|Year : 2018 | Volume
| Issue : 2 | Page : 308-313
Dosimetric properties of new formulation of PRESAGE® with tin organometal catalyst: Development of sensitivity and stability to megavoltage energy
Davood Khezerloo1, Hassan Ali Nedaie2, Abbas Takavar1, Alireza Zirak3, Bagher Farhood1, Nooshin Banaee4, Eisa Alidokht5
1 Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
2 Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences; Radiotherapy Oncology Department, Cancer Research Centre, Cancer Institute; Tehran, Iran
3 Laser and Optics Research School, NSTRI, Tehran, Iran
4 Department of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran
5 Department of Optics and Photonics, Shahid Beheshti University, Tehran, Iran
|Date of Web Publication||8-Mar-2018|
Dr. Hassan Ali Nedaie
Radiotherapy Oncology Department, Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran. Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran
Source of Support: None, Conflict of Interest: None
Aim: Tin-base catalyst is one of the widely used organometallic catalysts in polyurethane technology. The purpose of this study was to evaluate the effect of tin organometallic catalyst in the radiation response and radiological properties of a new formula of PRESAGE®.
Materials and Methods: In the study, two types of PRESAGE were fabricated. A very little amount of dibutyltindillaurate (DBTDL) (0.07% weight) was used as a catalyst in the fabrication of new PRESAGE (i.e., PRESAGE with catalyst), which components were: 93.93% weight polyurethane, 5% weight tetrachloride, and 1% weight leucomalachite green (LMG). For PRESAGE without catalyst, 94% weight polyurethane, 4% weight tetrachloride, and 2% weight LMG were used. Radiochromic response and postirradiation stability of PRESAGEs were determined. Also, radiological characteristics of PRESAGEs, such as mass density, electron density, mass attenuation coefficient, and mass stopping power in different photon energies were assessed and compared with water.
Results: The absorption peak of new PRESAGE compared to PRESAGE without catalyst was observed without change. Sensitivity of new PRESAGE was higher than PRESAGE without catalyst and its stability after the first 1 h was relatively constant. Also, Mass attenuation coefficient of new PRESAGE in energy ranges <0.1 MeV was 10% more than water, whereas the maximum difference of mass stopping power was only 3%.
Conclusions: Tin organometallic catalyst in very low concentration can be used in fabrication of radiochromic polymer gel to achieve high sensitivity and stability as well as good radiological properties in the megavoltage photon beam.
Keywords: Dosimetry, PRESAGE®, radiation, three-dimensional dosimetry
|How to cite this article:|
Khezerloo D, Nedaie HA, Takavar A, Zirak A, Farhood B, Banaee N, Alidokht E. Dosimetric properties of new formulation of PRESAGE® with tin organometal catalyst: Development of sensitivity and stability to megavoltage energy. J Can Res Ther 2018;14:308-13
|How to cite this URL:|
Khezerloo D, Nedaie HA, Takavar A, Zirak A, Farhood B, Banaee N, Alidokht E. Dosimetric properties of new formulation of PRESAGE® with tin organometal catalyst: Development of sensitivity and stability to megavoltage energy. J Can Res Ther [serial online] 2018 [cited 2019 Nov 15];14:308-13. Available from: http://www.cancerjournal.net/text.asp?2018/14/2/308/183550
| > Introduction|| |
With the advent of new radiotherapy techniques, such as intensity-modulated radiotherapy (IMRT), stereotactic radiosurgery, and volumetric modulated arc therapy (VMAT), the demand for an accurate, feasible three-dimensional (3D) dosimetry system has been increased. To date, treatment verification methods in IMRT techniques have been implemented by the allocation of point dosimeters, such as diode, ionization chamber, and the two-dimensional dosimeters; however, 3D verification is still associated with many challenges.,,,, Recently, a new generation of solid polymer gels was introduced which demonstrate a radiochromic response to ionizing radiation known as PRESAGE®. Some special features of this solid dosimeter make it an attractive candidate to be used as a robust 3D dosimeter. It can be fabricated in any desired shape without any container.,, It is also tissue equivalent, not sensitive to oxygen and its response is independent on the room temperature, energy, and dose rates.,
The 3D matrix of PRESAGE® is polyurethane, which is widely used in medical equipment, construction of coating equipment, adhesive, and sealant. Furthermore, the dose recorder part of PRESAGE® is a kind of leuco-dye such as leucomalachite green (LMG) and a free radical initiator (RI) such as chloroform or carbon tetrachloride. Generally, for designation of an optimal leuco-dye and RIs, four factors should be considered: Stability during fabrication, overall radiation sensitivity imparted to the dosimeter, and pre- and post-irradiation stability of dosimeter.
