A two-dimensional liquid-filled ionization chamber array prototype for small-field verification: characterization and first clinical tests

Abstract

In this work we present the design, characterization and first clinical tests of an in-house developed two-dimensional liquid-filled ionization chamber prototype for the verification of small radiotherapy fields and treatments containing such small fields as in radiosurgery, which consists of 2 mm × 2 mm pixels arranged on a 16×8 rectangular grid. The ionization medium is isooctane. The characterization of the device included the study of depth, field-size and dose-rate dependences, which are sufficiently moderate for a good operation at therapy radiation levels. However, the detector presents an important anisotropic response, up to ≃ 12% for front versus near-lateral incidence, which can impact the verification of full treatments with different incidences. In such a case, an anisotropy correction factor can be applied. Output factors of small square fields measured with the device show a small systematic over-response, less than 1%, when compared to unshielded diode measurements. An IMRT radiosurgery treatment has been acquired with the liquid-filled ionization chamber device and compared with film dosimetry by using the gamma method, showing good agreement: over 99% passing rates for 1.2% and 1.2 mm for an incidence-per-incidence analysis; 100% passing rates for tolerances 1.8% and 1.8 mm when the whole treatment is analysed and the anisotropy correction factor is applied. The point dose verification for each incidence of the treatment performed with the liquid-filled ionization chamber agrees within 1% with a CC01 ionization chamber. This prototype has shown the utility of this kind of technology for the verification of small fields/treatments. Currently, a larger device covering a 5 cm × 5 cm area is under development.

Tolerance limits and methodologies for IMRT measurement‐based verification QA: Recommendations of AAPM Task Group No. 218

“…Isodose overlay of two measurements, the solid line from radiochromic film and the dashed line from liquid‐filled ionization chambers. From Brulla–Gonzalez . The 20%, 50%, 70%, 80%, 90%, and 95% dose levels are shown.…”

Development and clinical characterization of a novel 2041 liquid‐filled ionization chambers array for high‐resolution verification of radiotherapy treatments

“…The detector also presents an important anisotropy due to its design, especially for lateral incidences. The observed anisotropy is similar to values reported for other LIC arrays . Interestingly, inter‐pixel variations of the anisotropic correction factor for each angle are much lower than the observed anisotropy trend.…”

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

“…Each pixel has an effective area of 2 mm  2 mm and a nominal gap of 0.5 mm. The readout is performed with the X-ray Data Acquisition System (Sens-Tech Ltd, UK), which has a sensitivity around 4200 pC/ADC and tunable integration time ranging from 0.01 ms to 0.5 s. More information on the design and characterization of this device (with isooctane as sensitive medium) can be found elsewhere [10]. The liquid used in this study is tetramethylsilane (Merck, NMR calibration grade) with a nominal purity 4 99:7%.…”

“…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.…”