|Year : 2018 | Volume
| Issue : 2 | Page : 287-291
Dose rate and energy dependence study of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel with flattened and unflattened photon beams
P Sathiyaraj, James Jebaseelan Samuel
Department of Physics, Medical Gel Dosimetry Laboratory, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India
|Date of Web Publication||8-Mar-2018|
Mr. P Sathiyaraj
Nuclear and Medical Physics Division, VIT University, Vellore, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Aims: The aim of this study is to evaluate the dose rate and energy dependency of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride (MAGAT) gel in unflattened photon beam using X-ray computed tomography (CT) and ultraviolet (UV)-visible spectroscopy.
Materials and Methods: MAGAT gel was prepared and it was exposed to 6 MV flattened and unflattened beams. The dose selected for irradiation was ranging from 3 to 15 Gy with an increment of 3 Gy. The dose rate dependency of the gel was investigated by exposing the gel to three different dose rates of 250, 500, and 1500 cGy/min for flattening filter free (FFF). To verify the energy dependency of the gel, it was exposed by both FFF and flattening filter (FF) for constant dose rate (250 cGy/min) and different energy (6 and 10 MV X-ray photons). The exposed gels were scanned by X-ray CT and UV-visible spectrophotometer.
Results: The change in dose sensitivity observed over the dose rate from 250 cGy/min to 1500 cGy/min was 58.00% and 57.89% using a UV-visible spectrophotometer and X-ray CT analysis method. Energy dependency was evaluated with respect to dose sensitivity and the variation between 6 MV FF and FFF photon beams was found to be 2.20% and 2.21% using UV-visible spectrophotometer analysis and X-ray CT, respectively. Similarly, the variation noticed with 10 MV FF and FFF was 2.30% using UV-visible spectrophotometer analysis and 2.22% using X-ray CT analysis.
Conclusions: The results clearly show that the MAGAT gel was highly dose rate-dependent and less dependent on energy. The beam quality variation between FF and FFF was less. The similar results obtained using X-ray CT scanner and UV-visible spectrophotometer indicate that this study can be recommended for polymer gel scanning procedure.
Keywords: Dose rate, methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel, ultraviolet-visible spectroscopy, X-ray computed tomography
|How to cite this article:|
Sathiyaraj P, Samuel JJ. Dose rate and energy dependence study of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel with flattened and unflattened photon beams. J Can Res Ther 2018;14:287-91
|How to cite this URL:|
Sathiyaraj P, Samuel JJ. Dose rate and energy dependence study of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel with flattened and unflattened photon beams. J Can Res Ther [serial online] 2018 [cited 2020 Jul 14];14:287-91. Available from: http://www.cancerjournal.net/text.asp?2018/14/2/287/191033
| > Introduction|| |
The flattening filter (FF), an integral part in the treatment head of a standard linear accelerator (linac), substantially reduces the dose rate and considered to be the major source of head scattered photons that cause the difference of in-air output with field size. Furthermore, FF beam delivers uniform fluence pattern and homogeneous dose distribution across the beam. The removal of FF from linac has major advantages such as operating at higher dose rates, nonuniform beam profile, and reduction in head-scattered radiation. The increase in dose rate can shorten the beam-on time, thereby enhancing the patient throughput.
The advent of modern radiotherapy techniques such as intensity-modulated radiotherapy and stereotactic radiosurgery/radiotherapy has stimulated a growing attention in operating medical linacs in an FF-free (FFF) mode where varying fluence and inhomogeneous dose distributions are indispensable. Uniform dose distribution across the beam is not necessary for executing these techniques. These interesting factors instigate to operate the linac with FFF mode in radiotherapy applications. The advantages of FFF beam over FF in many aspects are sharp penumbra, less head scatter, and less out-of-field dose.
This study presents the evaluation of the dependency of polymer gel dosimeter with dose rate and energy. The polymer gel is one of the best three-dimensional (3D) dosimeters than other available dosimeters such as ion chamber, film, diode, and thermoluminescence dosimeter. The advantages of gel dosimeter are tissue equivalency, angular independency, providing 3D dose information with high spatial resolution, and availability of many readout systems. In 1950, Day and Stein used the radiosensitive gels for radiation dosimetry purpose. Later, Fricke and polymer gel dosimeters were developed. Diffusion of ferric ion from the irradiated region is the major drawback of Fricke gel, and this limitation is not in polymer gel.
