R2-dose Sensitivity Research Articles

Characterization of an enhanced formulation N-(3-methoxypropyl) acrylamide polymer gel dosimeter by the addition of an organic sensitizer for clinical practice

Polymer gel dosimeters are being developed and improved to perform measurements of radiation dose distributions in 3-dimensions (3-D) for the purpose of patient specific quality assurance for cancer patients and the configuration of treatment planning systems. The effect of organic glucose sensitizer (GL) material on N-(3-methoxypropyl) acrylamide polymer gel dosimeter (NMPAGAT) was presented in this study and evaluated after irradiation using nuclear magnetic resonance (NMR) read-out technique in terms of spin–spin relaxation rate (R2) of hydrogen protons within the water molecule. The improved tissue equivalent polymers of GL-NMPAGAT were irradiated using a medical linear accelerator with photon beams with doses up to 20 Gy, energies range 6–15 MV, dose rates range 50–500 cGy/min, and irradiation temperatures range 15–25 °C. The R2 dose sensitivity of GL-NMPAGT polymer with a concentration of 25 % wt GL was improved significantly and found to be two times higher than the same gel without GL in the linear dose response of 0.07–8 Gy. The 0.07 Gy represents the minimum detectable dose in the improved dosimeters. The dosimeter was found stable within a period of up to two weeks after irradiation, which is significant. No significant dose rate, energy dependence, and irradiation temperature for the dosimeter was observed over the range studied. There is decrease in R2 values with increasing scanning temperature. The gel is water equivalent and has theoretical energy-independent response from 0.1 to 20 MeV with density of 1.023 ± 0.003 g/cm3 and Zeff of 7.53. The results of the improved dosimeter in this study (i.e., 100 %, 0.32 s−1 Gy−1, and 8 Gy linearity) are toward the upper limit compared to other polymer gel dosimeters even with sensitizer additives.

Enhancement the dosimetric properties of N-(3-methoxypropyl) acrylamide polymer gel dosimeter for cancer treatment

Recent studies related to the polymer gel dosimetry technique for radiation therapy have shown the importance of using additive of different concentrations to the polymer gel to improve the dose sensitivity. This work provides a significant improvement on the developed N-(3-methoxypropyl) acrylamide polymer gel dosimeter by incorporated with an inorganic salt of calcium chloride (CaCl2). Nuclear Magnetic Resonance relaxometry read out technique was used to examine dose-response after irradiation by measuring the change in the relaxation rate of R2. A medical linear accelerator with 6 MV photon beams and a 200 cGy/min dose rate was used to irradiate the developed gel with different absorbed doses. It was evidently clear from the findings that the R2 dose sensitivity of the developed polymer gel increased significantly by adding different concentrations of CaCl2 reaching 3.5 times higher than that without salt. The dose-response of the improved gel at various concentrations of co-monomer. Also, the dependency on the radiation dose rate, photon energy and irradiation temperature was investigated. Additionally, the stability of unirradiated as well as irradiated gel for 7-days post-irradiation was reported.

The impact of various amounts of fabricating components on the response of PASSAG polymer gel dosimeter: An optimization study

Polymer gel dosimeters are used to measure three-dimensional dose distribution with high spatial resolution. Recently, a novel polymer gel dosimeter, known as PASSAG, has been introduced in which has several advantages over other polymer gel dosimeters; however, its response-dose sensitivity is relatively low. It has been shown that the sensitivity of polymer gel dosimeters can be changed by various amounts of components used in their fabrication. This study aims to assess the dosimetric characteristics of PASSAG gel dosimeter for different amounts of components in its formulation. For this purpose, different formulations of PASSAG gel dosimeter were first fabricated, and then they were irradiated using 6 MV X-rays in a 0–10 Gy dose range. After the irradiation process, the responses (R2) of gel dosimeters were read out by a 1.5 T MRI scanner during one-month post irradiation time. According to the results, the maximum R2-dose sensitivity belonged to PASSAG gel dosimeter with 5% gelatin, 4% AMPS sodium salt (as monomer), and 4% Bis (as crosslinker) that showed a 31% increased sensitivity compared to conventional PASSAG gel dosimeter (5% gelatin, 3% monomer, and 3% crosslinker). Moreover, an excellent linear R2-dose response was observed for optimized PASSAG gel dosimeter. The dose resolution values of optimized PASSAG gel dosimeter also varied between 0.14 and 0.42 Gy in a 0–10 Gy dose range. Other findings revealed that the R2 values of the optimized gel dosimeter had a relative stability during 15–18 °C and 20–24 °C scanning temperatures. Furthermore, a temporal stability in the R2 values of the optimized gel dosimeter was observed during 15–30 days post irradiation time. The R2-dose sensitivity of optimized gel dosimeter also varied during 30 days post-irradiation time (from 31.00% to 47.97%).

