Dosimetric evaluation of PASSAG-U polymer gel dosimeter: Dependence of dose rate and photon energy

Abstract

OBJECTIVE:

Several physical factors such as dose rate and photon energy may change response and sensitivity of polymer gel dosimeters. This study aims to evaluate the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on dose rate and photon energy.

MATERIALS AND METHODS:

The PASSAG-U gel dosimeters were prepared under normal atmospheric conditions. The obtained gel dosimeters were irradiated to different dose rates (100, 200, and 300 cGy/min) and photon energies (6 and 15 MV). Finally, responses (R₂) of the PASSAG-U gel dosimeters with 3% and 5% urea were analyzed by MRI technique at 1, 10, 14 days after the irradiation process.

RESULTS:

The findings showed that the R₂-dose responses of PASSAG-U gel dosimeters with 3% and 5% urea do not vary under the differently evaluated dose rates and photon energies. The R₂-dose sensitivity of PASSAG-U polymer gel dosimeter with 3% urea does not change under the differently evaluated dose rates and photon energies, but it changes for PASSAG-U polymer gel dosimeter with 5% urea. The dose resolution values ranged from 0.20 to 0.86 Gy and from 0.27 to 2.20 Gy for the PASSAG-U gel dosimeter with 3% and 5% urea for the different dose rates and photon energies, respectively. Furthermore, it was revealed that the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on dose rate and photon energy can vary over post irradiation time.

CONCLUSIONS:

The study results demonstrated that dosimetric characteristics (dependence of dose rate and photon energy, and dose resolution) of PASSAG-U gel dosimeter with 3% were better than those of PASSAG-U gel dosimeter with 5% urea.

Keywords

Polymer gel dosimetry PASSAG-U urea photon energy dose rate

1 Introduction

Recent developments in radiation therapy techniques have resulted to create a conformal dose distribution in tumor target volume with lower dose values to surrounding normal tissues [1]. Because of complex dose distribution created in these modern radiotherapeutic, it is crucial to obtain three-dimensional (3D) dose distribution [2, 3]. Based on chemical mechanisms, the realistic 3D dosimetric systems are categorized in three group, including: a) ferric, b) polymer gel, and 3) radiochromic dosimeters [4].

In 1984, a ferrous sulfate gel dosimeter was used by Gore et al. for measurement of 3D dose distribution [5]. In these dosimetric systems, ferric ions diffuse over time and this effect can cause an inaccurate dose distribution; hence, dose reading must be immediately performed after irradiation process and it limits its applications and effectiveness [6 –8]. In 1993, Maryanski et al. introduced a type of polymer gel dosimeter (called as polyacrylamide gel) that its monomers were polymerized under radiation [6]. Thereafter, some researchers improved and optimized the polymer gel dosimeters.

Presence of toxic materials in the structure of polymer gel dosimeters (such as Acrylamide, Methacrylic acid, etc.) has led to limit their utilization in clinical applications [9, 10]. Several researchers have introduced monomers with less toxic. For example, the use of N-isopropylacrylamide monomer (with oral LD₅₀ of 375 mg kg^–1) in structure of NIPAM gel dosimeter was introduced by Senden et al. [11]. In another study, PAMPSGAT gel dosimeter (with oral LD₅₀ of 1830 mg kg^–1) was introduced by Abtahi [12]. In our recent study, we introduced PASSAG gel dosimeter with negligible toxicity (LD₅₀ > 16,000 mg/kg), as the safest polymer gel dosimeter so far [10]. The findings obtained from dosimetric evaluation of PASSAG gel dosimeter represented promising results for this gel dosimeter [10, 13].

In our recent study on optimization of PASSAG gel dosimeter, it was showed that the use of urea in structure of this gel dosimeter increased its R₂-dose sensitivity and the obtained new formula was named PASSAG-U (PASSAG and Urea) [14]. The urea, through further acceleration of the polymerization reaction of gel dosimeters, results to increase their R₂-dose sensitivity. However, R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeter on dose rate and photon energy have been not evaluated. For use of the PASSAG-U gel dosimeter in clinical applications, it is essential for evaluation of its dosimetric response in different physical conditions. Therefore, in the present study, R₂-dose response and sensitivity dependence of the PASSAG-U gel dosimeters on dose rate and photon energy were investigated by using MRI technique.

2 Materials and methods

2.1 Preparation process of polymer gel dosimeters

By considering the PASSAG-U gel dosimeter recipe presented by research group of Farhood et al. [14], the PASSAG-U gel dosimeters with 3% and 5% urea were prepared under normal atmospheric situation. The reason of this selection was that these two types of PASSAG-U gel dosimeters (3% and 5% urea) had more R₂-dose sensitivity than the PASSAG-U gel dosimeter with 1% urea; hence, the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on dose rate and photon energy were just evaluated in the present study.