Each ideal dosimeter should have a set of inherent characteristics to predict tissue dose accurately. In other words, each dosimeter should be a tissue equivalent; so for this reason, its physical characteristics, such as mass density, electron density, effective mass number, energy attenuation coefficients, stopping power and the scattering power of electrons should be the same as the tissue.,, The main elements of PRESAGE® are carbon, hydrogen, nitrogen, oxygen, and halogen, with the different weight fraction. The components and weight fraction of each part of PRESAGE® can have influence on being-tissue equivalent of dosimeter. Further studies were focused on the effect of the PRESAGE® components on dosimetric properties.,,,, By changing the type and concentration of leuco-dyes, RI, and maybe catalyst, PRESAGE® with different dosimetric properties will be fabricated. Since the effective atomic number of leuco-dye was very close to that of the polyurethane and according to the relatively low concentration of the material used, its effect on the density and atomic number of PRESAGE® is low when compared with other compounds. Therefore, the main factors that affect the density and atomic number are the type and concentration of the RI as well as the catalyst. However, a good compromise should be established between the concentration of PRESAGE® components and sensitivity, stability, linearity, and other dosimetric features.,,,,
In this study, a new formula of PRESAGE® was fabricated with a given weight fraction of component as well as tin organometallic catalyst that lead to increase in the sensitivity and improve stability. Then, dosimetric characteristic was investigated and compared with water and PRESAGE® without organometallic catalyst.
| > Materials and Methods|| |
Fabrication of new PRESAGE®
Generally, weight fraction of each part of new PRESAGE® was selected in such a way that the sensitivity of the dosimeter is relatively high and effective atomic number close to tissue. The effect of the concentration of carbon tetrachloride in PRESAGE® is more than other components, but because of its high atomic number, even a slight increment in concentration of it led to the strong growth of effective atomic number of new PRESAGE®. In this study, two types of PRESAGE® with different weight fraction fabricated. The first type was with organometallic catalyst (new PRESAGE). The weight fraction of polyurethane was selected at 93.93% of the total weight of PRESAGE, which is prepared by a combination of one equivalent of polyol (crystal clear 206, Part B) with two equivalent of a prepolymer (Crystal Clear 206, Made by Smooth-On Inc., Part A). For the dose reporter, 1% LMG was thoroughly dissolved in 5% CCl4 and then added to Part B; finally, the blend was added to the Part A and was thoroughly mixed to obtain a homogenous solution. For catalyst, 0.07% of DBTDL catalyst (C32H64O4 Sn) was added to the solution and finally it was poured in cuvettes, and then stored at 60 psi for 72 h. All process was performed in semi-dark condition and at room temperature (22–25°C). It is notable that effective atomic number was calculated by Mayneord formula:
The second type was a PRESAGE® without organometallic catalyst. It consist of 94% polyurethane, 2% LMG, 4% CCl4; so that these amount have been reported the literature, because of appropriate tissue equivalent and good dosimetric properties.
A Plexiglas container was designed to facilitate irradiation process [Figure 1]. To obtain the responses and calibration curves of both types of PRESAGE, the cuvettes were exposed at the step doses of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 Gy in 2.5 cm depth in the Plexiglas phantom, 10 cm × 10 cm field size, source to skin distance = 100 and dose rate of 300 MU/min with 6 MV photon beam energy emitted from a Siemens Primus accelerator (Siemens AG, Erlangen, Germany). It should be noted that for each dose step, three cuvettes were exposed. In order to study of both types of PRESAGE PRESAGE (i.e., with catalyst) stability, 3 cuvettes were exposed to 4 Gy in the same irradiation setup, then optical density was acquired in time intervals 0, 1, 2, 4, 6, 8, 24, 36, 48, and 72 h. Irradiation cuvettes were stored in a room temperature in the dark condition for 5 days. After the 5 days, the 3 cuvettes that were already irradiated were re-irradiated again at 4 Gy in the same setup and the optical density was acquired again in the same time intervals.
Measuring the optical absorption properties
Optical characterization of both types of PRESAGE was obtained by investigating the absorption spectrum with Varian Cary 50 spectrophotometer in the wavelength range of 400–800 nm.
