Severe dose inaccuracies caused by an oxygen-antioxidant imbalance in normoxic polymer gel dosimeters

Dosimetric properties of PASSAG polymer gel dosimeter in electron beam radiotherapy using magnetic resonance imaging

BACKGROUND

Several physical factors such as photon beam energy, electron beam energy, and dose rate may affect the dosimetric properties of polymer gel dosimeters. The photon beam energy and dose rate dependence of PASSAG gel dosimeter were previously evaluated.

OBJECTIVE

This study aims to assess the dosimetric properties of the optimized PASSAG gel samples in various electron beam energies.

METHODS

The optimized PASSAG gel samples are first fabricated and irradiated to various electron energies (5, 7, 10 and 12 MeV). Then, the response (R2) and sensitivity of gel samples are analyzed by magnetic resonance imaging technique at a dose range of 0 to 10 Gy, scanning room temperature range of 15 to 22 °C, and post-irradiation time range of 1 to 30 days.

RESULTS

The R2-dose response and sensitivity of gel samples do not change under the evaluated electron beam energies (the differences are less than 5%). Furthermore, a dose resolution range of 11 to 38 cGy is obtained for the gel samples irradiated to different electron beam energies. Moreover, the findings show that the R2-dose response and sensitivity dependence of gel samples on electron beam energy varies over different scanning room temperatures and post-irradiation times.

CONCLUSION

The dosimetric assessment of the optimized PASSAG gel samples provides the promising data for this dosimeter during electron beam radiotherapy.

Chemical Overview of Gel Dosimetry Systems: A Comprehensive Review

Advances in radiotherapy technology during the last 25 years have significantly improved both dose conformation to tumors and the preservation of healthy tissues, achieving almost real-time feedback by means of high-precision treatments and theranostics. Owing to this, developing high-performance systems capable of coping with the challenging requirements of modern ionizing radiation is a key issue to overcome the limitations of traditional dosimeters. In this regard, a deep understanding of the physicochemical basis of gel dosimetry, as one of the most promising tools for the evaluation of 3D high-spatial-resolution dose distributions, represents the starting point for developing new and innovative systems. This review aims to contribute thorough descriptions of the chemical processes and interactions that condition gel dosimetry outputs, often phenomenologically addressed, and particularly formulations reported since 2017.

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

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.

Study of the formulation optimization and reusability of a MAGAT gel dosimeter

PURPOSE

This study aims to optimize the formulation of a methacrylic acid gelatine and tetrakis (hydroxymethyl) phosphonium chloride (MAGAT) gel dosimeter to achieve acceptable dosimetric characteristics and the lowest final costs. This study also evaluates the reusability of the dosimeter.

METHODS

The MAGAT gel dosimeter formulation was optimized. Tetrakis (hydroxymethyl) phosphonium chloride (THPC) concentrations (2, 5, 8, 10, 20, and 65 mM), methacrylic acid (MA) concentrations (2.0, 2.5, 3.0, 3.5, and 4.0% w/w) and gelatin concentrations (4.36, 6.45, 8.36, and 10.45% w/w) were evaluated to provide an adequate dosimetric response. The final dosimeter formulation linearity and dose rate dependence were evaluated. The reutilization methodology of the optimized gel formulation, but containing 2 mM of THPC, which was previously irradiated with a dose of 2 Gy, is also presented.

RESULTS

The optimized mass concentration of the dosimeter consists of 88.60% deionized water, 8.36% gelatin, 3.00% of MA and 0.04% THPC (5 mM). It presents a linear response for doses up to 10 Gy with a 1.16 Gy^-1 s^-1 sensitivity. A maximum sensitivity variation of less than 4.0% was found when varying the dose rate of the radiation beams from 300 to 500 cGy/min. It was possible to reuse the dosimeter, however the sensitivity decreased by 15% from the first to the second irradiation.

