Development and operation of a pixel segmented liquid-filled linear array for radiotherapy quality assurance

Abstract

A liquid isooctane (C(8)H(18)) filled ionization linear array for radiotherapy quality assurance has been designed, built and tested. The detector consists of 128 pixels, each of them with an area of 1.7 mm x 1.7 mm and a gap of 0.5 mm. The small pixel size makes the detector ideal for high gradient beam profiles such as those present in intensity modulated radiation therapy (IMRT) and radiosurgery. As the read-out electronics we use the X-ray Data Acquisition System with the Xchip developed by the CCLRC. Studies concerning the collection efficiency dependence on the polarization voltage and on the dose rate have been made in order to optimize the device operation. In the first tests, we have studied dose rate and energy dependences. Dose rate dependence was found to be lower than 2.1% up to 5 Gy min(-1), and energy dependence lower than 2.5% up to 20 cm depth in solid water. Output factors and penumbras for several rectangular fields have been measured with the linear array and were compared with the results obtained with a 0.125 cm(3) air ionization chamber and radiographic film, respectively. Finally, we have acquired profiles for an IMRT field and for a virtual wedge. These profiles have also been compared with radiographic film measurements. All the comparisons show a good correspondence. The device has proved its capability to verify on-line therapy beams with good spatial resolution and signal-to-noise ratio.

Recombination in liquid‐filled ionization chambers beyond the Boag limit

“…We have used numerical simulations to complement the experimental study of collection efficiencies in LICs beyond the Boag limit for pulsed beams, and their dependence of doseper-pulse and voltage. The model we use has been presented elsewhere 4,25 and will not be discussed in detail. It discretizes space-time (1 µm -1 µs in this work) and simulates charge drift, recombination, and collection by using the transport equations of charge carriers in the medium.…”

“…3 This makes LIC devices especially suitable for the verification of clinical treatments involving steep dose gradients and small fields. Research in this field has led to the development of several prototypes, [4][5][6][7] and commercial devices used for dosimetry/quality assurance [8][9][10] and portal imaging. 11 However, the high mass density and low mobilities of charge carriers also cause a large recombination in LICs.…”

Liquid-filled ionization chamber temperature dependence

“…The liquid ionization chamber studied in this paper is a linear array of 128 pixels designed and developed by the Radiophysics Research Group of the University of Santiago de Compostela [6,7]. The LIC was designed to obtain a dose profile in a single beam shot.…”

Characterization of tetramethylsilane for liquid-filled ionization dosimeters: Ion mobilities, free-ion yield and general recombination

“…Research in LICs for radiotherapy has grown in the last two decades due to the good characteristics that they present for dosimetry, namely a near water-equivalent response, and the attainable high spatial resolution due to the high density of the ionization medium (when compared to air), which is especially important in the verification of small and/or high gradient fields such as those present in many advanced radiotherapy techniques. Several prototypes and commercial devices, as well as dosimetry methods, have been developed and characterized [1][2][3][4][5][6][7][8][9][10][11]. Isooctane (2,2,4-trimethylpentane, C 8 H 18 ) is the liquid most commonly used in LICs due to its good physico-chemical properties.…”