Dosimetric characterization of a large area pixel-segmented ionization chamber.

A pixel-segmented ionization chamber has been designed and built by Torino University and INFN. The detector features a 24 x 24 cm2 active area divided in 1024 independent cylindrical ionization chambers and can be read out in 500 micros 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 0.07 cm3. 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.

[1]  J. Boag.  The recombination correction for an ionisation chamber exposed to pulsed radiation in a 'swept beam' technique. I. Theory. , 1982, Physics in medicine and biology.

[2]  Bernard Gottschalk,et al.  Charge-balancing current integrator with large dynamic range , 1983 .

[3]  A. Swallow.  Book Review: The dosimetry of pulsed radiation. ICRU Report 34, International Commission on Radiation Units and Measurements, Bethesda, Maryland, 1982, pp. vii + 47. , 1984 .

[4]  P. Munro,et al.  A flatness and calibration monitor for accelerator photon and electron beams. , 1986, International journal of radiation oncology, biology, physics.

[5]  A consistency monitor for radiation therapy treatments. , 1996, Medical physics.

[6]  A. Cattai,et al.  Strip ionization chambers as 3-D detector for hadron therapy , 1997 .

[7]  T. Zhu,et al.  Performance evaluation of a diode array for enhanced dynamic wedge dosimetry. , 1997, Medical physics.

[8]  E. McCullough,et al.  Measuring dose distributions for enhanced dynamic wedges using a multichamber detector array. , 1997, Medical physics.

[9]  G. Mazza,et al.  Performances of a VLSI wide dynamic range current-to-frequency converter for strip ionization chambers , 1998 .

[10]  Ben J.M. Heijmen,et al.  Dosimetric verification of intensity modulated beams produced with dynamic multileaf collimation using an electronic portal imaging device. , 1999 .

[11]  C. De Wagter,et al.  The value of the LA48 linear ion chamber array for characterization of intensity-modulated beams. , 2001, Physics in medicine and biology.

[12]  Radiation damage studies of a recycling integrator VLSI chip for dosimetry and control of therapeutical beams , 2002 .

[13]  P. Jursinic,et al.  A 2-D diode array and analysis software for verification of intensity modulated radiation therapy delivery. , 2003, Medical physics.