Measuring the radiation transport parameters
To compare photon attenuation and stopping the power of PRESAGE types with water, photon cross section data were obtained using NIST XCOM X-ray attenuation database and the total stopping power was calculated from NIST ESTAR database.
| > Results|| |
[Table 1] shows the characteristics of PRESAGE types (new PRESAGE and PRESAGE without catalyst) compared to water. The difference of mass density and effective atomic number between new PRESAGE and water was 10% and 4%, respectively; and the PRESAGE without catalyst were 4.8% and 0.5%, respectively. The absorption peak of the both types of PRESAGE in some different doses normalized by reference cuvette is as shown in [Figure 2]. It is clearly observed that the absorption peak at different doses was in the range of 627–635 nm.
|Table 1: Effective atomic number, mass density and relative electron density to water of PRESAGE types|
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|Figure 2: Absorption spectrum of PRESAGE with catalyst in different doses. The peak was observed in the range of 625–635 nm in all doses. The curves were set by the reference cuvette to get net absorption data|
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Good linear correlation coefficients were obtained related to responses of both types of PRESAGE in the dose range of 0–20 Gy; so that for new PRESAGE and PRESAGE without catalyst were R2 = 0.97 and R2 = 0.99, respectively. Furthermore, the calibration curves of both types PRESAGE were shown in [Figure 3]; so that the sensitivity slopes for new PRESAGE and PRESAGE without catalyst were obtained 0.014 OD/cm/Gy and 0.008 OD/cm/Gy, respectively.
|Figure 3: Comparison of sensitivity of new PRESAGE (dotted-line) with PRESAGE without catalyst (solid-line)|
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[Figure 4] illustrates the stability of the PRESAGE types after the first and second 4 Gy irradiation. For new PRESAGE, the highest postirradiation color fading was observed in the first 1 h after irradiation as in this time the optical density relatively grew up to 20%, but after that, radiochromic postresponse remained constant until 72 h. The response of this PRESAGE that has been re-irradiated with 4 Gy after 5 days was similar to the response of the PRESAGE that exposed in a single 8 Gy; on the other hand, with the irradiation of the new PRESAGE that was already irradiated, after 5 days if the dose is doubled, the changes in optical density is also doubled. In this case, no postirradiation color fading was observed even in the 1st h. For the PRESAGE without catalyst, the highest postirradiation color fading was observed in the first 6 h after irradiation as in this time the optical density relatively grew up to 35%.
|Figure 4: Postresponse of PRESAGE types at the first and second irradiation. Dashed line represents stability of new PRESAGE after first and second irradiation and solid line shows stability of the PRESAGE without catalyst irradiated after 5 days|
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The mass attenuation coefficient and mass stopping power ratio of both types of PRESAGE were compared to water and the results were shown in [Figure 5] and [Figure 6]. The maximum discrepancy of the total mass attenuation coefficient between new PRESAGE and water was obtained 10% in the energies between 0.01 and 0.1 MeV whereas there was no significant difference between water and PRESAGE without catalyst in this range of energies (difference was <3%). However, in the energies after 0.1 MeV, the discrepancy between PRESAGE types was <2%. The mass stopping power was relatively constant since the maximum discrepancy between new PRESAGE and water at energies of about 1 MeV was <2%, even though after 1 MeV energy, the discrepancy between water and the PRESAGE types relatively increased to 3%.
|Figure 5: Mass stopping power ratio in photon interaction for new PRESAGE, PRESAGE without catalyst and water|
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|Figure 6: Mass attenuation coefficient in photon interaction for new PRESAGE, PRESAGE without catalyst and water|
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| > Discussion|| |
One of the methods to increase the sensitivity of PRESAGE dosimeter is an increment of halocarbon concentration, but excessive increment will lead to oxidation of LMG which continues in even hours after irradiation; however, this phenomenon also occurs at low concentrations of halocarbon. Furthermore, organometallic catalyst can cause increased sensitivity of gel too, because the metal component is bonded to halocarbon and since they include a high atomic number (ZSn= 50); therefore, the probability of production of secondary electrons due to the interaction of ionizing radiation increases and consequently the production of free radicals in halocarbon and oxidation of LMG increases. In this study, the high percentage of carbon tetrachloride was used to increase PRESAGE dosimeter sensitivity, but high concentration of free RI can reduce the stability. By adding a very small concentration of organometal catalyst, PRESAGE dosimeter sensitivity remained high. In addition, the dosimeter stability remained high as well.