In this study, methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride (MAGAT), which is methacrylic acid (MAA)-based gel, has been chosen as the dosimeter as it is more water equivalent than the other MAA family gel. MAGAT comes under the polymer gel dosimeter type, and the whole process contains three steps:First, preparation of gel using radiosensitive chemicals; second, irradiation of the gel using gamma or X-ray photons; and third, extract the dose information using different scanning methods such as optical computed tomography, X-ray computed tomography (CT), magnetic resonance imaging, and ultraviolet (UV)-visible spectroscopy. Many authors have reported the dose rate dependency of polymer gel for FF.,,,, In this study, the dose rate and energy dependency of polymer gel in FFF beam have been reported and compared with FF photon beams.
| > Materials and Methods|| |
Elekta Versa HD linac was used for this study; it is capable of delivering 6 and 10 MV FFF and FF beams. Each of the FFF energies has its own independent energy set different from any flattened beam; this leads to beam quality of FFF beam restored to that nominal value for that energy. If the penetrative quality of FFF and FF is same, then it may not affect the dosimetric parameters such as energy and dose rate dependency. In this study, two different gel readout modalities such as CT and UV-visible spectrophotometer were used.
MAGAT gel was prepared under normal atmospheric condition. The ingredients used for the preparation of MAGAT are shown in [Table 1]. Initially, gelatin powder was sprinkled little by little on the water surface. After gelatin gets deposited in the bottom of the water container, it was soaked for 15 min. After that, the container was placed over the magnetic stirrer. The gelatin was mixed with water, and 50°C temperature was maintained. The clear solution indicates the complete melting of gelatin, and the temperature of the gel solution was reduced to 35°C. At this stage, MAA was added and the solution was stirred continuously for 10 min. The temperature was further reduced to 30°C and tetrakis (hydroxymethyl) phosphonium chloride (THPC) was added. Stirring was continued till the entire process of preparation of gel solution. In the MAGAT gel, MAA acts as a monomer which is able to form a polymer network.
|Table 1: Methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride composition and their role on polymerization|
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After the polymerization, gelatin matrix enables to localize the exposed region. The oxygen which inhibits the growth of polymerization can be removed by adding THPC to the solution. Water plays a major role in the gel as it is a source of free radicals through radiolysis process. The plastic cuvettes were filled with the prepared gel solution and covered by polythene sheet. The cuvettes were placed in the refrigerator for 12 h to solidify the gel solution.
A wax phantom was fabricated to place the gel cuvettes to provide uniform dose distribution. The dimension of the wax phantom was 20 cm × 20 cm and 4-cm thickness. Slab phantoms were used for backscattering and buildup purpose where 9 cm was used for backscattering and 1 cm slab for buildup thickness. The whole phantom setup was scanned by X-ray CT and the images were transferred to the Monaco treatment planning system (TPS). In Monaco TPS, plans with two parallel opposed beams were generated and the dose was prescribed to the center of the gel cuvettes.
The delivered dose was ranging from 3 to 15 Gy with an increment of 3 Gy. Before irradiation, the gel cuvettes were kept in the treatment room for 2 h to obtain thermal equilibrium. Gel was irradiated after 14 h of preparation (12 h for gel solidification +2 h to obtain thermal equilibrium). The dose rate dependency was evaluated by exposing the gel to three different dose rates, such as 250, 500, and 1500 cGy/min, for FFF and the energy dependency was evaluated for 6 and 10 MV of FFF and FF beams by exposing the gel with the constant dose rate of 250 cGy/min.
The exposed gels were placed in the refrigerator for 12 h and the temperature was maintained at 4°C. The complete polymerization of the gel was achieved during this period. The postirradiation scan was done after attaining the thermal equilibrium by keeping the polymerized gel for 2 h in the scanning room. The scanning of gel was performed with two different modalities such as X-ray CT (12 KvP, 200 mA, and 3-mm slice thickness) and UV-visible spectroscopy (1 nm interval). The scanned data were transferred to TPS and the CT – numbers of the exposed gels were extracted for different doses. The unexposed gel was taken as a control. In a UV-visible spectrophotometer, gels were scanned from 400 to 800 nm (1 nm interval). The maximum absorbance values of the gel were noted for every gel cuvette.