CONCLUSIONS

A low-cost MAGAT gel dosimeter with optimized formulation that responds to radiation in a dose range of 0 to 10 Gy with small dose-rate dependence is presented. The MAGAT gel can be reused after a 2 Gy irradiation.

A systematic review of clinical applications of polymer gel dosimeters in radiotherapy

Radiotherapy has rapidly improved because of the use of new equipment and techniques. Hence, the appeal for a feasible and accurate three-dimensional (3D) dosimetry system has increased. In this regard, gel dosimetry systems are accurate 3D dosimeters with high resolution. This systematic review evaluates the clinical applications of polymer gel dosimeters in radiotherapy. To find the clinical applications of polymer gel dosimeters in radiotherapy, a full systematic literature search was performed on the basis of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines in electronic databases up to January 31, 2017, with use of search-related terms in the titles and abstracts of articles. A total of 765 articles were screened in accordance with our inclusion and exclusion criteria. Eventually, 53 articles were included in the study. The findings show that most clinical applications of polymer gel dosimeters relate to external radiotherapy. Most of the gel dosimeters studied have acceptable dose accuracy as a 3D dosimeter with high resolution. It is difficult to judge which is the best polymer gel dosimeter to use in a clinical setting, because each gel dosimeter has advantages and limitations. For example, methacrylic acid-based gel dosimeters have high dose sensitivity and low toxicity, while their dose response is beam energy dependent; in contrast, N-isopropylacrylamide gel dosimeters have low dose resolution, but their sensitivity is lower and they are relatively toxic.

New normoxic N-(Hydroxymethyl)acrylamide based polymer gel for 3D dosimetry in radiation therapy

A novel composition of normoxic polymer gel dosimeters based on radiation-induced polymerization of N-(Hydroxymethyl)acrylamide (NHMA) is introduced in this study for 3D dosimetry for Quality Assurance (QA) in radiation therapy. Dosimeters were irradiated by 6, 10 and 18MV photon beams of a medical linear accelerator at various dose rates to doses of up to 20Gy. The dose response of polymer gel dosimeters was studied using nuclear magnetic resonance (NMR) spin-spin relaxation rate (R₂) of hydrogen protons within the water molecule. Also, we measured gel response using absorption spectroscopy and found that this novel gel can be successfully utilized for both MRI- and OCT- (Optical Computed Tomography) based 3D dosimetry. We investigated dosimetric properties of six different compositions of the new NHMA-based gel in terms of dose rate, radiation beam quality and stability of dose-dependent polymerization after irradiation. We found no significant effects of these parameters on the novel gel dosimeter performance in both relaxation rate and absorbance measurements.

Leuco-crystal-violet micelle gel dosimeters: I. Influence of recipe components and potential sensitizers

Radiochromic leuco crystal violet (LCV) micelle gel dosimeters are promising three-dimensional radiation dosimeters because of their spatial stability and suitability for optical readout. The effects of surfactant type and surfactant concentration on dose sensitivity of LCV micelle gels are tested, demonstrating that dose sensitivity and initial colour of the gel increases with increasing Triton x-100 (Tx100) concentration. Using Cetyl Trimethyl Ammonium Bromide (CTAB) in place of Tx100 produces gels that are nearly colourless prior to irradiation, but reduces the dose sensitivity. The separate effects of Tri-chloro acetic acid concentration and pH are investigated, revealing that controlling the pH near 3.6 is crucial for achieving high dose sensitivity. The sensitizing effect of chlorinated species on dose sensitivity is tested using 2,2,2-trichloroethanol (TCE), chloroform, and 1,1,1-trichloro-2-methyl-2-propanol hemihydrate. TCE gives the largest improvement in dose sensitivity and is recommended for use in micelle gel dosimeters because it is less volatile and safer to use than chloroform. Preliminary experiments on a new gel containing CTAB as the surfactant and TCE show that this new gel gives a dose sensitivity that is 24% higher than that of previous LCV micelle gels and is nearly colourless prior to irradiation.

