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Original Citation:
D-IMRT verification with a 2D pixel ionization chamber: dosimetric and clinical results in head and
neck cancer
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D-IMRT verification with a 2D pixel ionization chamber: dosimetric and clinical results in head
and neck cancer
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2005 Phys. Med. Biol. 50 4681
(http://iopscience.iop.org/0031-9155/50/19/017)
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INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY
Phys. Med. Biol. 50 (2005) 4681–4694 doi:10.1088/0031-9155/50/19/017
D-IMRT verification with a 2D pixel ionization
chamber: dosimetric and clinical results in head
and neck cancer
M Stasi
1
, S Giordanengo
2
,RCirio
2
, A Boriano
2,3
, F Bourhaleb
4
,
I Cornelius
2
, M Donetti
2,5
, E Garelli
2,6
,IGomola
7
, F Marchetto
2
,
M Porzio
2,3,8
, C J Sanz Freire
2,3,
10
, A Sardo
2,3,9
and C Peroni
2,3
1
Institute for Cancer Research and Treatment (IRCC), Candiolo and A.S.O. Ordine Maurizano,
Torino, Italy
2
Istituto Nazionale di Fisica Nucleare (INFN), Via P. Giuria 1, I-10125 Torino, Italy
3
Experimental Physics Department, University of Torino, Via P. Giuria 1, I-10125 Torino, Italy
4
TERA Foundation, Via Puccini 1, I-28100 Novara, Italy
5
CNAO Foundation, Via Caminadella 16, I-20123 Milano, Italy
6
ASP, Viale S. Severo 65, I-10125 Torino, Italy
7
Scanditronix-Wellh
¨
ofer, Bahnhofstrasse 5, D-90592 Schwarzenbruck, Germany
8
A.O. S.S. Antonio, Biagio e C. Arrigo, Via Venezia 16, I-15100 Alessandria, Italy
9
A.O. O.I.R.M. S. Anna, C. so Spezia 60, I-10126 Torino, Italy
E-mail: cirio@to.infn.it
Received 11 February 2005, in final form 5 July 2005
Published 21 September 2005
Online at stacks.iop.org/PMB/50/4681
Abstract
Dynamic intensity-modulated radiotherapy (D-IMRT) using the sliding-
window technique is currently applied for selected treatments of head and
neck cancer at Institute for Cancer Research and Treatment of Candiolo (Turin,
Italy). In the present work, a PiXel-segmented ionization Chamber (PXC)
has been used for the verification of 19 fields used for four different head and
neck cancers. The device consists of a 32 × 32 matrix of 1024 parallel-plate
ionization chambers arranged in a square of 24 × 24 cm
2
area. Each chamber
has 0.4 cm diameter and 0.55 cm height; a distance of 0.75 cm separates the
centre of adjacent chambers. The sensitive volume of each single ionization
chamber is 0.07 cm
3
. Each of the 1024 independent ionization chambers is
read out with a custom microelectronics chip.
The output factors in water obtained with the PXC at a depth of 10 cm
were compared to other detectors and the maximum difference was 1.9% for
field sizes down to 3 × 3cm
2
. Beam profiles for different field dimensions
were measured with the PXC and two other types of ionization chambers; the
maximum distance to agreement (DTA) in the 20–80% penumbra region of a
3 × 3cm
2
field was 0.09 cm. The leaf speed of the multileaf collimator was
varied between 0.07 and 2 cm s
−1
and the detector response was constant to
10
Present address: Hospital Universitario de Salamanca, Paseo de San Vicente 182, Salamanca, Spain.
0031-9155/05/194681+14$30.00 © 2005 IOP Publishing Ltd Printed in the UK 4681
4682 M Stasi et al
better than 0.6%. The behaviour of the PXC was measured while varying the
dose rate between 0.21 and 1.21 Gy min
−1
; the mean difference was 0.50% and
the maximum difference was 0.96%. Using fields obtained with an enhanced
dynamic wedge and a staircase-like (step) IMRT field, the PXC has been tested
for simple 1D modulated beams; comparison with film gave a maximum DTA
of 0.12 cm. The PXC was then used to check four different IMRT plans for
head and neck cancer treatment: cervical chordoma, parotid, ethmoid and skull
base. In the comparison of the PXC versus film and PXC versus treatment
planning system, the number of pixels with γ parameter 1 was 97.7% and
97.6%, respectively.
1. Introduction
In recent years, several techniques in beam delivery have been developed in an attempt to obtain
precisely conformed dose profiles. Beam intensities can be modified with the use of wedges
(static or dynamic) (Khan 1994) and compensator filters (Ellis et al 1959). Non-uniform
beam intensities can be delivered with a static multileaf collimator (S-MLC) using multiple
coplanar or non-coplanar beams to achieve good conformation of dose distributions; moreover,
there has been great interest in implementing intensity-modulated radiation therapy (IMRT)
in external beam therapy (Ezzell 2003, Purdy 2001, Webb 2000). Dose distributions with
improved conformity to the target can be obtained with this technique, leading to a reduced
dose to the surrounding healthy tissue and critical organs. The sliding-window technique with
dynamic multileaf collimators (DMLC) (Convery et al 1992, Webb 1989, 1992, 1994) is based
on independently moving each leaf pair of a MLC during treatment while the beam is on,
to obtain sweeping apertures of variable width across the treatment field. The width of the
aperture varies among leaf pairs and for each leaf pair the width is also a function of time.
Sliding-window D-IMRT is currently applied at Institute for Cancer Research and
Treatment (IRCC) of Candiolo (Turin, Italy), in particular for the treatment of head and
neck cancers.