One of the most widely used catalysts in industry for manufacturing polyurethane is the tin organometallic catalysts. This catalyst includes carboxylate groups (RCO−2) that act as a single oxygen scavenger which prevents the oxidation of LMG and consequently increases the poststability of the PRESAGE®. It seems that by the excessive increment of the concentration of organometallic catalysts, the stability of the new PRESAGE® also increases steadily. However, it should be noted that the effective atomic number and mass density of PRESAGE® strongly depend on the concentration of organometallic catalyst. The high concentration of metal catalyst as well as acts like a layer of shield against free radicals and secondary electrons in PRESAGE®, thus lead to further reduction in the sensitivity of the gel. In this study, for postirradiation stability, a small amount of tin-base catalyst was used which not only accelerated in the polymerization of the PRESAGE® and reduction of the fabrication time also increased its postirradiation stability. Stability after re-irradiation of the new PRESAGE with tin-catalyst after 5 days however was seen with different behavior, such that there was not any color bleaching in postresponse after second irradiation even in the 1st min. As another result, the sensitivity of the new PRESAGE in this case during postirradiation remained without change.
Recently, studies have focused on the effect of PRESAGE® components type and concentration on its dosimetric properties. Mostaar et al. reported a very good dose response linearity in 1 and 2% weight concentration of LMG and by increasing CCl4 up to 20% weight, the sensitivity of PRESAGE® improved, but after being increased up to 30% weight, significant sensitivity enhancement was not observed. However, they showed that with increase in RI, the poststability of PRESAGE® is reduced. PRESAGE® with 20% weight RI and 2% weight LMG showed the lowest postirradiation stability, whereas 5% weight RI and 1% or 2% LMG demonstrated a good stability. Eznaveh et al. also investigated the sensitivity of the PRESAGE® with the change concentration of LMG from 2% to 6%, and CCl4 from 28% to 32% as RI. They concluded that ideal PRESAGE® with high sensitivity was obtained as LMG = 4% and CCl4 =32% concentration. Moreover, the stability of the PRESAGE® response until 9 days after irradiation showed that this PRESAGE composition had an acceptable stability. However, they did not survey other dosimetric properties. In a comprehensive study, Alqathami et al. investigated the influences of very small fraction (<0.1% weight) of three commercially available bismuth (Bi Neo), tin (DBTDL), and zinc (Zn Oct) metal compounds on dosimeter characteristics of PRESAGE®, that its RI was chloroform. The chemical composition of PRESAGE® consists of LMG (2% weight), crystal clear 206/catalyst (93% weight), and chloroform (5% weight) as RI. Bi Neo showed higher sensitivity than with DBTDL and Zn Oct at the same concentrations. Nevertheless, because of the high atomic numbers of the metal atoms of Bi: 83; Sn: 50; Zn: 30, the Zeff number for all gels was higher than water.
For assessment of attenuation coefficient of PRESAGE types and water, it is notable that approximately more than 60% of the elemental composition of the PRESAGE® are carbon, which has a significantly lower attenuation coefficient when compared with oxygen in low photon energies (0.01–0.1 MeV), with predominant photoelectric absorption. However, the probability of photoelectric interactions is also proportional to the atomic number (Z) by Z3 and is predominantly in the kilovoltage energy range (0.01–0.1 MeV), so conceivably, a peak in attenuation coefficient will be justifiable since the mass density of the new PRESAGE was 10% higher than the water [Figure 5]. Hence, to use PRESAGE in kilovoltage energies, it is necessary to define a correction factor. In addition, the Compton scattering is independent of the atomic number (Z) of the absorbing material since the process involves only free electrons, so it just depends on electron per gram of the material. Indeed, electron per gram of water is higher than both types of PRESAGE, so in the range of energy that Compton scattering is predominant (>0.1 MeV), the attenuation coefficient of the both types of PRESAGEs was slightly lower than water [Figure 5].
Moreover, mass stopping power indicates the energy lost in the density of electron field in the medium, thereby the low Z material have relatively higher stopping power. In this study, the effective atomic number of new PRESAGE and PRESAGE without catalyst was obtained 4 and 1% higher than water.
| > Conclusions|| |
In this study, the effect of tin organometallic catalyst in the radiation response and radiological properties of a new formula of PRESAGE was evaluated. In general, inherent unique features of new PRESAGE, such as, linearity, dose, and dose rate independence, oxygen insensitivity, tissue equivalent, and low variations of response in room temperature (22–50°C) as well as high level of stability in this gel formula; it takes a special place in 3D dosimetry in new complex and precise techniques of radiotherapy, such as IMRT, cyber knife, and VMAT.
This article is based on the data extracted from the Ph.D. Desecration Code No. 25118 presented to Medical Physics Department of Tehran University of Medical Sciences.
Financial support and sponsorship
The authors are grateful for the financial support provided by Tehran University of Medical Sciences, Tehran, Iran. This work is done at Cancer Institute and the authors would like to thank the staff of Radiotherapy Physics Dept. for their valuable assistance.
Conflicts of interest
There are no conflicts of interest.
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