Assessment of photon energy and dose rate dependence of U-NIPAM polymer gel dosimeter

One of the main drawbacks of NPAM gel dosimeter is its relatively low R2-dose sensitivity. Recently, the sensitivity of NIPAM gel dosimeter has been improved by adding urea and this new formula gel dosimeter was named U-NIPAM (Urea and NIPAM). In the current project, photon energy and dose rate dependence of optimized U-NIPAM gel dosimeter (with 3% urea) was assessed by MRI technique. The gel samples were fabricated under normal atmospheric conditions and after 1 day, they were exposed to photon energies of 6 and 15 MV and dose rates of 300, 400, and 500 cGy/min. Then, MRI responses (R2) of the U-NIPAM gel samples were analyzed at 12, 36, 72, 168 h after the irradiation process. The dosimetric evaluation of optimized U-NIPAM gel dosimeter characteristics at 12 h after the irradiation process resulted to following findings: 1) different photon energies (6 and 15 MV) have significant effect on the R2-dose sensitivity of gel dosimeter in 0–7 Gy dose range (P < 0.05), 2) the R2-dose sensitivity of gel dosimeter does not significantly change with different dose rate (300–500 cGy/min) in 0–7 Gy dose range (P˃0.05), 3) the obtained dose resolution for evaluated various photon energies and dose rates ranges from 0.06 Gy to 0.20 Gy. Moreover, it was found that the R2-dose sensitivity dependence of gel dosimeter on photon energy can vary over post irradiation; nevertheless, the R2-dose sensitivity dependence of dosimeter on dose rate dose not vary over post irradiation time.

Improvement of the sensitivity of PASSAG polymer gel dosimeter by urea

The severe toxicity in compound of polymer gel dosimeters has been reported as one of their major limitations for utilization in clinical applications. Recently, PASSAG polymer gel dosimeter has been introduced as a safe polymer gel dosimeter. Despite the excellent dosimetric results reported for this gel dosimeter, its R2-dose sensitivity is relatively low. Therefore, the present study is aimed to improve the sensitivity of PASSAG gel dosimeter by adding urea to its structure. Moreover, it was tried to obtain the optimal amount of urea for the new gel dosimeter. After preparation of the PASSAG-U (PASSAG and Urea) gel dosimeters, they were irradiated using 6 MV photon energy and their responses were read by a 1.5 T MRI scanner. Then, the R2-dose response and the R2-dose sensitivity of the PASSAG-U gel dosimeters with various percentages of the urea were assessed at a 0–10 Gy dose range, various scanning temperatures (15–24 °C), and post irradiation times (1–30 days). The radiological properties of PASSAG-U gel dosimeters confirmed soft tissue and water equivalence of the new gel dosimeters. Compared to the PASSAG gel dosimeter, the R2-dose sensitivities of PASSAG-U gel dosimeters with 1%, 3%, and 5% urea were improved by 12.14%, 25.15%, and 27.90%, respectively. Although the addition of urea improves the R2-dose sensitivity of the gel dosimeter, it leads to the degradation of dose resolution (especially for 5% urea). Moreover, the dosimetric evaluation of characteristics related to the PASSAG-U gel dosimeters with various urea concentrations resulted to following conclusions: 1) the optimal amount of urea was determined 3%; 2) there was a stability in the R2 values for 18–22 °C scanning temperatures; 3) there was a temporal stability at the response of PASSAG-U gel dosimeters from 14 to 30 days after irradiation; 4) the R2-dose sensitivity of PASSAG-U gel dosimeters varied over post irradiation time.