The influence of antioxidant THPC on the properties of polymer gel dosimeter

In order to decrease the negative influence of oxygen to the response of PAG dosimeters THPC has been added to the gel in the role of scavenger. Apart from the decreased influence of oxygen, THPC also influences other properties of gel dosimeters. This study examines these influences and their quantification. Previous studies have shown that increasing the concentration of THPC causes a decreasing response of the dosimeter (as measured in the relaxation rate R2). Evaluation of the IR spectrum of gels irradiated by a variety of doses has shown that it is caused mostly by the changed structure of the arising polymer, not due to the decreased polymerization. THPC also changes the kinetics of the subsequent reactions in the gel after the end of irradiation. THPC has its influence also on the size of the dose response overshoot that happens in the areas of steep dose gradients. An easy model of action in the gel was suggested, which allows one to estimate the size and kinetics of the changed response of the dosimeter after the end of irradiation depending on the content of THPC, the size of the dose and the dose gradient.

3D evaluation of 3DVH program using BANG3 polymer gel dosimeter

PURPOSE

With the recent introduction of intensity modulated arc therapy techniques, there is an increasing need for validation of treatment delivery in three-dimensional (3D) space. A commercial dosimetry device ArcCHECK™ (Sun Nuclear Corporation, Melbourne, FL, USA) can be used in conjunction with 3DVH program. With this system, one can reconstruct the 3D dose distribution produced in the actual patient. In this work the authors evaluate the relative accuracy of the ArcCHECK™-3DVH system using BANG3 (MGS Research, Guilford, CT, USA) polymer gel dosimeter.

METHODS

About 15-cm diameter and 20-cm long cylindrical phantoms filled with BANG3 was used to simulate a patient, to which a volumetrically modulated arc therapy plan was created with Pinnacle3 treatment planning software (Philips Healthcare, Andover, MA, USA). The plan (76 Gy total in 38 fractions) was designed for prostate radiotherapy using a 6 MV photon beam from an Elekta Synergy linear accelerator (Elekta AB, Stockholm, Sweden). The treatment was delivered to the simulated patient. The same plan was used to irradiate an ArcCHECK™ device with an insert plug. The point dose at the isocenter was measured using a Farmer-type ionization chamber. The measured dose data were imported into the 3DVH program, which generated the 3D dose distributions projected onto the simulated patient. The dose data recorded in the polymer gel were read out using a MRI scanner and the 3D dose distribution delivered to the simulated patient was analyzed and compared with those from the 3DVH program and the Pinnacle3 software. The comparison was accomplished by using the gamma index, overlaying the isodose lines for a set of data on selected planes, and computing dose-volume histogram of structures.

RESULTS

The dose at the center of the ArcCHECK™ device measured with an ionization chamber was 1.82% lower than the dose predicted by Pinnacle3. The 3D dose distribution generated by Pinnacle3 was compared with those obtained by the ArcCHECK™-3DVH system and BANG3. The gamma passing rates for criteria of 3% dose difference, 3 mm distance-to-agreement, and 25% lower dose threshold were 99.1% for the former and 95.7% for the latter. The mean and maximum PTV doses estimated by the 3DVH were 74.0 and 79.3 Gy in comparison to 74.4 and 76.5 Gy with Pinnacle3. Those values for BANG3 measurements were 74.7 and 79.5 Gy. The mean doses to rectum were 40.2, 39.8, and 38.8 Gy for Pinnacle3, 3DVH, and BANG3, whereas the mean doses to the bladder were 26.7, 25.7, and 21.7 Gy, respectively.

CONCLUSIONS

The ArcCHECK™-3DVH system provides an accurate estimation of 3D dose distribution in an actual patient within a clinically meaningful tolerance level. However, both 3DVH and BANG3 showed two noticeable differences from Pinnacle3. First, the measured dose throughout the PTV region was less uniform than Pinnacle3. Second, the dose gradient at the interface between PTV and rectum was steeper than Pinnacle3 prediction. Further investigation may be able to identify the cause for these findings.