Evaluation of the basic properties of the BANGkit gel dosimeter

Validation of complex radiotherapy techniques using polymer gel dosimetry

Modern radiotherapy delivers highly conformal dose distributions to irregularly shaped target volumes while sparing the surrounding normal tissue. Due to the complex planning and delivery techniques, dose verification and validation of the whole treatment workflow by end-to-end tests became much more important and polymer gel dosimeters are one of the few possibilities to capture the delivered dose distribution in 3D. The basic principles and formulations of gel dosimetry and its evaluation methods are described and the available studies validating device-specific geometrical parameters as well as the dose delivery by advanced radiotherapy techniques, such as 3D-CRT/IMRT and stereotactic radiosurgery treatments, the treatment of moving targets, online-adaptive magnetic resonance-guided radiotherapy as well as proton and ion beam treatments, are reviewed. The present status and limitations as well as future challenges of polymer gel dosimetry for the validation of complex radiotherapy techniques are discussed.

Evaluation of two calibration methods for MRI-based polymer gel dosimetry

Polymer gel dosimetry (PGD) can provide three-dimensional (3D) dose data for evaluation of the dose calculation algorithms used by treatment planning systems (TPS). Although the PGD technique, particularly with MRI, is now ready for clinical applications, an accurate calibration method is vital for treatment validation in 3D. This study evaluated the single-phantom electron beam (SPE) method that used the depth-dose data of a 9 MeV electron beam. This technique was compared with the multi-vial x-ray (MVX) method that used nine small vials irradiated with various doses. We tested two regression equations, i.e., third-order polynomial and tangent functions, and two dose-normalization methods, i.e., one-point and two-point methods. These methods were evaluated using a dose distribution generated by a 3 cm × 3 cm open arc beam. We used MAGAT polymer gel manufactured in-house. We found that the SPE method required a smaller dose scaling for the dose comparison. The tangent function showed better data fitting than the polynomial function with smaller uncertainty of the estimated coefficients. We did not observe a distinct advantage of the SPE method over the MVX method for the 3D dose comparison with the test case. From this study, we infer that the SPE method with the tangent function as the regression equation and one-point dose normalization is a good calibration option for the MRI-based polymer gel dosimetry.

Quantitative magnetic resonance elastography for polymer-gel dosimetry phantoms

Commonly dose-responses of conventional dosimetric methods are affected by a saturation dose and are known to be limited when the delivered dose is relatively high. In contrast, elastic properties of polymer-gel dosimeter phantoms play major roles in a new dosimetry technique using magnetic resonance elastography (MRE). A single volume of polymer-gel dosimeter solution containing methacrylic and ascorbic acid in gelatin initiated by copper was prepared. The material was subsequently stored in cylindrical containers for future use as a biological tissue-mimicking phantom material. The phantom material was irradiated with gamma rays, where absorbed doses of 10-50 Gy were delivered. To study the dynamic elastic behaviour, periodic mechanical external forces of 100-400 Hz were applied to generate shear waves in the samples. The radiation-induced changes in the shear modulus of the samples were estimated from wave-displacement images and converted to elastograms. The smallest and largest shear modulus values were approximately 2.10 ± 0.64 and 35.26 ± 2.85 kPa, respectively. The dynamic elastic response of the polymer-gel dosimeters showed an increased dependency with frequency. A linear relationship (R² = 0.996) was observed between the integrated area and the absorbed dose of the samples. The elastograms clearly showed that the largest shear modulus values were in the irradiated region of the polymer-gel dosimeter phantoms. Quantitative values of the shear modulus of polymer-gel dosimeters were estimated using MRE.

