Ion recombination for ionization chamber dosimetry in a helical tomotherapy unit

Abstract

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.

Ion recombination and polarity corrections for small‐volume ionization chambers in high‐dose‐rate, flattening‐filter‐free pulsed photon beams

“…yields C init values of 3.3 × 10 −8 and 2.6 × 10 −8 for positive and negative charge collection, respectively, indicating that exponential excess charge accounts for 0.3% and 0.2% of the chamber signal at applied voltages of −300 V and +300 V, respectively. The value at +300 V agrees within uncertainty with the value reported by Palmans et al at the same applied voltage …”

“…The exponential term, e − γV , was proposed by Zankowski and Podgorsak to account for exponential phenomena such as charge multiplication . These phenomena have been shown to play a major role in ionization chamber measurements at clinically applied voltages, especially in small‐volume chambers where strong electric fields are present . All fits described in this work were obtained using the non‐linear curve fitting function in Grace plotting software.…”

A general algorithm for calculation of recombination losses in ionization chambers exposed to ion beams

“…These large fields require high beam current in order to complete field delivery in short time. Complex multi-field optimized treatment plan validation may consequently require careful assessment of recombination effects as a function of time and position, already stressed by Palmans et al [15] for photon fields used in Tomotherapy.…”

NCS Report 27: Quality assurance for Tomotherapy systems

“…In TG148 a target volume of diameter 8 cm and 10 cm in length is recommended, together with a 5 cm treatment slit width and a pitch of 0.287 (Palmans et al, 2010).…”

“…Take ionisation readings per time unit at high (U2) and low voltage (U1) of the chamber to define the recombination correction ks as described in NCS report 18-appendix 2 (NCS 18, 2008). Valuable information on this topic can also be found in Palmans (Palmans et al, 2010) where the need for a correct determination of the recombination correction is expressed, especially for the ion chambers smaller than the Farmer type chambers. .…”