The chemical components used to prepare the gel dosimeters and their concentrations are listed in Table 1.

Table 1
Chemical materials and their concentrations used in structure of PASSAG-U gel dosimeters with 3% and 5% urea

Substance type Chemical concentration (w)

Urea (Merck, Germany) 3.0 or 5.0±0.0 %

Deionized water (Obtained from water deionizer, Absaz Co., Iran) 86.0 or 84.0±0.8 %

Gelatin (from porcine skin, type A, 300 Bloom, Sigma Aldrich, USA) 5.0±0.0 %

AMPS (Merck,≤% 100) + NaOH (Merck, ≤%100) 3.0±0.0 %

Bis (≥99.5%, Sigma Aldrich, USA) 3.0±0.0 %

THPC (80% solution in water, Sigma Aldrich, USA) 10 mM

Substance type	Chemical concentration (w)
Urea (Merck, Germany)	3.0 or 5.0±0.0 %
Deionized water (Obtained from water deionizer, Absaz Co., Iran)	86.0 or 84.0±0.8 %
Gelatin (from porcine skin, type A, 300 Bloom, Sigma Aldrich, USA)	5.0±0.0 %
AMPS (Merck,≤% 100) + NaOH (Merck, ≤%100)	3.0±0.0 %
Bis (≥99.5%, Sigma Aldrich, USA)	3.0±0.0 %
THPC (80% solution in water, Sigma Aldrich, USA)	10 mM

The preparation process of PASSAG-U gel dosimeters was as follows: 1) 3% or 5% urea amount was completely dissolved in 80% of the water at room temperature; 2) the AMPS sodium salt (as monomer) was produced at room temperature; 3) the gelatin was swelled in the solution resulted from the first step for ten minutes, before its temperature increases to 50 °C; 4) at 48 °C, the Bis (as crosslinker) was added and completely dissolved in the solution resulted from the third step; 5) at 37 °C, the monomer was added to the mixture obtained from forth step; 6) at 35 °C, the THPC (as antioxidant) was blended with 10% of the remaining water, and was added to the final mixture. By visual inspection of the obtained PASSAG-U gel dosimeters, they were transparent and clear. Immediately after preparing the gel dosimeters, they were transferred into the vials with dimension of 6 cm length and 1.2 cm outer diameter. Then, the lids of these vials were closed with their caps and parafilm was used to seal them. The vials were finally stored for approximately 1 day at 4–5°C in a refrigerator.

2.2 Irradiation process of polymer gel dosimeters

A Siemens Primus linear accelerator (Siemens AG, Erlangen, Germany) installed in Yasrebi Radiation Oncology Center (Kashan, Iran) was used to irradiate the PASSAG-U gel dosimeters about 1 day after their preparation. The vials filled with the gel dosimeters were located in the central part and distance of 5 cm from the wall of a large water phantom (50×50×40 cm³). To stabilize the temperature of vials with room temperature, they were stored within the water phantom for 1 h before the irradiation process. The properties of the irradiation field include: field size of 20×20 cm², source to axis distance of 1 m, gantry angle of 90°. A 2–10 Gy dose range (by step of 2 Gy) was delivered to various separate vials and one vial was considered as reference/control vial (0 Gy). Figure 1 shows the experimental set-up to irradiate PASSAG-U gel dosimeters located inside the water phantom.

Fig.1

The experimental set-up to irradiate PASSAG-U gel dosimeters located inside the water phantom.

Before the irradiation process, we verified the absolute dose delivered to the vials by a calibrated ionization chamber (Farmer type, PTW, Germany). Also, the dose measurements were performed in accordance with Technical Reports Series No. 398 dosimetry protocol [15].

2.3 Evaluating the dose rate and photon energy dependence of polymer gel dosimeters

To investigate the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters on dose rate, they were irradiated by 6 MV photon energy at three dose rates of 100, 200, and 300 cGy/min. To evaluate the photon energy dependence, the gel dosimeters were irradiated by 6 and 15 MV photon energies at 200 cGy/min dose rate.

For each dose rate/photon energy, five vials were used and irradiated to the various dose values of 2, 4, 6, 8, and 10 Gy.