A novel method for increasing the sensitivity of NIPAM polymer gel dosimeter

NIPAM polymer gel dosimeter has low toxicity and its response does not particularly depend on dose rate and beam energy. However, a chief drawback of the NPAM gel dosimeter is its relatively low sensitivity. In the current study, the sensitivity of the NIPAM gel dosimeter was improved by adding urea which this new formula gel dosimeter was called U-NIPAM (Urea and NIPAM) and optimal amount of urea for this new formula was evaluated. For this purpose, various concentrations of the urea (1%, 2%, 3% and 4% (w/w)) were tested. The samples were irradiated using 1.25 MeV and 6 MV photon energies and imaged by a 1.5 T MRI scanner. Then, the MRI response (R2) and the sensitivity of the conventional NIPAM and U-NIPAM gel dosimeters with different percentages of urea were analyzed at a 0–6 Gy dose range, different post irradiation time, and temperature during scanning. The radiological properties of U-NIPAM polymer gel dosimeter reveal that this substance can be considered as a water/soft tissue equivalent material. With analyzing the findings, it was found that the optimal amount of urea is 3%, because after this concentration, the R2-dose sensitivity of U-NIPAM gel dosimeter does not significantly changed (P > 0.05). Furthermore, the results showed that using 3% urea in conventional NIPAM gel formulation leads to a sensitivity increase of around 37% (0.242 vs. 0.177 s−1 Gy−1). In addition, other results for optimized U-NIPAM gel dosimeter include: a) an excellent linear R2-dose response in 0–6 Gy dose range, and b) stability in the R2 value and the sensitivity for 18–21 °C temperatures.

Evaluation of dose rate and photon energy dependence of PASSAG polymer gel dosimeter

In the current study, dose rate and photon energy dependence of PASSAG gel dosimeter is evaluated by using MRI technique. The gel samples were readies under normal atmospheric conditions and then were irradiated after 24 h with 100, 200, 300, and 400 cGy/min dose rates and photon energies of 6 and 18 MV from a linear accelerator. The results demonstrate that there is no considerable effect of dose rate and photon energy dependence on the R2-dose response and sensitivity of PASSAG gel dosimeter in 0–10 Gy dose range, as there are 0.01–2.80 and 1.46–2.52% difference ranges between the R2 values of the gel samples at four different dose rates and two various photon energies, respectively.

Dosimetric characteristics of PASSAG as a new polymer gel dosimeter with negligible toxicity

Despite many advantages of polymer gel dosimeters, their clinical use is only not realized now. Toxicity of polymer gel dosimeters can be considered as one of their main limitations for use in routine clinical applications. In the current study, a new polymer gel dosimeter is introduced with negligible toxicity. For this purpose, 2-Acrylamido-2-Methy-1-PropaneSulfonic acid (AMPS) sodium salt monomer was replaced instead of acrylamide monomer used in PAGAT gel dosimeter by using %6 T and %50 C to the gel formula and the new formulation is called PASSAG (Poly AMPS Sodium Salt and Gelatin) polymer gel dosimeter. The irradiation of gel dosimeters was carried out using a Co-60 therapy machine. MRI technique was used to quantify the dose responses of the PASSAG gel dosimeter. Then, the MRI responses (R2) of the gel dosimeter was analyzed at different dose values, post-irradiation times, and scanning temperatures. The results showed that the new gel formulation has a negligible toxicity and it is also eco-friendly. In addition, carcinogenicity and genetic toxicity tests are negative for the monomer used in PASSAG. The radiological properties of PASSAG gel dosimeter showed that this substance can be considered as a soft tissue/water equivalent material. Furthermore, dosimetric evaluation of the new polymer gel dosimeter revealed an excellent linear R2-dose response in the evaluated dose range (0–15 Gy). The R2-dose sensitivity and dose resolution of PASSAG gel dosimeter were 0.081 s−1Gy−1 (in 0–15 Gy dose range) and 1 Gy (in 0–10 Gy dose range), respectively. Moreover, it was shown that the R2-dose sensitivity and dose resolution of the new gel dosimeter improves over time after irradiation. It was also found that the R2 response of the PASSAG gel dosimeter has less dependency to the 18, 20, and 24 °C scanning temperature in comparison to that of room temperature (22 °C).