Due to the complexity of the dynamic IMRT (D-IMRT) techniques, the verification of
dose delivery is crucial (Webb 1997) and successful clinical implementation of IMRT requires
verifying the consistency between calculated and delivered dose distributions for each patient
(Tsai et al 1998). Nowadays, one of the most widely used methods for IMRT verification
is to compare the dose distribution calculated by the treatment planning system (TPS) in a
simple-geometry phantom with the dose distribution measured with films (Ting and Davis
2001,Xinget al 1999). If film data are normalized to ionization chamber measurements, the
dose distribution can be expressed in absolute values. IMRT verification can also be performed
using arrays of silicon diodes (Watts 1998,Zhuet al 1997) and matrices of silicon diodes
(Jursinic and Nelms 2003, Letourneau et al 2004). A check of the beam fluence has also
been carried out using 2D beam imaging systems (Li et al 2001,Maet al 1998). Electronic
portal imaging devices (EPID) are also being used for IMRT verification (Greer and Popescu
2003,Pasmaet al 1999, Van Esch et al 2004, Warkentin et al 2003, Zeidan et al 2004).
An active-matrix flat-panel dosimeter has been recently tested for in-phantom dosimetric
measurements (Moran et al 2005). All of these detectors have some advantages but, on the
other hand, are characterized by some disadvantages. Films have very good spatial resolution
and granularity but have to be carefully calibrated (Burch et al 1997,Olch2002, Sykes et al
1999, Yeo and Wang 1997,Zhuet al 2002) and the measurement is not available in real time.
D-IMRT verification with a 2D pixel ionization chamber 4683
EPIDs have a good spatial resolution and granularity and provide a real-time measurement;
on the other hand, calibration, ageing due to radiation and dead time in electronics read out
are to be taken into account. Matrices of detectors (diodes or ionization chambers) have poor
granularity. Diodes feature a good spatial resolution, but have the calibration and ageing
features of silicon detectors. Ionization chambers have worse spatial resolution than diodes,
but provide a direct measurement of the dose, without need for frequent calibration. Indeed,
none of these detectors is able to provide every kind of measurement. For example, a matrix,
due to the fact that not the whole surface is sensitive, would not allow the correct verification
of a plan with very small spikes in dose distribution and would not be able to quantitatively
measure the displacement of a single leaf unless appropriate corrections were applied to raw
data.
The present work is aimed at verifying the behaviour of a new device for the measurement
of head and neck IMRT treatments used in practical clinic at IRCC. The detector is a
PiXel-segmented ionization Chamber (PXC); it was designed and built by Torino University
and Istituto Nazionale di Fisica Nucleare (INFN). Basic dosimetric properties of the PXC
(i.e. detector stability, dose and dose-rate dependence, tissue maximum ratio (TMR)) were
described in a previous paper (Amerio et al 2004). Recently, Scanditronix-Wellh
¨
ofer
(Schwarzenbruck, Germany) has commercialized a detector, MatriXX, which is based on the
present R&D. PTW (Freiburg, Germany) is also producing a matrix of ionization chambers.
The main differences between the two detectors stand in the number of independent ionization
chambers (1020 versus 729), their arrangement with respect to the MLC and the dimension of
each chamber (4.5 mm diameter × 5 mm height versus 5 × 5 × 5mm
3
).
2. Materials and methods
Measurements were performed on a Varian Clinac 600 C/D delivering a 6 MV x-ray beam,
equipped with Varian Millennium 120-leaf MLC. The 120 leaves are of different sizes: the
40 central leaves are 0.5 cm wide at isocentre and produce a field of up to 20 × 20 cm
2
;
the remaining leaves are 1 cm wide, except for the four external ones that are 1.4 cm wide.
The maximum obtainable field size is 40 × 40 cm
2
.
Absolute dose calibration of the linac was performed according to the AAPM TG-51
code of practice (Almond et al 1999); the measured percent-depth-dose (PDD) was 66.1% for
x rays at 10 cm depth in water. Calibration conditions were: a dose rate of 0.01 Gy MU
−1
at the
depth-of-maximum dose (1.5 cm), a source-to-surface distance (SSD) of 100 cm and a field
size of 10 × 10 cm
2
. A PTW 30010 cylindrical ionization chamber was used for the calibration.
This chamber has a 0.6 cm
3
volume and was calibrated at the German National Laboratory,
PTB (Braunschweig, Germany). It was used connected to a PTW Unidos electrometer with a
+400 V bias voltage. Beam profiles were measured at 10 cm water-equivalent depth using a
water-phantom Wellh
¨
ofer Blue-Phantom with two detectors: a cylindrical ionization chamber
Wellh
¨
ofer IC15 (0.125 cm
3
volume, 0.55 cm internal diameter) and an Exradin (Standard
Imaging, Middleton, USA) A16 Micropoint chamber (0.007 cm
3
volume, 0.24 cm internal
diameter). IC15 is the detector that is used for the measurement of the data input to the TPS
(PDD, relative profiles at five different water-equivalent depths, diagonal profile, output factor
(OF)), CadPlan-Helios version 6.3.5 (Varian, Zug, Switzerland).
The two-dimensional dose-measuring detector used in this work is a pixel-segmented
ionization chamber. Similar detectors were previously tested on proton (Brusasco et al 1997),
electron (Belletti et al 1999), carbon ion (Bonin et al 2004) and x-ray (Amerio et al 2004)
beams, where they were used as a dosimeter or beam monitor. In this work, results are shown
for dosimetry of x-ray IMRT used in four head and neck pathologies. While details of the