Dosimetric verification for intensity-modulated arc therapy plans by use of 2D diode array, radiochromic film and radiosensitive polymer gel

Several tools are used for the dosimetric verification of intensity-modulated arc therapy (IMAT) treatment delivery. However, limited information is available for composite on-line evaluation of these tools. The purpose of this study was to evaluate the dosimetric verification of IMAT treatment plans using a 2D diode array detector (2D array), radiochromic film (RCF) and radiosensitive polymer gel dosimeter (RPGD). The specific verification plans were created for IMAT for two prostate cancer patients by use of the clinical treatment plans. Accordingly, the IMAT deliveries were performed with the 2D array on a gantry-mounting device, RCF in a cylindrical acrylic phantom, and the RPGD in two cylindrical phantoms. After the irradiation, the planar dose distributions from the 2D array and the RCFs, and the 3D dose distributions from the RPGD measurements were compared with the calculated dose distributions using the gamma analysis method (3% dose difference and 3-mm distance-to-agreement criterion), dose-dependent dose difference diagrams, dose difference histograms, and isodose distributions. The gamma passing rates of 2D array, RCFs and RPGD for one patient were 99.5%, 96.5% and 93.7%, respectively; the corresponding values for the second patient were 97.5%, 92.6% and 92.9%. Mean percentage differences between the RPGD measured and calculated doses in 3D volumes containing PTVs were -0.29 ± 7.1% and 0.97 ± 7.6% for the two patients, respectively. In conclusion, IMAT prostate plans can be delivered with high accuracy, although the 3D measurements indicated less satisfactory agreement with the treatment plans, mainly due to the dosimetric inaccuracy in low-dose regions of the RPGD measurements.

[Investigation of polymer gel dosimetry for small circular irradiated fields]

Polymer gels can be used as tissue equivalent dosimeters, and polymer gel dosimetry can be employed without perturbation of the radiation field. In this study, polymer gel dosimetry was used for small circular irradiation fields 10-30 mm in diameter using a radiation planning system. The irradiated gels were compared with planned data for a 50% dose width of 6 Gy dose maximum, and for the dose difference between gels and planned data over an 80% dose maximum area. The present study investigated magnetic resonance imaging (MRI) conditions based on an optimal dose-R2 calibration curve. The average difference between the full width half maximum of the 50% dose width between gels and planned data was 11%. The average dose difference over 80% of the dose was 5.6%. Optimal dose-R2 calibration curves were acquired using images with echo times of 30 and 60 ms. For cases of larger thicknesses and an increasing number of averages, the coefficients of variance of the curves were smaller than under other conditions. Compared to other traditional dosimetric tools, polymer gels have the advantage of providing three-dimensional dosimetric data. An arbitrary profile from the gel's data can be compared with the profile of the planned data. In the future, new gel dosimeters will be needed that demonstrate improved dose evaluation under 1 Gy and stability in high dose areas.

Evaluation of three-dimensional polymer gel dosimetry using X-ray CT and R2 MRI

It is difficult to obtain images of thin slices from measurement of spin-spin relaxation (R2) with magnetic resonance imaging (MRI) using the traditional dose reading method of polymer gel dosimetry. In this study, the dose reading method was performed using X-ray computed tomography (CT) for proton beam measurements in order to enable collection of thin slices. In addition, three-dimensional (3D) images of polymer gels were constructed using volume rendering. As a result of acquisition of thin slices, more detailed 3D data consisting of smaller voxel sizes compared to R2 were acquired. However, it was found that with thin slice thicknesses and small voxels, the signal-to-noise ratio around the voxels deteriorated. In addition, the coefficient of variation of non-irradiated gels with CT was smaller than that with R2 MRI.

