Characterization of a MLIC Detector for QA in Scanned Proton and Carbon Ion Beams

Abstract

Purpose: Beam energy validation is a fundamental aspect of the routine quality assurance (QA) protocol of a particle therapy facility. A multilayer ionization chamber (MLIC) detector provides the optimal tradeoff between achieving accuracy in particle range determination and saving operational time in measurements and analysis procedures. We propose the characterization of a commercial MLIC as a suitable QA tool for a clinical environment with proton and carbon-ion scanning beams. Materials and Methods: Commercial MLIC Giraffe (IBA Dosimetry, Schwarzenbruck, Germany) was primarily evaluated in terms of short-term and long-term stability, linearity with dose, and dose-rate independence. Accuracy was tested by analyzing range of integrated depth-dose curves for a set of representative energies against reference acquisitions in water for proton and carbon ion beams; in addition, 2 modulated proton spread-out Bragg peaks were also measured. Possible methods to increase the native spatial resolution of the detector were also investigated. Results: Measurements showed a high repeatability: mean relative standard deviation was within 0.5% for all channels and both particle types. The long-term stability of the gain calibration showed discrepancies less than 1% at different times. The detector response was linear with dose (R2 > 0.99) and independent on the dose rate. Measurements of integrated depth-dose curve ranges revealed a mean deviation from reference measurements in water of 0.1 ± 0.3 mm for protons with a maximum difference of 0.4 mm and 0.2 ± 0.6 mm with maximum difference of 0.85 mm for carbon ion beams. For the 2 modulated proton spread-out Bragg peaks, measured differences in distal dose falloff were ≤0.5 mm against calculated values. Conclusions: The detector is stable, linearly responding with dose, precise, and easy to handle for QA beam energy checks of proton and carbon ion beams.

Impact of magnetic field regulation in conjunction with the volumetric repainting technique on the spot positions and beam range in pencil beam scanning proton therapy

“…It consists of 180 independent air‐vented, plane‐parallel ionization chambers with a radius of 6.0 cm. More details on Giraffe can be found in the publication by Vai et al 18 For range measurements in FR mode, a spot map was generated representing “down” direction followed by “up” direction. Specifically, “down” direction consisted of 32 layers for energies ranging from 225 to 70 MeV at decrements of 5 MeV, whereas the “up” direction consisted of the same number of layers, but the energies ranged from 70 to 225 MeV at increments of 5 MeV.…”

Evaluation of treatment planning system accuracy in estimating the stopping‐power ratio of immobilization devices for proton therapy

“…The ground-truth WET of immobilization devices was measured using the Giraffe MLIC, 12 comprising 180 air-vented plane-parallel ionization chambers, with an inter-chamber spacing of 2 mm. The MLIC was laid horizontally or vertically on the treatment couch, and a single 200-MeV pencil beam was directed from a 270 • or 0 • gantry angle perpendicular to the center of the front entrance of the chamber sensitive volume.…”

Determination of Integral Depth Dose in Proton Pencil Beam Using Plane-parallel Ionization Chambers

“…The effective points of measurements range from about 2 mm to 330 mm in a depth axis. A uniformity calibration should be performed before the operation of the multi-layer ionization chamber to correct the relative dose of each channel to match the reference measurement in water [4,11]. The IBA Giraffe was calibrated in this study by using 220 MeV for PTW Bragg peak chamber type 34070, measured in a water phantom.…”