Y De Deene

Polymer gel dosimetry

Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

An investigation of the chemical stability of a monomer/polymer gel dosimeter.

The aim of this work is to investigate the temporal stability of a polyacrylamide gelatin hydrogel used for 3D monomer/polymer gel dosimetry techniques involving different methods of analysis. Long-term instabilities for a similar gel have recently been reported, but differ markedly from those described in this work. Two kinds of long-term instabilities are described. One affects the slope of the dose-R2 plot and is related to post-irradiation polymerization of the comonomer/polymer aggregates. It is observed that post-irradiation polymerization only lasts 12 hours after irradiation. The other instability affects the intercept of the dose-R2 plot, lasts for up to 30 days and is related to the gelation process of gelatin. Further studies were performed on gelatin gels of varying compositions to obtain a better understanding of the molecular mechanism that causes the instability due to gelation. The studies included observations of the spin-spin and spin-lattice relaxation rates in combination with diffusion measurements and optical measurements. It is shown that the heating history during the manufacture of the gel affects the absolute R2 value of the gel but not its variation. The findings presented in this study may help in producing more stable and reproducible monomer/polymer gel dosimeters.

Three-dimensional dosimetry using polymer gel and magnetic resonance imaging applied to the verification of conformal radiation therapy in head-and-neck cancer

BACKGROUND AND PURPOSE It was our aim to investigate NMR-based BANG gel dosimetry as a three-dimensional dosimetry technique in conformal radiotherapy. MATERIALS AND METHODS The BANG gel consisting of gelatin, water and co-monomers was first validated in a cylindrical glass flask for a single standard beam. Next, the gel contained in a human neck-shaped cast was used to verify a treatment plan for the conformal irradiation of a concave tumour in the lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions measured using radiographic film. RESULTS The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, the difference map between gel-measured and film-measured dose distributions revealed a noise component and a more systematic deviation including structural or space-coherent patterns. The mean absolute value of the difference amounted to 8%. A number of possible causes for this deviation are designated. CONCLUSIONS Polymer gel dosimetry in combination with magnetic resonance imaging is a promising method for dosimetric verification of conformal radiotherapy.

On the accuracy of monomer/polymer gel dosimetry in the proximity of a high-dose-rate 192Ir source.

The aim of this work was to investigate the applicability of MR-based polymer gel dosimetry to measure the absorbed dose distribution at short distance from an iridium-192 brachytherapy point source. In this paper, different methodological problems that may result in significant errors in the measured dose distribution are discussed. First of all the extent to which physicochemical mechanisms alter the dose response is discussed. The most important among these are the oxygen permeability of the catheter material and monomer-diffusion-related effects during irradiation. The effect of oxygen on the dose-R2 curve has been determined quantitatively and an oxygen map is performed using a well-defined external irradiation beam. The effect of diffusion of monomer during irradiation has been simulated. Another contribution of errors is related to magnetic susceptibility differences between the catheter and the gel during scanning the irradiated gel. The magnetic field distortion has been mapped by using both an experimental MRI technique and by simulation. Moreover, in constructing a dose-versus-distance curve by polar averaging, the sensitivity to the exact distance between source and point of measurement and to partial volume effects is illustrated. An optimization program is proposed to determine the location of the source on a sub-pixel scale.

Validation of MR-Based Polymer Gel Dosimetry as a Preclinical Three-Dimensional Verification Tool in Conformal Radiotherapy

The aim of this work was to investigate MR‐based polymer gel dosimetry as a three‐dimensional (3D) dosimetry technique in conformal radiotherapy. A cylindrical container filled with polymer gel was placed in a water‐filled torso phantom to verify a treatment plan for the conformal irradiation of a mediastinal tumor located near the esophagus. Magnetic resonance spin‐spin relaxation rate images were acquired and, after calibration, converted to absorbed dose distributions. The dose maps were compared with dose distributions measured using radiographic film. The average root‐mean‐square structural deviation, for the complete dose distribution, amounted to less than 3% between gel and film dose maps. It may be expected that MR gel dosimetry will become a valuable tool in the verification of 3D dose distributions. The influence of imaging artifacts arising from eddy currents, temperature drift during scanning, and B1 field inhomogeneity on the dose maps was taken into account and minimized. Magn Reson Med 43:116–125, 2000.

