D-IMRT VERIFICATION WITH A 2D PIXEL
IONISATION CHAMBER: DOSIMETRIC AND CLINICAL
RESULTS IN HEAD AND NECK CANCER | 1
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M. Stasi , S. Giordanengo , R. Cirio , A. Boriano , F. Bourhaleb , I. Cornelius , M. Donetti , E. Garelli , I. Gomola ,
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F. Marchetto , M. Porzio , C. J. Sanz Freire , A. Sardo , C. Peroni
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Institute for Cancer Research and Treatment (IRCC), Candiolo and A.S.O. Ordine Maurizano, Torino, Italy
Istituto Nazionale di Fisica Nucleare (INFN), Via P. Giuria 1, I-10125 Torino, Italy
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Experimental Physics Department, University of Torino, Via P. Giuria 1, I-10125 Torino, Italy
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TERA Foundation, Via Puccini 1, I-28100 Novara, Italy
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CNAO Foundation, Via Caminadella 16, I-20123 Milano, Italy
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ASP, Viale S.Severo 65, I-10125 Torino, Italy
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IBA Dosimetry, Bahnhofstrasse 5, D-90592 Schwarzenbruck, Germany
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A.O. S.S. Antonio, Biagio e C. Arrigo, Via Venezia 16, I-15100 Alessandria, Italy
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A.O. O.I.R.M. S. Anna, C.so Spezia 60, I-10126 Torino, Italy
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Partially supported by IBA Dosimetry
Present address: Hospital Universitario de Salamanca, Paseo de San Vicente 182, Salamanca, Spain
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Corresponding author: [email protected] , fax: +39.011.6699579
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(Received 11 February 2005; in final form 5 July 2005; published 21 September 2005)
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
(IRCC) of Candiolo (Turin, Italy). In the present work a PiXel-segmented ionisation Chamber (PXC) has
been used for the verification of 19 fields used for 4 different head and neck cancers. The device
consists of a 32x32 matrix of 1024 parallel plate ionisation chambers arranged in a square 24x24 cm2 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 ionisation chamber is 0.07 cm3.
Each of the 1024 independent ionisation chambers is read out with a custom microelectronics chip.
The Output Factors (OF) 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 3x3 cm2. Beam profiles for
different field dimensions were measured with the PXC and two other types of ionisation chambers;
the maximum Distance-To-Agreement (DTA) in the 20 - 80 % penumbra region of a 3x3 cm2 field was
0.9 mm. The leaf speed of the MultiLeaf Collimator (MLC) was varied between 0.07 and 2 cm/s and the
detector response was constant to better than 0.6 %. The behaviour of the PXC was measured whilst
varying the dose rate between 0.21 and 1.21 Gy/min; the mean difference was 0.50 % and the
maximum difference was 0.96 %. Using fields obtained with an Enhanced Dynamic Wedge (EDW) and
a staircase-like (step) IMRT field, the PXC has been tested for simple 1-D modulated beams; comparison
with film gave a maximum DTA of 1.2 mm. 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 (TPS) the number
of pixels with  parameter ≤ 1 was 97.7 % and 97.6 % respectively.
Keywords: 2D ionisation chamber matrix, dosimetry, Intensity Modulated Radiation Therapy, IMRT
©2005 Physics in Medicine and Biology
DOSIMETRIC CHARACTERIZATION OF A LARGE
AREA PIXEL-SEGMENTED IONIZATION CHAMBER |
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S. Amerio , A. Boriano , F. Bourhaleb , R. Cirio , M. Donetti , A. Fidanzio , E. Garelli , S. Giordanengo , E. Madon ,
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F. Marchetto , U. Nastasi , C. Peroni , A. Piermattei , C. J. Sanz Freire , A. Sardo and E. Trevisiol
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A.O.S. Giovanni Battista, Presidio S. Giovanni A.S., Via Cavour 31, I-10100 Torino, Italy
Dipartimento di Fisica Sperimentale and INFN, Via P. Giuria 1, I-10125 Torino, Italy
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Fondazione TERA, Via Puccini 1, I-28100 Novara, Italy
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Istituto di Fisica, Università Cattolica S.C. di Roma, Largo F. Vito 1, I-00168 Roma, Italy
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ASP, Viale S. Severo 65, I-10125 Torino, Italy
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A.O. O.I.R.M. S. Anna, C.so Spezia 60, I-10126 Torino, Italy
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(Received 12 August 2003; revised 9 October 2003; accepted 19 November 2003;
published 28 January 2004)
ABSTRACT
A pixel-segmented ionization chamber has been designed and built by Torino University and INFN. The
detector features a 24x24 cm2 active area divided in 1024 independent cylindrical ionization chambers and
can be read out in 500 µs without introducing dead time; the digital charge quantum can be
adjusted between 100 fC and 800 fC. The sensitive volume of each single ionization chamber is
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0.07 cm . The purpose of the detector is to ease the two-dimensional (2D) verifications of fields with
complex shapes and large gradients. The detector was characterized in a PMMA phantom using 60Co and
6 MV x-ray photon beams. It has shown good signal linearity with respect to dose and dose rate to water.
The average sensitivity of a single ionization chamber was 2.1 nC/Gy, constant within 0.5% over one
month of daily measurements. Charge collection efficiency was 0.985 at the operating polarization voltage
of 400 V and 3.5 Gy/min dose rate. Tissue maximum ratio and output factor have been
compared with a Farmer ionization chamber and were found in good agreement. The dose profiles have
been compared with the ones obtained with an ionization chamber in water phantom for the field sizes
supplied by a 3D-Line dynamic multileaf collimator. These results show that this detector can be used for
2D dosimetry of X-ray photon beams, supplying a good spatial resolution and sensibly reducing the time
spent in dosimetric verification of complex radiation fields.
Keywords: biomedical equipment; ionisation chambers; dosimetry; phantoms; X-ray detection
©2004 American Association of Physicists in Medicine
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