The saturation loss for plane parallel ionization chambers at high dose per pulse values

Abstract

The use of plane parallel ionization chambers with electron beams with high dose per pulse entails dose uncertainties due to the overestimation of the ion recombination factor, k, up to 20% if conventional dosimetric protocols are used. In this work MD-55-2 radiochromic films have been used as reference dosimeters to obtain dose to water per pulse DGAF(w) values for three Novac7 (Hitesys) electron beams of E0 = 5.8 MeV. However, the beam calibration by MD-55-2 films is time consuming and the use of plane parallel chambers is fundamental for a periodic quality control procedure. Three plane parallel chambers have been used and the general formula for the k determination has been tested using the calibration doses, DGAF(w). In particular, consistent ion recombination factors ksat(V0) (with the ion chamber polarized at V0), that follow the Boag theory, have been estimated at different dose per pulse values for the three plane parallel ionization chambers. This means that at present any ion chamber needs a specific ksat (V0) determination by using a reference dosimeter for which the response is independent of the dose rate. An accurate determination of ksat(V0), using a reference quality beam, can be used to determine the dose to water per pulse for electron beams of different quality and geometrical configuration.

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

“…This calculation procedure has been considered to be appropriate once the linear dependence of $1/M$ and $1/V$ required by the Boag theory is determined to be satisfied. Some publications show that this linear dependence is not valid over a certain threshold voltage for various chambers, both plane‐parallel ( 15 , 20 – 23 ) and cylindrical. ( 24 , 25 ) …”

Guidelines for quality assurance in intra-operative radiation therapy

“…This fact sets a limit to the employ of ionisation chambers for the beams calibration in terms of dose per MU. In particular, due to the high density of electric charge produced in the chamber's volume per radiation pulse, the correction factor for ion recombination can be largely overestimated if the correction methods recommended by the international protocols are used 15,39,40 . For this reason, for the measure of dose to water in reference conditions, ionisation chambers cannot be employed and no published dosimetry protocol can be used.…”

“…Ionisation chambers, as previously reported, are advised against: due to the high density of electric charges produced in the volume of the chamber for each radiation pulse, in fact, the correction factor for the ion recombination 11,15,39,40 can be largely overestimated when the correction methods recommended in the international dosimetry protocols are applied. The recombination factor, moreover, may vary depending on the depth in the phantom of the point of measurement.…”

“…Defining the physical characteristics of the electron beams used for IORT requires an accurate initial dosimetric study 14 , particularly with the new mobile accelerator 15 ; moreover, an accurate protocol for a regular quality control of the equipment based on international recommendations has to be prepared 16 . The definition of the procedures to be followed during the execution of IORT and its documentation is essential for the optimisation of the programme of quality assurance in this radiotherapy treatment.…”

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

“…However, the two‐voltage method has been shown to be notably inaccurate for both low electric field strength (6) and very large doses per pulse 14 , 15 . Piermattei et al, (14) for example, showed substantial error in the recombination correction estimated by the two‐voltage technique for a high dose‐per‐pulse intraoperative electron therapy accelerator. That error corresponded to a 20% difference in dose to water measured by several ion chambers, as compared to a dose‐per‐pulse independent dosimeter.…”