Evaluation of ferrous Methylthymol blue gelatin gel dosimeters using nuclear magnetic resonance and optical techniques

Signal development of irradiated Methylthymol blue (MTB) Fricke gelatin gel dosimeter was studied using nuclear magnetic resonance (NMR) in terms of the spin-spin relaxation rate (R2) of hydrogen protons within the water molecule. The optical properties of MTB gel were also studied using a spectrophotometric technique. The gel dosimeter presents a linear response for doses up to 20 Gy with ultraviolet visible (UV-Vis) spectrophotometry and 40 Gy with NMR. For a 0.1 mM MTB concentration and dose of up to 10 Gy, the sensitivity of the gel response analyzed by a spectrophotometer is 0.077 a.u. Gy−1 at 620 nm with a linear coefficient of (r2 = 0.998), while the R2 sensitivity is 0.02 s−1 Gy−1 with a linear coefficient of (r2 = 0.998). The sensitivity of the Fricke-MTB dosimeter was higher when compared with the Fricke-XO dosimeter. The dose rate and photon energy beams were dependent of the absorbance and R2 dose sensitivity, as observed over the range studied. The Fricke-MTB gelatin dosimeters can be scanned at higher wave lengths than the conventional Fricke-XO gelatin formulations.

Investigation of an improved MAA-based polymer gel for thermal neutron dosimetry

The aims of this study are to increase the melting point of a normoxic Methacrylic acid-based polymer gel as well as investigation of its responses to thermal neutron and gamma irradiation. New polymer gel was named MAGIC-A (MAGIC and agarose). The R 2-dose sensitivity of MAGIC-A polymer gel for gamma irradiation was 0.381 ± 0.010 Gy−1 s−1 at 24 h post irradiation time which decreased to 0.180 ± 0.004 Gy−1 s−1 for thermal neutron irradiation. Results of this research confirmed the applicability of the MAGIC-A polymer gels for dosimetry of thermal neutrons as well as gamma irradiation.

Validation of a prototype optical computed tomography system

In radiation cancer treatments, the most of the side effects could be minimized using a proper dosimeter. Gel dosimeter is the only three-dimensional dosimeter and magnetic resonance imaging (MRI) is the gold standard method for gel dosimeter readout. Because of hard accessibility and high cost of sample reading by MRI systems, some other alternative methods were developed. The optical computed tomography (OCT) method could be considered as the most promising alternative method that has been studied widely. In the current study, gel dosimeter scanning using a prototype optical scanner and validation of this optical scanner was performed. Optical absorbance of the irradiated gel samples was determined by both of conventional spectrophotometer and the fabricated OCT system at 632 nm. Furthermore, these irradiated vials were scanned by a 1.5 T MRI. The slope of the curves was extracted as the dose-response sensitivity. The R2-dose sensitivity measured by MRI method was 0.1904 and 0.113 for NIPAM and PAGAT gels, respectively. The optical dose sensitivity obtained by conventional spectrophotometer and the fabricated optical scanner was 0.0453 and 0.0442 for NIPAM gels and 0.0244 and 0.0242 for PAGAT gels, respectively. The scanning results of the absorbed dose values showed that the new OCT and conventional spectrophotometer were in fair agreement. From the results, it could be concluded that the fabricated system is able to quantize the absorbed dose values in polymer gel samples with acceptable accuracy.

Optical and NMR dose response of N-isopropylacrylamide normoxic polymer gel for radiation therapy dosimetry

BackgroundApplication of less toxic normoxic polymer gel of N-isopropyl acrylamide (NIPAM) for radiation therapy has been studied in recent years. AimIn the current study the optical and NMR properties of NIPAM were studied for radiation therapy dosimetry application. Materials and methodsNIPAM normoxic polymer gel was prepared and irradiated by 9MV photon beam of a medical linac. The optical absorbance was measured using a conventional laboratory spectrophotometer in different wavelengths ranging from 390 to 860nm. R2 measurements of NIPAM gels were performed using a 1.5T scanner and R2–dose curve was obtained. ResultsOur results showed R2 dose sensitivity of 0.193±0.01s−1Gy−1 for NIPAM gel. Both R2 and optical absorbance showed a linear relationship with dose from 1.5 to 11Gy for NIPAM gel dosimeter. Moreover, absorbance–dose response varied considerably with light wavelength and highest sensitivity was seen for the blue part of the spectrum. ConclusionOur results showed that both optical and NMR approaches have acceptable sensitivity and accuracy for dose determination with NIPAM gel. However, for optical reading of the gel, utilization of an optimum optical wavelength is recommended.

