Ion recombination for ionization chamber dosimetry in a helical tomotherapy unit
- 24 May 2010
- journal article
- Published by Wiley in Medical Physics
- Vol. 37 (6Part1), 2876-2889
- https://doi.org/10.1118/1.3427411
Abstract
Purpose: Ion recombination for ionization chambers in pulsed high-energy photon beams is a well-studied phenomenon. Despite this, the correction for ion recombination is often determined inaccurately due to the inappropriate combination of using a high polarizing voltage and the simple two-voltage method. An additional complication arises in new treatment modalities such as IMRT and tomotherapy, where the dosimetry of a superposition of many constituting fields becomes more relevant than of single static fields. For these treatment modalities, the irradiation of the ion chamber geometry can be instantaneously inhomogeneous and time dependent. Methods: This article presents a study of ion recombination in ionization chambers used for dosimetry in a helical tomotherapy beam. Models are presented for studying the recombination correction factors in a continuous beam, in pulsed large and small fields, and in helical fields. Measurements using Exradin A1SL, NE2571, and NE2611 type chambers and Monte Carlo simulations using PENELOPE are performed in support of these models. Results: Initial recombination and charge multiplication are found to be the same in Co-60 and in the pulsed high-energy photon beam for the chambers and operating voltages used in this study. Applying the two-voltage technique for the A1SL chamber at its recommended operating voltage of 300 V leads to an overestimation of the recombination. Operating at a voltage of 100 V yields larger but more accurate values for the recombination correction. The recombination correction measured for this chamber in the TomoTherapy HiArt unit is lower than the 1% applied in the routine dosimetry for this treatment unit. For a helical dose delivery with a small slice width, lateral electron scatter in the cavity makes that the recombination is smaller than for an open beam delivering the same total dose. In a Farmer type chamber, a helical delivery with a 1 cm slice field results in a time and spatially integrated volume recombination of 55% of that with a 2.5 cm slice field. The relative recombination corrections for different slice widths and different field offsets with respect to the chamber center obtained from the developed models are in good agreement with experimental data. Conclusions: Because of the presence of charge multiplication, it is more accurate to determine the recombination correction at lower operating voltages than are often applied using the two-voltage method. Models and experiments for partial irradiation conditions of the ion chamber show that resulting recombination corrections are reduced compared to those for an open field. A model for helical dose deliveries results in recombination corrections that get lower with smaller slice widths. This model could be adapted to any IMRT delivery where the ion chamber is instantaneously partial and/or inhomogeneously irradiated, and could provide a practical procedure to calculate the recombination for complex deliveries for which it is difficult to be measured. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3427411Keywords
Funding Information
- U.K. National Measurement System's Acoustics and Ionising Radiation Metrology Program
- European Community's Seventh Framework Programme
- ERA-NET Plus (217257)
This publication has 21 references indexed in Scilit:
- Monte Carlo simulation of helical tomotherapy with PENELOPEPhysics in Medicine & Biology, 2008
- SU-FF-T-195: Dosimetry Audit for Tomotherapy Using Alanine/EPRMedical Physics, 2006
- Absorbed dose to water based dosimetry versus air kerma based dosimetry for high-energy photon beams: an experimental studyPhysics in Medicine & Biology, 2002
- Measurement of saturation correction factors of thimble-type ionization chambers in pulsed photon beamsPhysics in Medicine & Biology, 1993
- Ion recombination corrections for plane-parallel and thimble chambers in electron and photon radiationPhysics in Medicine & Biology, 1993
- Saturation correction for the NE 2560/1 dosemeter in photon dosimetryPhysics in Medicine & Biology, 1990
- The recombination correction for an ionisation chamber exposed to pulsed radiation in a 'swept beam' technique. I. TheoryPhysics in Medicine & Biology, 1982
- The Saturation Curve for Ionization Measurements in Pulsed Radiation BeamsThe British Journal of Radiology, 1952
- On the Theory of RecombinationPhysical Review B, 1940
- Mesure de la valence des ions dans les gazLe Radium, 1913