Errors introduced by dose scaling for relative dosimetry

Some dosimeters require a relationship between detector signal and delivered dose. The relationship (characteristic curve or calibration equation) usually depends on the environment under which the dosimeters are manufactured or stored. To compensate for the difference in radiation response among different batches of dosimeters, the measured dose can be scaled by normalizing the measured dose to a specific dose. Such a procedure, often called “relative dosimetry”, allows us to skip the time‐consuming production of a calibration curve for each irradiation. In this study, the magnitudes of errors due to the dose scaling procedure were evaluated by using the characteristic curves of BANG3 polymer gel dosimeter, radiographic EDR2 films, and GAFCHROMIC EBT2 films. Several sets of calibration data were obtained for each type of dosimeters, and a calibration equation of one set of data was used to estimate doses of the other dosimeters from different batches. The scaled doses were then compared with expected doses, which were obtained by using the true calibration equation specific to each batch. In general, the magnitude of errors increased with increasing deviation of the dose scaling factor from unity. Also, the errors strongly depended on the difference in the shape of the true and reference calibration curves. For example, for the BANG3 polymer gel, of which the characteristic curve can be approximated with a linear equation, the error for a batch requiring a dose scaling factor of 0.87 was larger than the errors for other batches requiring smaller magnitudes of dose scaling, or scaling factors of 0.93 or 1.02. The characteristic curves of EDR2 and EBT2 films required nonlinear equations. With those dosimeters, errors larger than 5% were commonly observed in the dose ranges of below 50% and above 150% of the normalization dose. In conclusion, the dose scaling for relative dosimetry introduces large errors in the measured doses when a large dose scaling is applied, and this procedure should be applied with special care.

PACS numbers: 87.56.Da, 06.20.Dk, 06.20.fb

A variable echo-number method for estimating R2 in MRI-based polymer gel dosimetry

PURPOSE

Spin-spin relaxation rate R2 is commonly used to quantify absorbed dose for magnetic resonance imaging (MRI)-based polymer gel dosimetry. R2 is estimated by applying a parameter fitting algorithm to a train of spin-echo signals. However, a careless application of a large number of echoes can result in anomalous R2 values because the echo signal intensity decreases to the background signal offset level for a long echo time. In this article, the authors proposed and evaluated a variable echo-number (VAREC) method to remedy the problem.

METHODS

The VAREC algorithm uses only echo signals, whose intensities are greater than a preset threshold. Here, the threshold is defined as the standard deviation of Gaussian noise times a multiplier alpha. The authors implemented three R2 estimation methods in an in-house program: The nonlinear least-squares algorithm (NLLS), the VAREC method, and the maximum likelihood estimator with the Rician signal intensity distribution (MLE_R). Those methods were used to estimate the R2 values of test phantoms with known R2 values and BANG3-type polymer gels, which were irradiated to 12 different doses ranging from 0 to 50 Gy. The R2 values were measured by using a 32-echo CPMG pulse sequence on 3 T MRI scanners. The R2 values of the VAREC method were compared with those of NLLS and MLE_R.

RESULTS

The R2 values of the NLLS method incorrectly decreased to the zero-dose level for doses greater than 10 Gy. The R2 values of the VAREC method with alpha=2 agreed with those of MLE_R within the measurement uncertainty. The uncertainties of the R2 values were the smallest for alpha=2 or 3 among various alpha values.

CONCLUSIONS

The VAREC algorithm is simple, fast, and robust for the R2 estimation. The authors recommend this method with alpha=2 or 3 for R2 estimation using multispin echo MRI protocols.

Clinical application of the Fricke-glucomannan gel dosimeter for high-dose-rate (192)Ir brachytherapy

This study investigates the efficacy of a new Fricke dosimeter formulation consisting of a standard Fricke gel dosimeter gelled with glucomannan (FrGDG). FrGDG was irradiated using a (192)Ir gamma-ray source with a remote afterloading system based on computed tomography images. (60)Co irradiation was performed for measuring the absorption of FrGDG and water. The distribution maps of T2 values from the irradiated containers were obtained by MR imaging and converted to the absorbed dose to visualize the dose distribution. We found that FrGDG was produced easily and quickly at room temperature. R2 (1/T2) values were reproducible and linearly correlated with the absorbed doses in the range from 0 to 30 Gy for irradiation with (192)Ir (the correlation coefficient was 0.99). The mean deviation between the doses obtained from the MR images of the FrGDG and those calculated by the treatment planning system for doses of 37.5, 40, 50, 62.5 and 75 Gy was 4.9%, 4.8%, 3.5%, 2.3% and 2.4%, respectively. In conclusion, MR imaging of FrGDG can visualize the dose distribution successfully, and thus serves as a useful quality assurance tool for complicated three-dimensional radiotherapy treatments.