2.4 Reading process of polymer gel dosimeters

We used a 1.5 T MRI scanner (Siemens Avanto, Germany) to obtain the responses of unirradiated/irradiated PASSAG-U gel dosimeters during 1, 10, 14 days after the irradiation process. For this aim, we used from a standard RF head coil to record the signals. Also, we applied a 32-echo Carr–Purcell–Meiboom–Gill pulse sequence. It is notable that MRI scanning parameters used in the current study were similar to those used in our previous study [14], as listed in Table 2.

Table 2
MRI scanning parameters used in response reading process of PASSAG-U gel dosimeters

Features Properties

First echo time (TE) 22 ms

Space between echoes (ES) 22 ms

Repetition time (TR) 4000 ms

Matrix size 232×256

Feld of view 35.4×38.0 cm²

Slice thickness 2 mm

Number of averages 2

Pixel band-width 100 Hz

Features	Properties
First echo time (TE)	22 ms
Space between echoes (ES)	22 ms
Repetition time (TR)	4000 ms
Matrix size	232×256
Feld of view	35.4×38.0 cm²
Slice thickness	2 mm
Number of averages	2
Pixel band-width	100 Hz

The R₂ maps and R₂-dose curves of PASSAG-U gel dosimeters with 3% and 5% urea were obtained by the methods presented in our previous studies [10 , 14]. Briefly, an image processing program was first written in MATLAB (Mathworks, Natic, MA), and then the MRI images were imported to it. Then, R₂ maps were produced by fitting a mono-exponential decay curve to pixel intensities in the consecutive base images on a pixel-by-pixel basis. Finally, the R₂-dose curve was obtained by plotting the average calculated R₂ in the irradiated area of the gel samples as a function of the absorbed radiation dose.

2.5 R₂-dose sensitivity and dose resolution

The term of “R₂–dose sensitivity (α)” is specified as slope of the linear region of the gel dosimeter response (R₂) to absorbed dose values and it can be obtained by differentiating the R₂-dose curve [14].

The dose resolution value ( $D_{Δ}^{p}$ ) of PASSAG-U gel dosimeters in different dose values were obtained by following equation [16]: $D_{Δ}^{p} = k_{p} \sqrt{2} \frac{σ_{R_{2}}}{α}$ (1)

Where, σ_R2 is standard deviation resulted from R₂ values of gel dosimeters, and k_p is coverage factor.

3 Results and Discussion

3.1 Dose rate dependence

The R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on three dose rates (100, 200, and 300 cGy/min) were evaluated and illustrated in Fig. 2. The resulted presented in this section are related to 1-day post irradiation time.

Fig.2

R₂-dose curves of PASSAG-U polymer gel dosimeters with 3% (a) and 5% (b) urea as a function of dose value for 100, 200, and 300 cGy/min dose rates at 6 MV photon energy, scanning room temperature (18 °C), and 24 h after irradiation process.

According to the findings (Fig. 2), it was found that the R₂-dose responses of PASSAG-U gel dosimeters with 3% and 5% urea are independent of dose rate. There were 0.24–5.90% and 0.19–5.43% difference ranges between the R₂ values of PASSAG-U gel dosimeters with 3% and 5% urea, respectively, at the above-mentioned dose rates.

The equations resulted from R₂-dose curves of Fig. 2a and b show exact linear fittings for three dose rates in 0–10 Gy dose range. The Equations (2)–(4) and (5)–(7) are belonged to the PASSAG-U gel dosimeters with 3% and 5% urea at 100, 200, and 300 cGy/min dose rates, respectively. Also, the goodness of the fit parameters related to these equations are listed in Table 3.

Table 3

Goodness of the linear fit to R₂-dose data of PASSAG-U gel dosimeters with 3% and 5% urea at various dose rates

Parameters	PASSAG-U, 3%			PASSAG-U, 5%
	100 cGy/min	200 cGy/min	300 cGy/min	100 cGy/min	200 cGy/min	300 cGy/min
SSE	0.010	0.055	0.054	0.017	0.046	0.039
R-square	0.994	0.973	0.973	0.992	0.981	0.981
Adjusted R-square	0.993	0.966	0.967	0.989	0.976	0.976
RMSE	0.052	0.118	0.116	0.065	0.108	0.098

$R_{2} = 0 . 164 \times Dose + 3 . 344)$ (2) $R_{2} = 0 . 168 \times Dose + 3 . 435$ (3) $R_{2} = 0 . 168 \times Dose + 3 . 470$ (4) $R_{2} = 0 . 169 \times Dose + 4 . 026$ (5) $R_{2} = 0 . 184 \times Dose + 4 . 079$ (6) $R_{2} = 0 . 169 \times Dose + 4 . 027$ (7)