MAGIC-f Gel in Nuclear Medicine Dosimetry: study in an external beam of Iodine-131

MAGIC-f gel applicability in Nuclear Medicine dosimetry was investigated by exposure to a 131I source. Calibration was made to provide known absorbed doses in different positions around the source. The absorbed dose in gel was compared with a Monte Carlo Simulation using PENELOPE code and a thermoluminescent dosimetry (TLD). Using MRI analysis for the gel a R2-dose sensitivity of 0.23 s−1Gy−1was obtained. The agreement between dose-distance curves obtained with Monte Carlo simulation and TLD was better than 97% and for MAGIC-f and TLD was better than 98%. The results show the potential of polymer gel for application in nuclear medicine where three dimensional dose distribution is demanded.

Evaluation of the radiation-sensitizer/protector and/or antioxidant efficiencies using Fricke and PAG dosimeters

In this study, our aim is to assess the potential of Fricke and polyacrylamide gel (PAG) dosimeters to quantitatively evaluate the efficiency of potential radiation sensitizers/protectors and antioxidants. These compounds are of importance in radiotherapy as well as in disease prevention and promotion of health. The basic principle of the Fricke dosimeter is the radiation-induced oxidation of Fe2+ to Fe3+ in an aerated aqueous 0.4 M H2SO4. The production of ferric ions is most sensitive to the radical species produced in the radiolysis of water. Using this method, we observed that cystamine (one of the best of the known radioprotectors) can prevent oxydation of Fe2+ from reactive radiolysis species. However, one obvious disadvantage of the Fricke dosimeter is that it operates under highly acidic conditions (pH 0.46), which may degrade biological compounds. In contrast, the pH of the polyacrylamide gel (PAG) dosimeter is almost neutral, such that degradation of compounds is less probable. A change in R2-dose sensitivity was observed in the presence of radiosensitizers/radioprotectors and antioxidants. The protective effect of Trolox (a well-known antioxidant) and thiourea (a radioprotector) was readily observed using the PAG dosimeter. Incorporation of iodinated radiation sensitizers such as NaI and an iodine contrast agent led to a quantifiable sensitizer enhancement ratio. These studies suggest that the Fricke and the PAG dosimeters have the potential to evaluate the efficiency of radiation sensitizers/protectors and antioxidants.

Response verification of dose rate and time dependence of PAGAT polymer gel dosimeters by photon beams using magnetic resonance imaging

The purpose of this study was to evaluate dependence of PAGAT polymer gel dosimeter 1/T2 on different post time imaging as well as on different mean dose rates for a standard clinically used Co-60 therapy unit and an electa linear accelerator. Using MRI, the formulation to give the maximum change in the transverse relaxation rate R2(1/T2) was determined to be 4.5% N,N'-methylen-bis-acrylamide(bis), 4.5% acrylamid(AA), 5% gelatine, 5 mM tetrakis (hydroxymethyl) phosphonium chloride (THPC), 0.01 mM hydroquinone (HQ) and 86% HPLC(Water).When the preparation of final polymer gel solution is completed, it is transferred into phantoms and allowed to set by storage in a refrigerator at about 4°C. The sensitivity of the dosimeter was represented by the slope of calibration curve in the linear region measured for each modality. A calibration curve (in the linear region) based on 16 dosimeters (15 irradiated and one background) was obtained for 1.25 MV photon beam. To determine the calibration curve of the PAGAT polymer gel dosimeter, there are two linear responses between 2–10Gy and 10–30Gy and the R2-dose sensitivity showed stability with imaging post time after 38 days. Dose rate of dependence was studied in 6 MV photon beam with the use of dose rates 80, 160, 240, 320, 400 and 480 cGy/min. Evaluation of dosimeters were performed on Siemens Symphony, Germany 1.5T Scanner in the head coil. A multi echo sequence with 32 equidistant echoes was used for the evaluation of irradiated polymer gel dosimeters. The parameters of the sequence were as follows: TR 3000ms, TE 20ms, Slice Thickness 4 mm and FOV 256mm. No trend in polymer-gel dosimeter 1/T2 dependence was found on mean dose rate for photon beams.