Validation and application of polymer gel dosimetry for the dose verification of an intensity-modulated arc therapy (IMAT) treatment

Polymer gel dosimetry was used to assess an intensity-modulated arc therapy (IMAT) treatment for whole abdominopelvic radiotherapy. Prior to the actual dosimetry experiment, a uniformity study on an unirradiated anthropomorphic phantom was carried out. A correction was performed to minimize deviations in the R2 maps due to radiofrequency non-uniformities. In addition, compensation strategies were implemented to limit R2 deviations caused by temperature drift during scanning. Inter- and intra-slice R2 deviations in the phantom were thereby significantly reduced. This was verified in an investigative study where the same phantom was irradiated with two rectangular superimposed beams: structural deviations between gel measurements and computational results remained below 3% outside high dose gradient regions; the spatial shift in those regions was within 2.5 mm. When comparing gel measurements with computational results for the IMAT treatment, dose deviations were noted in the liver and right kidney, but the dose-volume constraints were met. Root-mean-square differences between both dose distributions were within 5% with spatial deviations not more than 2.5 mm. Dose fluctuations due to gantry angle discretization in the dose computation algorithm were particularly noticeable in the low-dose region.

Mathematical analysis and experimental investigation of noise in quantitative magnetic resonance imaging applied in polymer gel dosimetry

Abstract In polymer gel dosimetry, the spin–spin relaxation rate R 2=1/ T 2, is related to the absorbed dose that is delivered to a gel phantom by high-energy radiation beams. In a two-points method, R 2 is calculated from two differently T 2-weighted images. In the many-points method, R 2 is calculated by fitting the pixel intensities of a set of differently exponentially T 2-weighted images. An analysis of the influence of noise on the resulting R 2 image may contribute considerably to the enhancement of the accuracy of the dose map. The relation between the noise level in the differently T 2-weighted images and the noise level in the R 2 image is derived mathematically. This relation is dependent on the actual R 2, on the choice of the echo times in the sequences used, and the fitting algorithm. Both a least square fit to the semi-logarithmic T 2-relaxation plot and a maximum-likelihood estimation on the T 2-relaxation plot were investigated. It was found that dose images obtained from R 2 images through calibration will contain noise that originates from the noise in the R 2 image and from the probability distribution of the coefficients of the calibration curve. We present a method for optimizing the echo times in order to maximize the signal-to-noise ratio (SNR) in the resulting R 2 image for both methods and for both fitting algorithms. The mathematical considerations on the SNR as presented, can also be applied on other data sets which display a mono-exponential behavior (e.g. diffusion measurements, T 1 relaxation).

Radiological properties of the PRESAGE and PAGAT polymer dosimeters

The radiological properties of the PRESAGE and PAGAT polymer dosimeters have been investigated and their water equivalence determined for use in radiotherapy dosimetry. The radiological water equivalence of each of the polymer dosimeters was determined by comparing the photon and electron interaction cross-sections over the 10 keV-20 MeV energy range and by Monte Carlo modelling the depth dose from a linear accelerator using the BEAMnrc software package. PRESAGE was found to have an effective Z-value and mass density (kgm(-3)) approximately 17% and 10% higher than water, respectively. A maximum difference of 85% was discovered in the photoelectric interaction probability curve of PRESAGE when compared to water over the energy range 10-100 keV, partially due to the Z(3) dependence of the photoelectric effect. The mass radiative stopping power ratios and mass scattering power ratios were both found to have less than 9% difference from water. The depth dose for PRESAGE from a 6MV photon beam had an absolute percentage difference to water of less than 2% and a relative percentage difference of less than 8%. The mass density of PAGAT was found to be 2.6% higher than water due to its high gelatine and monomer concentration. The cross-sectional attenuation and absorption coefficient ratios were found to be within 5% for energies between 10 and 100 keV and within 1% for energies between 100 keV and 20 MeV. The mass collisional stopping power, mass radiative stopping power and mass scattering power ratios were all less than 1% over the energy range studied. The depth dose had an absolute percentage difference to water of less than 1% and a relative percentage difference of less than 2.5%. These results indicate that the PAGAT polymer gel formulation is more radiological water equivalent than the PRESAGE formulation. However, the PRESAGE dosimeter offers some advantages in terms of ease of use and its lack of water equivalence may be overcome with dosimetric correction factors.

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: II. Analysis of B1-field inhomogeneity

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = 1/T2, is related to radiation dose that has been delivered to a gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. The accuracy that is aimed for in this quantitative MR application is about 5% relative to the maximum dose. In a conventional imaging MR scanner, however, several imaging artefacts may perturb the final dose map. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose maps are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B1-field inhomogeneities as a source of dose inaccuracy. First, the influence of B1-field inhomogeneities on slice profiles is investigated using a thin-slice phantom. Secondly, a FLASH sequence is used to map the B1-field by assessing the effective flip angle in each voxel of a homogeneous phantom. In addition, both experiments and computer simulations revealed the effects of B1 field inhomogeneities on the measured R2. This work offers a method to correct R2 maps for B1 -field inhomogeneities.

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: I. Analysis and compensation of eddy currents

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = I/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower, is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.