Investigating potential physicochemical errors in polymer gel dosimeters

2012

Phys. Med. Biol.

The intra- and inter-batch accuracy and precision of MRI (polyacrylamide gelatin gel fabricated at atmospheric conditions) polymer gel dosimeters are assessed in full 3D. In the intra-batch study, eight spherical flasks were filled with the same polymer gel along with a set of test tubes that served as calibration phantoms. In the inter-batch study, the eight spherical flasks were filled with different batches of gel. For each spherical phantom, a separate set of calibration phantoms was used. The spherical phantoms were irradiated using a three-field coplanar beam configuration in a very reproducible manner. The calibration phantoms were irradiated to known doses to obtain a dose-R₂ calibration plot which was applied on the corresponding R₂ maps of all spherical phantoms on an individual basis. The intra-batch study showed high dosimetric precision (3.1%) notwithstanding poor accuracy (mean dose discrepancies up to 13.0%). In the inter-batch study, a similar dosimetric precision (4.3%) and accuracy (mean dose discrepancies up to 13.7%) were found. The poor dosimetric accuracy was attributed to a systematic fault that was related to the calibration method. Therefore, the dose maps were renormalized using an independent ion chamber dose measurement. It is illustrated that with this renormalization, excellent agreement between the gel measured and TPS calculated 3D dose maps is achievable: 97% and 99% of the pixels meet the 3%/3 mm criteria for the intra- and inter-batch experiments, respectively. However renormalization will result in significant dose deviations inside a realistically sized anthropomorphic phantom as will be shown in a concurrent paper.

Section: Introductionmentioning

confidence: 99%

On the validity of 3D polymer gel dosimetry: I. Reproducibility study

Vandecasteele

2012

Phys. Med. Biol.

“…To elucidate the origins of these dose discrepancies caused by the calibration using small calibration phantoms, several experiments were set up to investigate possible physico-chemical effects that may influence the accuracy of polymer gel dosimetry. Several hypotheses were already proposed in the scientific literature for these differences (MacDougall et al 2008, Dumas et al 2006, Xu et al 2010, De Deene et al 2007, Salomons et al 2002, Sedaghat et al 2010, Hepworth et al 1999, Sedaghat et al 2011a, 2011b. These hypotheses included the effect of inhomogeneous distribution of oxygen before irradiation (Sedaghat et al 2011a(Sedaghat et al , 2011b and temperature effects before and during irradiation (Cosgrove et al 2000, Salomons et al 2002, De Deene et al 2006, 2007, Sedaghat et al 2010.…”

Section: Introductionmentioning

confidence: 99%

On the validity of 3D polymer gel dosimetry: II. Physico-chemical effects

Vandecasteele

2012

Phys. Med. Biol.

This study quantifies some major physico-chemical factors that influence the validity of MRI (PAGAT) polymer gel dosimetry: temperature history (pre-, during and post-irradiation), oxygen exposure (post-irradiation) and volumetric effects (experiment with phantom in which a small test tube is inserted). Present results confirm the effects of thermal history prior to irradiation. By exposing a polymer gel sample to a linear temperature gradient of ∼2.8 °C cm⁻¹ and following the dose deviation as a function of post-irradiation time new insights into temporal variations were added. A clear influence of the temperature treatment on the measured dose distribution is seen during the first hours post-irradiation (resulting in dose deviations up to 12%). This effect diminishes to 5% after 54 h post-irradiation. Imposing a temperature offset (maximum 6 °C for 3 h) during and following irradiation on a series of calibration phantoms results in only a small dose deviation of maximum 4%. Surprisingly, oxygen diffusing in a gel dosimeter up to 48 h post-irradiation was shown to have no effect. Volumetric effects were studied by comparing the dose distribution in a homogeneous phantom compared to the dose distribution in a phantom in which a small test tube was inserted. This study showed that the dose measured inside the test tube was closer to the ion chamber measurement in comparison to the reference phantom without test tube by almost 7%. It is demonstrated that physico-chemical effects are not the major causes for the dose discrepancies encountered in the reproducibility study discussed in the concurrent paper (Vandecasteele and De Deene 2013a Phys. Med. Biol. 58 19-42). However, it is concluded that these physico-chemical effects are important factors that should be addressed to further improve the dosimetric accuracy of 3D MRI polymer gel dosimetry.

“…However, it is a mistake to assume that normoxic gel dosimeters are completely insensitive to oxygen. Normoxic gel dosimeters still require casting materials that avoid infiltration of ‘fresh’ oxygen [ 259 , 260 ]. Large amounts of oxygen in the gel dosimeter results in inhibition of the polymerization reaction, while small amounts result in a promotion of polymerization.…”

Section: Polymer Gel Dosimetersmentioning

confidence: 99%

Radiation Dosimetry by Use of Radiosensitive Hydrogels and Polymers: Mechanisms, State-of-the-Art and Perspective from 3D to 4D

2022

Gels

Gel dosimetry was developed in the 1990s in response to a growing need for methods to validate the radiation dose distribution delivered to cancer patients receiving high-precision radiotherapy. Three different classes of gel dosimeters were developed and extensively studied. The first class of gel dosimeters is the Fricke gel dosimeters, which consist of a hydrogel with dissolved ferrous ions that oxidize upon exposure to ionizing radiation. The oxidation results in a change in the nuclear magnetic resonance (NMR) relaxation, which makes it possible to read out Fricke gel dosimeters by use of quantitative magnetic resonance imaging (MRI). The radiation-induced oxidation in Fricke gel dosimeters can also be visualized by adding an indicator such as xylenol orange. The second class of gel dosimeters is the radiochromic gel dosimeters, which also exhibit a color change upon irradiation but do not use a metal ion. These radiochromic gel dosimeters do not demonstrate a significant radiation-induced change in NMR properties. The third class is the polymer gel dosimeters, which contain vinyl monomers that polymerize upon irradiation. Polymer gel dosimeters are predominantly read out by quantitative MRI or X-ray CT. The accuracy of the dosimeters depends on both the physico-chemical properties of the gel dosimeters and on the readout technique. Many different gel formulations have been proposed and discussed in the scientific literature in the last three decades, and scanning methods have been optimized to achieve an acceptable accuracy for clinical dosimetry. More recently, with the introduction of the MR-Linac, which combines an MRI-scanner and a clinical linear accelerator in one, it was shown possible to acquire dose maps during radiation, but new challenges arise.