According to the Equations (2)–(4), the R₂-dose sensitivities of PASSAG-U gel dosimeter with 3% urea are 0.164±0.017, 0.168±0.039, and 0.168±0.039 s^–1 Gy^–1, respectively, and for PASSAG-U gel dosimeter with 5% urea (equations (5)–(7)) are 0.169±0.022, 0.184±0.036, and 0.169±0.033 s^–1 Gy^–1, respectively. For the PASSAG-U gel dosimeters with 3% and 5% urea, there were 0.18–2.53% and 0.24–8.44% difference ranges between the R₂-dose sensitivities of gel dosimeters, respectively, at the evaluated dose rates; this means the R₂-dose sensitivity of PASSAG-U polymer gel dosimeter with 3% urea does not change under different dose rates (<5%), but it changes for PASSAG-U polymer gel dosimeter with 5% urea (>5%).

The results of several studies showed that depending on the type of polymer gel dosimeter, its R₂-dose response and sensitivity can be independent/dependent on dose rate [9 , 17–23]. For example, it was reported that R₂-dose sensitivity of BANG-2 gel dosimeter is independent of dose rate [17]. In other reports on MAGIC-A and PAGAT polymer gel dosimeters, the findings showed that the R₂-dose responses of these two gel dosimeters are independent of evaluated dose rates [19, 20]. In another study, it was showed that R₂-dose response of NIPAM gel dosimeter is dependent of dose rate [9]. Assessment of R₂-dose response and sensitivity dependency of PASSAG gel dosimeter on dose rate showed that the R₂-dose response and sensitivity of this gel dosimeter are independent of dose rate [13].

In this study, we evaluated the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters on three dose rates (100–300 cGy/min), which are used almost in clinical applications of external beam radiation therapy. Assessment on other dose rates is also suggested as a future study.

3.2 Photon energy dependence

To assess the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on photon energy, 6 and 15 MV photon energies were applied, as seen in Fig. 3. The resulted presented in this section are related to 1-day post irradiation time.

Fig.3

R₂-dose curves of PASSAG-U polymer gel dosimeters with 3% (a) and 5% (b) urea as a function of dose value for 6 and 15 MV photon energies at 200 cGy/min dose rate, scanning room temperature (18 °C), and 24 h after irradiation process.

There were 0.24–3.16% and 1.24–6.75% difference ranges between the R₂ values of PASSAG-U gel dosimeters with 3% and 5% urea, respectively, at 6 and 15 MV photon energies. By analyzing the results (Fig. 3), it was found that these differences were less than 5% for all evaluated cases, except for the PASSAG-U gel dosimeter with 5% urea at dose value of 8 Gy (6.75%). It means that the R₂-dose responses of PASSAG-U gel dosimeters with 3% and 5% urea are not depended on different photon energies.

The following equations (8)-(9) and (10)-(11) result from the R₂-dose curves of PASSAG-U gel dosimeters with 3% and 5% urea at 6 and 15 MV photon energies, respectively (Fig. 3). The equations demonstrate exact linear fittings for the above-mentioned gel dosimeters at two photon energies in the 0–10 Gy dose range. Also, the goodness of the fit parameters related to these equations is listed in Table 4. $R_{2} = 0 . 168 \times Dose + 3 . 435$ (8) $R_{2} = 0 . 159 \times Dose + 3 . 497$ (9) $R_{2} = 0 . 184 \times Dose + 4 . 079$ (10) $R_{2} = 0 . 161 \times Dose + 4 . 076$ (11)

Table 4

Goodness of the linear fit to R₂-dose data of PASSAG-U gel dosimeters with 3% and 5% urea at various photon energies

Parameters	PASSAG-U, 3%		PASSAG-U, 5%
	6 MV	15 MV	6 MV	15 MV
SSE	0.055	0.074	0.046	0.027
R-square	0.973	0.960	0.981	0.985
Adjusted R-square	0.966	0.950	0.976	0.981
RMSE	0.118	0.136	0.108	0.083

The R₂-dose sensitivities of PASSAG-U gel dosimeter with 3% urea (equations (8)–(9)) are 0.168±0.039 and 0.159±0.045 s^–¹ Gy^–¹, respectively, and for PASSAG-U gel dosimeter with 5% urea (equations (10)–(11)) are 0.184±0.036 and 0.161±0.027 s^–¹ Gy^–¹, respectively. There was a 5.68% difference between the R₂-dose sensitivities of PASSAG-U gel dosimeter with 3% at 6 and 15 MV photon energies and the difference increased to 13.34% for that with 5% urea. This means the R₂-dose sensitivity of PASSAG-U polymer gel dosimeter with 3% urea does not change under different photon energies, but it changes for the PASSAG-U polymer gel dosimeter with 5% urea.