An evaluation of the dosimetric performance characteristics of N-vinylpyrrolidone-based polymer gels

The aim of this work was to investigate the dosimetric performance properties of the N-vinylpyrrolidone argon (VIPAR) based polymer gel as a dosimetric tool in clinical radiotherapy. VIPAR gels with a larger concentration of gelatin than the standard recipe were manufactured and irradiated up to 68 Gy using a 6 and 18 MV linear accelerator. Using MRI, the R2-dose response was recorded at different imaging sessions within a 34 day time period post-irradiation. The R2-dose response was found to be linear between 5 and 68 Gy. Although dose sensitivity did not show significant variation with time, the measured R2-dose values showed an increasing trend, which was less evident beyond 17 days. At one day post-irradiation, calculated dose standard uncertainties at 20 Gy and 56 Gy were 2.2% and 1.7%, providing a dose resolution of 0.45 Gy and 0.97 Gy, respectively. Although these values fulfilled the 2% limit of ICRU, when gels were imaged at one day post-irradiation, it was shown that the temporal evolution of the R2 values deteriorated the per cent standard uncertainty and the dose resolution by ∼57%, when imaged 17 days post-irradiation. Variation in the coagulation temperature of the gels did not impact the R2-dose sensitivity. This study has shown that the VIPAR gel has the properties of a dosimetric tool required in clinical radiotherapy, especially in applications where a wide dose dynamic range is employed. For results with the lowest per cent uncertainty and the optimum dose resolution, the dosimetry gels used in this work should be MR scanned at one day post-irradiation. Furthermore, a preliminary study on the R2-dose response of a new normoxic N-vinylpyrrolidone-based polymer gel showed that it could potentially replace the traditional VIPAR gel formulation, while preserving the wide dynamic dose response inherent to that monomer.

Investigation of vacuum pumping on the dose response of the MAGAS normoxic polymer gel dosimeter

The effect of vacuum pumping on the dose response of the MAGAS polymer gel dosimeter has been investigated. A delay of several days post-manufacture before irradiation was previously necessary due to the slow oxygen scavenging of ascorbic acid. The MAGAS polymer gel dosimeter was vacuum pumped before gelation to remove dissolved oxygen. The MAGAS polymer gel dosimeter was poured into glass screw-top vials, which were irradiated at various times, post-manufacture to a range of doses. Magnetic resonance imaging techniques were used to determine the R2-dose response and R2-dose sensitivity of the MAGAS polymer gel. The results were compared with a control batch of MAGAS polymer gel that was not vacuum pumped. It was shown that vacuum pumping on the MAGAS polymer gel solution immediately prior to sealing in glass screw-top vials initially increases the R2-dose response and R2-dose sensitivity of the dosimeter. An increase in the R2-dose response and R2-dose sensitivity was observed with increasing time between manufacture and irradiation. Over the range of post-manufacture irradiation times investigated, the greatest R2-dose response and R2-dose sensitivity occurred at 96 hours.

Dose resolution optimization of polymer gel dosimeters using different monomers

Polymer gel dosimeters of different formulations were manufactured from different monomers of acrylamide, acrylic acid, methacrylic acid, 1-vinyl-2-pyrrolidinone, 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate. Gelatin and agarose were used as the gelling agents and N,N '-methylene-bis-acrylamide was used as a co-monomer in each polymer gel dosimeter. The T2 dependence of each dosimeter was analysed using a model of fast exchange of magnetization. The influence of the half-dose and the apparent T2 of the polymer proton pool on the dose resolution (DΔp) were examined. Comparisons are made with the commonly employed R2–dose sensitivity. Differences exist suggesting that experiments reported in the literature using what were thought to be more optimal dosimeters may not actually be so. Based on DΔp of each formulation, conclusions are drawn on the optimal formulation required for a specific range of absorbed doses. In addition, information about the extent of polymerization of the monomers used along with some characteristics of the polymer network formed are reported. The influence of the concentration of monomers and gelling agent was subsequently evaluated using a model of fast exchange of magnetization. Based on these calculations, further improvement in DΔp can be expected.