imaging of absorbed dose
in conformal radiotherapy
M. Carrara*, G. Gambarini *,†, S. Gay*,†, L. Pirola* and M. Valente*,†
* Dipartimento
di Fisica, Università di Milano
† INFN – Sezione di Milano
Proposed method
Introduction
In a tissue-equivalent gel matrix, a ferrous sulphate
solution and the metal-ion indicator Xylenol Orange (XO)
are infused. Ionizing radiation causes a conversion of
ferrous ions Fe2+ into ferric ions Fe3+.
RADIATION
Fe2+
Fe3+
The complex of XO with Fe3+ produces visible light
absorption around 585nm, with yield proportional to
the absorbed dose. Then, this system (gel-dosimeter)
acts as a continuum dosimeter.
Some gel-dosimeters of
different shapes and exposures
Planned dose distributions for
a patient suffering prostate
cancer.
These
methodological
improvements
require
corresponding
improvements in the dosimetry methods, in order to ensure that
the values calculated with computer treatment planning systems
(TPS), adopted in the clincal praxis, agree with the delivered dose
distributions.
Considering that traditional dosimeters (i.e. ion chambers, films,
diodes or thermoluminescent detectors) are not well suited to this
task, an alternative technique is proposed.
Variation of optical density between
irradiated samples and reference
sample
Gel dosimeters are in form of layers of convenient shape
and thickness (1-3 mm).
For the analysis, they are placed on a plane light source
near a grey-level calibration standard.
Grey-level (GL) images of light transmittance at 585nm are
detected by a CCD camera, before and after the irradiation of
the dosimeters. With the properly developed software, the
stored images are propcessed with a semi-automatical
procedure (recognition and registration through reference
points; filtering and noise removal; artefacts removal;
conversion to dose matrices…), until 3D dose distributions are
obtained.
The instrumentation for the optical imaging of gel dosimeters
is transportable, and can be set up close to the radiation
source, in order to perform image detection short time before
and after gel dosimeter exposure.
Goal
Experimental 3D rendering
of
in-phantom
absorbed
dose, with high reliability
and good spatial resolution.
The developed software
for dose distributions’ rendering
A suitable software has been developed, able to
properly process the acquired dosimeters’
images to get the interactive rendering of dose
profiles, surfaces and volumes, as well as
isodose curves.
Examples of graphical user interfaces:
Test of the proposed method
The method’s reliability has been verified comparing the obtained dose distributions with those
measured by means of ionization chambers or calculated with Monte Carlo (MC) simulations
(Penelope), adopting field geometries in which the last are reliable.
1. Depth dose profiles
30
Dose reliability has been tested by intercomparing the obtained depth dose
profiles with those measured with a
cylindrical ionization chamber (Farmer,
0.6cc), in the same field configuration.
The good agreement between results is
evident.
2. Dose distributions’ imaging
Dose (Gy)
In the last decade, technological
improvements of radiotherapy (RT)
hardware and software have been
significant and consequently the use
and importance of RT in cancer
treatment have increased greatly.
A fundamental advance has been the
development
of
external
beam
techniques aimed at dose delivery that
is highly localized on the tumour
volume, sparing at the same time most
of the surrounding healthy tissues.
These techniques include conformal RT
as well as intensity modulated RT
(IMRT).
60Co
Siemens Mevatron MX2
Unit
20
10
Ionisation Chamber
Gel
Eγ=6MV
0
0
2
4
6
8
10
Depth in Phantom (cm)
Two different depth dose profiles measured with a single
gel-layer and with a ionization chamber
In two different experimental configurations (both with Varian Clinac 2100C, Eγ=18MV), dose profiles,
dose surfaces, isodose curves and 3D isodose distributions have been obtained. For inter-comparisons,
experimental results are reported together with treatment planning system (TPS) (Prowess 3D)
calculations and with MC simulations (Penelope).
Dose profiles along
Configuration 1 (C1):
(b)
beam direction (a)
and orthogonal to it
(b) [C1]
gel layers
12 3 4 5 6
(a)
3D distribution of
relative isodoses for
C1 (95% blue, 80%
yellow, 40% red)
3x2 field
phantom
Configuration 2 (C2):
(a)
(b)
Dose profiles along
beam direction (a)
and orthogonal to
it (b) [C2]
gel layers
12 3 4 5 6
270°
90°
3x2 field
phantom
Relative isodose curves obtained with a single gel-layer
(a), TPS (b) and MC (c) (95% red, 90% blue, 85% green,
80% yellow, 60% light blue, 40% orange) [C2]
Phantom for test measurements
CCD
Controller
CCD
This phantom offers the opportunity to select the
number of gel dosimeter layers to be inserted.
Both phantom’s component (poystirene) and geldosimeter matrix have good tissue-equivalence to
high energetic X-rays.
Filter
585nm
gel
Assembling the phantom
before its irradiation.
Illuminator
The phantom, ready
to be
irradiated at a 60Co unit.
(a)
(a)
(b)
Dose surface obtained with a single gellayer (a) and with MC simulation (b) [C2]
Conclusions
The proposed method
allows
reliable
3D
imaging of absorbed
dose with good spatial
resolution.
(b)
(c)
3D distribution of relative isodoses for
C2 (95% blue, 80% yellow, 40% red)