Direct determination ofkQfor Farmer-type ionization chambers in a clinical scanned carbon-ion beam using water calorimetry.

Within two studies,kQfactors for two Farmer-type ionization chambers have been experimentally determined by means of water calorimetry in the entrance channel (EC) of a monoenergetic carbon-ion beam (Osinga-Blättermannet al2017Phys. Med. Biol.622033-54) and for a passively modulated spread-out Bragg peak (SOBP) (Holmet al2021Phys. Med. Biol.66145012). Both studies were performed at the Heidelberg Ion Beam Therapy Center (HIT) using the PTB portable water calorimeter but applying different initial beam energies of 429 MeV u-1for the EC and 278 MeV u-1for the SOBP as well as different scanning patterns of the irradiated field. Comparing their results revealed differences between the experimentalkQfactors of up to 1.9% between the EC and the SOBP. To further investigate this unexpected difference, we performed additionalkQdeterminations for the EC of an 278 MeV u-1monoenergetic carbon-ion beam and reevaluated the original data of Osinga-Blättermannet al(2017Phys. Med. Biol.622033-54). This new experimental data indicated no difference between thekQfactors for the EC and the SOBP and the reevaluation led to a substantial reduction of the originally publishedkQfactors for the EC of the 429 MeV u-1beam (Osinga-Blättermannet al2017Phys. Med. Biol.622033-54). Finally, no significant difference between the data for the EC and the data for the SOBP can be found within the standard measurement uncertainty of experimentalkQfactors of 0.8%. The results presented here are intended to correct and replace thekQdata published by Osinga-Blättermannet al(2017Phys. Med. Biol.622033-54) and in Osinga-Blättermann and Krauss (2018Phys. Med. Biol.64015009).

Triple channel analysis of Gafchromic EBT3 irradiated with clinical carbon-ion beams.

Self-developing radiochromic film is widely used in radiotherapy QA procedures. To compensate for typical film inhomogeneities, the triple channel analysis method is commonly used for photon-irradiated film. We investigated the applicability of this method for GafchromicTMEBT3 (Ashland) film irradiated with a clinically used carbon-ion beam. Calibration curves were taken from EBT3 film specimens irradiated with monoenergetic carbon-ion beams of different doses. Measurements of the lateral field shape and homogeneity were performed in the middle of a passively modulated spread-out Bragg peak and compared to simultaneous characterization by means of a 2D ionization chamber array. Additional measurements to investigate the applicability of EBT3 for quality assurance (QA) measurement in carbon-ion beams were performed. The triple-channel analysis reduced the relative standard deviation of the doses in a uniform carbon ion field by 30% (from 1.9% to 1.3%) and reduced the maximum deviation by almost a factor of 3 (from 28.6% to 9.8%), demonstrating the elimination of film artifacts. The corrected film signal showed considerably improved image quality and quantitative agreement with the ionization chamber data, thus providing a clear rationale for the usage of the triple channel analysis in carbon-beam QA.

Water calorimetry-basedkQfactors for Farmer-type ionization chambers in the SOBP of a carbon-ion beam.

The dosimetry of carbon-ion beams based on calibrated ionization chambers (ICs) still shows a significantly higher uncertainty compared to high-energy photon beams, a fact influenced mainly by the uncertainty of the correction factor for the beam qualitykQ. Due to a lack of experimental data,kQfactors in carbon-ion beams used today are based on theoretical calculations whose standard uncertainty is three times higher than that of photon beams. To reduce their uncertainty, in this work,kQfactors for two ICs were determined experimentally by means of water calorimetry for the spread-out Bragg peak of a carbon-ion beam, these factors are presented here for the first time. To this end, the absorbed dose to water in the12C-SOBP is measured using the water calorimeter developed at Physikalisch-Technische Bundesanstalt, allowing a direct calibration of the ICs used (PTW 30013 and IBA FC65G) and thereby an experimental determination of the chamber-specifickQfactors. Based on a detailed characterization of the irradiation field, correction factors for several effects that influence calorimetric and ionometric measurements were determined. Their contribution to an overall uncertainty budget of the finalkQfactors was determined, leading to a standard uncertainty forkQof 0.69%, which means a reduction by a factor of three compared to the theoretically calculated values. The experimentally determined values were expressed in accordance with TRS-398 and DIN 6801-1 and compared to the values given there. A maximum deviation of 2.3% was found between the experiment and the literature.

2D range modulator for high-precision water calorimetry in scanned carbon-ion beams.

Ionization chamber-based dosimetry for carbon-ion beams still shows a significantly higher standard uncertainty than high-energy photon dosimetry. This is mainly caused by the high standard uncertainty of the correction factor for beam quality [Formula: see text]. Due to a lack of experimental data, the given values for [Formula: see text] are based on theoretical calculations. To reduce this standard uncertainty, [Formula: see text] factors for different irradiation conditions and ionization chambers (ICs) can be determined experimentally by means of water calorimetry. To perform such measurements in a spread-out Bragg peak (SOBP) for a scanned carbon-ion beam, we describe the process of creating an almost cubic dose distribution of about 6 × 6 × 6 cm3 using a 2D range modulator. The aim is to achieve a field homogeneity with a standard deviation of measured dose values in the middle of the SOBP (over a lateral range and a depth of about 4 cm) below 2% within a scanning time of under 100 s, applying a dose larger than 1 Gy. This paper describes the optimization and characterization of the dose distribution in detail.