| > Results|| |
Dose rate dependence
The dose rate dependency of the gel has been shown in graphs where [Figure 1] shows the plot between the mean CT number and absorbed dose with constant dose rates to obtain the dose sensitivity from the slop. [Figure 2] shows the plot between the mean CT number and different dose rate with fixed doses. The change in dose sensitivity over the dose rate from 250 to 1500 cGy/min was 57.89% with CT analysis.
|Figure 1: Variation of the mean computed tomography number of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride as a function of absorbed dose with different dose rate|
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|Figure 2: Dose rate dependency of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride gel with various doses|
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Absorbance values were obtained by a UV-visible spectrophotometer for different doses. At 400 nm, optical absorbance was higher when compared to other wavelengths. [Figure 3] shows the plot between the absorbance and absorbed dose with constant dose rate to obtain the dose sensitivity from the slop while [Figure 4] shows the plot between absorbance and different dose rates with fixed doses. The change in dose sensitivity over the dose rate from 250 to 1500 cGy/min was 58% with a UV-visible spectrophotometer. The R2 value of 0.99945 at 250 cGy/min indicates the good dose linearity that was superior to the other dose rates.
|Figure 3: Absorbance of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride as a function of absorbed dose with different dose rate|
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|Figure 4: Absorbance of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride versus dose rate with various doses|
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The energy dependency was represented by plotting a graph between the mean CT number and absorbed dose for fixed dose rate and is shown in [Figure 5]. Furthermore, the energy dependency was represented by plotting a graph between the absorbance and absorbed dose for fixed dose rate and is shown in [Figure 6]. The variation between 6 MV FF and FFF photon beams was found to be 2.20% using UV-visible spectrophotometer analysis, whereas using X-ray CT, the variation was 2.21%. Similarly, for 10 MV FF and FFF, it was 2.30% and 2.22% using UV-visible spectrophotometer analysis and X-ray CT analysis, respectively.
|Figure 5: Energy response of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride from the mean computed tomography number versus absorbed dose|
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|Figure 6: Energy response of methacrylic acid gelatin tetrakis (hydroxymethyl) phosphonium chloride from the absorbance versus absorbed dose|
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| > Discussion|| |
In MAGAT polymer gel, the generation of polymer network is caused by radicals through the process of radiolysis. These radicals are also involved in the termination process of polymerization. The rate of termination is proportional to the square of the concentration of the radicals present in the gel. To avoid the termination process in MAGAT gel, the rate of generation of radicals should be low to increase the rate of polymerization. As dose rate increases, the dose response of the MAGAT reduces due to the production of more radicals, and the growth of polymer chain gets terminated. In this investigation, a very high concentration of radicals due to the maximum dose rate (1500 cGy/min) leads to more termination process in MAGAT gel. De Deene et al. have reported that the changes in dose sensitivity in MAGAT were 66% for FF photon beams using with the dose rate from 30 to 400 cGy/min. In this study, the change in dose sensitivity over the dose rate from 250 to 1500 cGy/min was 58.00% and 57.89% using a UV-visible spectrophotometer and X-ray CT analysis method, respectively, indicating the dose rate dependency of the MAGAT gel. These results are consistent with the results reported by De Deene for MAGAT gel. The energy dependency of MAGAT gel was found to be slightly higher in 10 MV X-rays than 6 MV X-rays. Dose sensitivity variation between FF and FFF beams was within 3%.
The standard deviation between the two readout modalities (CT and UV) was found to be 0.077 in dose rate dependency study and 0.067 in energy dependency study for FF and FFF. The negligible variation observed indicates that both these modalities are capable to obtain the dose information of the gel. Both X-ray CT and UV-Visible spectrophotometer provide reliable results for the readout of the polymer gel, and it could be recommended for clinical applications.
| > Conclusions|| |
Energy dependency of a water-equivalent MAGAT gel has not shown any significant variation with both FF and FFF beams of 6 MV and 10 MV. Nonetheless, the dose rate dependency makes the MAGAT gel inferior to other gel dosimeters. This dosimetric property has to be considered while performing dosimetric measurements in clinical radiotherapy. Hence, further research work is required to make the MAGAT independent of dose rate and also to explore the potential use of MAGAT gel in clinical radiotherapy settings.
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Conflicts of interest
There are no conflicts of interest.
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