Determination of the recombination correction factork_Sfor some specific plane-parallel and cylindrical ionization chambers in pulsed photon and electron beams

Abstract

It has been shown from an evaluation of the inverse reading of the dosemeter (1/M) against the inverse of the polarizing voltage (1/V), obtained with a number of commercially available ionization chambers, using dose per pulse values between 0.16 and 5 mGy, that a linear relationship between the recombination correction factor kS and dose per pulse (DPP) can be found. At dose per pulse values above 1 mGy the method of a general equation with coefficients dependent on the chamber type gives more accurate results than the Boag method. This method was already proposed by Burns and McEwen (1998, Phys. Med. Biol. 43 2033) and avoids comprehensive and time-consuming measurements of Jaffé plots which are a prerequisite for the application of the multi-voltage analysis (MVA) or the two-voltage analysis (TVA). We evaluated and verified the response of ionization chambers on the recombination effect in pulsed accelerator beams for both photons and electrons. Our main conclusions are: (1) The correction factor k(S) depends only on the DPP and the chamber type. There is no influence of radiation type and energy. (2) For all the chambers investigated there is a linear relationship between kS and DPP up to 5 mGy/pulse, and for two chambers we could show linearity up to 40 mGy/pulse. (3) A general formalism, such as that of Boag, characterizes chambers exclusively by the distance of the electrodes and gives a trend for the correction factor, and therefore (4) a general formalism has to reflect the influence of the chamber construction on the recombination by the introduction of chamber-type dependent coefficients.

Ion recombination corrections of ionization chambers in flattening filter‐free photon radiation

“…Weinhouse and Meli (15) provided a convenient method for determining $P_{\text{ion}}$ using the two‐voltage technique with a voltage ratio of two, which was also used in TG‐51 protocol for ion recombination correction. Bruggmoser et al (14) have proposed that the recombination correction factor depends only on dose per pulse and the chamber type, and it is not affected by radiation type or energy.…”

“…The ion recombination correction factor, $P_{\text{ion}}$, is defined to account for incomplete collection of charges and it is a function of dose per pulse in a linear accelerator 10 , 14 . Dose per pulse in the unit of monitor units per pulse (MU/pulse) is dose rate (MU/min) divided by pulse rate (pulse/min).…”

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

“…Saturation curves were also obtained at multiple dose‐per‐pulse values for one Farmer‐type chamber and one microionization chamber in order to investigate P ion as a function of dose‐per‐pulse, and to benchmark this methodology against reported values. Plots of this relationship were fitted according to the TG‐51 addendum (based on the formalism of Bruggmoser et al): where C init is a measure of initial ion recombination, C gen is the coefficient of general ion recombination, and D w,pp is the dose‐to‐water per linac pulse. For the higher dose‐per‐pulse setups, which were achieved by varying the energy and source‐to‐surface distance (SSD) at a constant 300 MU/min, acquisition time was shortened to 8 min of pre‐irradiation followed by 2 min of data acquisition.…”

Determination of the ion recombination correction factor for intraoperative electron beams

“…Unfortunately, these values could not be compared with our results. On the other hand, different absolute values of (k s ) jaff th could be the result of different measurement conditions, especially the electrode distance of the applied chamber and the dose-per-pulse value investigated [21]. As reported by Burns and McEwen, a difference in the parallel-plate separation of 0.16 mm causes changes in the ion recombination factor of up to 8% [4].…”