Use of a liquid ionization chamber for stereotactic radiotherapy dosimetry

Abstract

Liquid ionization chambers (LICs) offer an interesting tool in the field of small beam dosimetry, allowing better spatial resolution and reduced perturbation effects. However, some aspects remain to be addressed, such as the higher recombination and the effects from the materials of the detector. Our aim was to investigate these issues and their impact. The first step was the evaluation of the recombination effects. Measurements were performed at different SSDs to vary the dose per pulse, and the collection efficiency was obtained. The BEAMnrc code was then used to model the Cyberknife head. Finally, the liquid ionization chamber itself was modelled using the EGSnrc-based code Cavity allowing the evaluation of the influence of the volume and the chamber materials. The liquid ionization charge collection efficiency is approximately 0.98 at 1.5 mGy pulse(-1), the highest dose per pulse that we have measured. Its impact on the accuracy of output factors is less than half a per cent. The detector modelling showed a significant contribution from the graphite electrode, up to 6% for the 5 mm collimator. The dependence of the average electronic mass collision stopping power of iso-octane with beam collimation is negligible and thus has no influence on output factor measurements. Finally, the volume effect reaches 5% for the small 5 mm collimator and becomes much smaller (<0.5%) for diameters above 10 mm. LICs can effectively be used for small beam relative dosimetry as long as adequate correction factors are applied, especially for the electrode and volume effects.

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

“…the fact that the sensitive volume is not small compared to the dimensions of the beam). This is possible through the use of Monte Carlo simulations 5 . With those aspects taken into account, global correction factors can be applied to the LIC readings obtained in clinical routine measurements (output factors, percentage depth doses, dose profiles) in order to fully eliminate the perturbations.…”

“…Moreover the sensitive medium has a density close to that of water, reducing the fluence perturbations associated with an air cavity. These aspects make the LIC an interesting candidate for small beam dosimetry [3][4][5] .…”

Evaluation of a synthetic single-crystal diamond detector for relative dosimetry measurements on a CyberKnife™

“…3 The use of natural diamond for radiation detectors has been investigated for such fields, owing to its high sensitivity, low leakage, resistance to radiation damage and near tissue equivalence; however, dose-rate dependence and variability of individual detector volume of these detectors makes depth-dose determination difficult. 4 In this work, a new commercially available diamond detector, PTW-60019 microDiamond™ (PTW, Freiberg, Germany), has been evaluated for dosimetry measurements on a clinical CyberKnife VSI system.…”

Monte Carlo calculation based on hydrogen composition of the tissue for MV photon radiotherapy

“…In the current work, dosimetrically equivalent tissue subsets are generated using Monte Carlo calculations in homogeneous phantoms and from preconceived results of Vanderstraeten et al (5) regarding the approximate number of intervals and the boundaries between them. Simulations were performed with DOSXYZnrc (11) using a phase space of a CyberKnife (Accuray Inc., Sunnyvale, CA) unit, previously modeled (12) . The result is an almost continuously changing tissue composition in consecutive subsets and, thus, a continuously changing hydrogen content, with a continuously changing density (from the mass density calibration curve).…”

“…The CyberKnife accelerator has been modeled using BEAMnrc (and the egs++ geometry package to model the iris collimator), generating phase space files at the exit plane of the secondary collimator (12) . The system allows selecting a treatment plan established on the MultiPlan (Accuray, Inc.) treatment planning system (TPS), and to compare the dose distribution and dose‐volume histograms (DVH) between MultiPlan and the Monte Carlo dose engine.…”