Direct determination of k Q for Farmer-type ionization chambers in a clinical scanned carbon ion beam using water calorimetry

• Published in: Physics in medicine & biology Vol. 62; no. 6; p. 2033
• Main Authors: Osinga-Blättermann, J-M, Brons, S, Greilich, S, Jäkel, O, Krauss, A
• Format: Journal Article
• Language: English
• Published: England 21-03-2017
• Subjects: Calibration; Calorimetry - instrumentation; Calorimetry - methods; Heavy Ion Radiotherapy; Humans; Phantoms, Imaging; Photons; Radiation Dosage; Radiometry - instrumentation; Radiometry - methods; Radiotherapy Dosage; Water - chemistry
• ISSN: 1361-6560
• DOI: 10.1088/1361-6560/aa5bac

Until now, the dosimetry of carbon ions with ionization chambers has not reached the same level of accuracy as that of high-energy photons. This is mainly caused by the approximately threefold larger uncertainty of the k
factor of ionization chambers, which, due to the lack of experimental data, is still derived by calculations. Measurements of absorbed dose to water, D
, by means of water calorimetry have now been performed in the entrance channel of a scanned 6 cm  ×  6 cm radiation field of 429 MeV/u carbon ions, allowing the direct calibration of ionization chambers and thus the experimental determination of k
. Within this work, values for k
have been determined for the Farmer-type ionization chambers FC65-G and TM30013. A detailed investigation of the radiation field enabled the accurate determination of correction factors needed for both calorimetric and ionometric measurements. Finally, a relative standard measurement uncertainty of 0.8% (k  =  1) could be achieved for the experimental k
values. For both chambers, the experimental k
factors were found to be about 1% larger than those tabulated in the German DIN 6801-1 protocol, whereas compared to the theoretical values stated in the TRS-398 protocol, the experimental k
value agrees within 0.4% for the TM30013 chamber but is about 1% lower in the case of the FC65-G chamber.