Technical note: Experimental determination of the effective point of measurement of the PTW‐31010 ionization chamber in proton and carbon ion beams

Abstract

Purpose:The accurate knowledge of the effective point of measurement (P eff ) is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off of particle beams. For plane-parallel ionization chambers (ICs), P eff is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, P eff is shifted from the chamber's center toward the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75⋅r cyl for light ions with r cyl being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined P eff for the 0.125 cc Semiflex IC in both proton and carbon ion beams. Methods: The first method consisted of simultaneous irradiation of a planeparallel IC and the Semiflex in a 4-cm wide spread-out Bragg peak.In the second method, a single-energy beam was used, and both ICs were positioned successively at the same measurement depths. For both approaches, the shift of the distal edge of the depth ionization distributions recorded by the two chambers at different reference points was used to calculate P eff of the Semiflex. Both methods were applied in carbon ion beams, and only the latter was applied in proton beams. Results: Both methods yielded a similar P eff for carbon ions, 0.88⋅r cyl , and 0.84⋅r cyl , which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75⋅r cyl is 0.4 and 0.3 mm, respectively, which is larger than the positioning uncertainty. In the proton beam, a P eff of 0.92⋅r cyl was obtained. Conclusions: The P eff for the 0.125 cc Semiflex IC is shifted further from the cavity center as recommended by IAEA TRS-398 for light ions, with the shift for proton beams being even larger than for carbon ion beams.

Experimental comparison of cylindrical and plane parallel ionization chambers for reference dosimetry in continuous and pulsed scanned proton beams

“…Palmans et al (2020) made a comprehensive analysis of experimental and Monte Carlo (MC) determined values of the EPOM for proton beams and confirmed the value of 0.75 x R for Farmer-type ICs. Barna et al (2022), however, found for a smaller cylindrical IC (Semiflex) in protons the EPOM to be greater than the value of 0.75 x R.…”

Diamond based integrated detection system for dosimetric and microdosimetric characterization of radiotherapy ion beams

“…All the measurements were performed in water, irradiating perpendicularly to the detector surface. The overall uncertainty on the detector position was assessed to be 0.3 mm 40 …”

“…The overall uncertainty on the detector position was assessed to be 0.3 mm. 40 The Dos-μDos device was connected to the commercial electronic readout as depicted schematically in Figure 3b. On one side, the total charge generated in the SV of the dosimeter was acquired by a PTW UNIDOS electrometer using a triaxial cable once the irradiation field was completely delivered and, on the other side, the energy-deposition distribution in the SV of the microdosimeter was collected event by event by using a standard charge sensitive electronic chain.…”

Cellular and Molecular Biological Alterations after Photon, Proton, and Carbon Ions Irradiation in Human Chondrosarcoma Cells Linked with High-Quality Physics Data

“…Repeated measurements were carried out along the entire depth dose curve, resulting in a reproducibility of 0.55% expressed as one standard deviation. The general positioning uncertainty was assessed by taking into account the reproducibility, resolution of the water phantom scale, and positioning of both ionization chambers [ 10 ].…”

“…In order to create the most physiological environment possible to enable reproducible in vitro studies, a novel water phantom-based setup was developed by our working group. The dosimetric characterization for both particle types has been carried out [ 10 ]. This setup allows comprehensive physics characterization of particle beams for pre-clinical research, in accordance with the report of a National Cancer Institute special panel [ 11 ] suggesting the linkage between high-quality biology data and high-quality physics data that go beyond the macroscopic parameter absorbed dose D in units of Gray.…”