Evaluation of chamber response function influence on IMRT verification using 2D commercial detector arrays

Abstract

This work is devoted to studying the influence of chamber response functions on the standard IMRT verification for the different detector technologies available on commercial devices. We have tested three of the most used 2D detector arrays for radiotherapy dosimetry verification, based on air-ionization chambers and diode detectors. The response function has been carefully simulated using the Monte Carlo method and measured through slit and pinhole collimators. Although the response function of air-ionization detectors is considerably different with respect to that of standard diodes, the impact on a verification based in the gamma function with tolerances 3 mm and 3% is quite limited. The results show that the standard air-ionization detector arrays perform in a similar way whenever the tolerances for the gamma function are not lowered below 1.5 mm and 1.5%. Additionally, the sensitivity of these devices to fluence perturbations was measured by intentionally modifying some leaf positions in the multileaf collimator. The wider response function of air-ionization chamber arrays made them slightly more sensitive to random fluence perturbations, although silicon diode arrays are more accurate to describe the dose distribution in a point by point basis.

Dosimetric characteristics of the novel 2D ionization chamber array OCTAVIUS Detector 1500

“…12 The fill factor of the OD1500 array, which characterizes its sensitivity to misaligned MLC leaves, was determined as 0.86 with a single OD1500 measurement and 1.00 with two merged OD1500 measurements. For a single measurement with the OD729 array, a fill factor of 0.5 has been obtained, which meets the value published by Gago-Arias et al, 14 whereas fill factor 1.00 is achieved merging four measurements by which a geometry with adjacent chambers is simulated.…”

Depth dependence of the single chamber response function of the I’mRT MatriXX array in a 6 MV photon beam

“…At the depth beyond the dose maximum, the dose response function of the I'mRT MatriXX can be characterized by a Gaussian function. Similar results were obtained by Garcia-Vicente et al [27], Bednarz et al [12], Ulmer and Kaissl [29], Pappas et al [30], Yan et al [19], Fox et al [28], Gago-Arias et al [31] and Looe et al [32]. They concluded that the Gaussian convolution kernels are the best fit function to describe the detector response function beyond d max .…”

“…The single detector's dose response function of the I'mRT MatriXX array can be defined as the convolution kernel K(x) that transforms the true dose profile D(x) into the measured signal profile M(x) and thereby characterizes the broadening of the slit-beam dose profile [16,17,31,32]. It is affected by the detector size, the replacement of water by air, the wall and the central electrode of the detector as well as the geometrical form of the chamber itself [16,32].…”

“…The limited spatial resolution of ionization chamber arrays has been attributed to the finite size of the single detectors as well as to the transport of secondary electrons from the walls into the detector volume [8,9,31,32]. This disadvantage can be overcome by the method proposed by Poppe et al [10], who convolved the calculated dose profile provided by the TPS with the dose response function of the single ionization chamber and used the convolution product for the gammaindex based comparison with the measured 2D array profile.…”

On flattening filter‐free portal dosimetry

“…The most commonly used dose distribution measurement devices currently available are film 5 , 6 (radiochromic), arrays 7 , 8 (both ion chamber and diodes), and EPIDs. Film has a high resolution, but its use is very time‐consuming; it needs time before scanning, the scanning itself, and a calibration process.…”