Some researchers have already evaluated the response/sensitivity dependence of different polymer gel dosimeters on photon energy [1 , 24–33]. In two reports on BANG gel dosimeter irradiated with several photon and electron energies, it was found that there is not any dose response dependence of BANG gel dosimeter on evaluated photon and electron energies [24, 25]. In other studies, the dose response dependences of nMAG, PAG and nPAG gel dosimeters on photon energy (6 and 25 MV) were evaluated. Their findings showed that the dose response of acrylamide-based gel dosimeters are not dependent on photon energy, but there is a small dose response dependence of methacrylic acid-based gel dosimeter on photon energy [30, 31]. In a study on assessment of response dependence of MAGAT gel dosimeter on photon energy (6 and 10 MV), it was reported that response dependence of this gel dosimeter on photon energy is not significant [32]. In a recent study on investigation of R₂-dose response and sensitivity dependence of PASSAG gel dosimeter on photon energy (6 and 18 MV), it was found that the response and sensitivity of this gel dosimeter do not change over different photon energies [13].

As a result, it can be mentioned that depending on the type polymer gel dosimeter, its R₂-dose response and sensitivity can change over different photon/electron energies. In the current study, we evaluated the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters on two photon energies (6 and 15 MV), as assessment on other photon energies and electron energies be suggested as a future study.

3.3 Dose resolution

The data presented in Fig. 4 demonstrated that the dose resolution value of PASSAG-U gel dosimeters with 3% and 5% urea for 100, 200, and 300 cGy/min dose rates, as a function of dose value, varies from 0.20 to 0.86 Gy and 0.27 to 1.69 Gy, respectively. Corresponding dose resolution value ranges for 6 and 15 MV photon energies were 0.20–0.85 Gy and 0.27–2.20 Gy, respectively. As seen from Figs. 4 and 5, the dose resolution values for PASSAG-U gel dosimeter with 5% urea are more than that of 3% urea.

Fig.4

Dose resolution values of PASSAG-U gel dosimeters with 3% (a) and 5% (b) urea as a function of dose value for 100, 200, and 300 cGy/min dose rates at 6 MV photon energy, scanning room temperature (18 °C), and 24 h after irradiation process.

Fig.5

Dose resolution values of PASSAG-U gel dosimeters with 3% (a) and 5% (b) urea as a function of dose value for 6 and 15 MV photon energies at 200 cGy/min dose rate, scanning room temperature (18 °C), and 24 h after irradiation process.

The “dose resolution” quantity is defined as the minimum separation between two dose values and it can be applied as a criterion to optimize and compare different types of polymer gel dosimeters [34]. As reported in our previous study [14], the addition of urea to formula of PASSAG gel dosimeter causes the degradation of dose resolution value. Although the R₂-dose sensitivity of PASSAG-U gel dosimeter with 5% urea is better (more) than that of 3%, there is an opposite pattern in term of dose resolution value for these two gel dosimeters. When urea is added to the gelatin used in the compound of gel dosimeter, it can cause the denaturation of its structure (especially proteins). In this state, the monomers/crosslinkers disperse non-uniformly at all the mixture of gel which it leads to increase the value of σ_R2, thereby the degradation of $D_{Δ}^{p}$ (Equation 1).

The dose resolution values of PASSAG-U gel dosimeter with 3% urea are almost the same with those of VIPAR [35], MAGIC [36], MAGIC-A [37], and acrylamide-based [38] polymer gel dosimeters. Nevertheless, the dose resolution values reported for MAGIC-f [39], NIPAM [40], and PVABAT [41] polymer gel dosimeters were less than those of PASSAG-U gel dosimeter with 3% urea.

3.4 Post irradiation time effect on dependence of dose rate and photon energy

In the present study, the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on dose rate and photon energy were investigated for three times after irradiation process (1, 10, and 14 days) and the obtained results are showed in Figs. 6–9. Also, the MR images and the R₂ maps related to the above-mentioned post irradiations for PASSAG-U gel dosimeters with 3% urea at dose rate of 300 cGy/min and 6 MV photon energy are illustrated in Fig. 10.

Fig.6

R₂-dose curves of PASSAG-U gel dosimeter with 3% urea as a function of dose value for various dose rates (with 6 MV photon energy) at 1 day (a), 10 days (b) and 14 days (c) after irradiation process at scanning room temperature (18°C).

Fig.7

R₂-dose curves of PASSAG-U gel dosimeter with 3% urea as a function of dose value for various photon energies (with 200 cGy/min) at 1 day (a), 1 days (b) and 14 days (c) after irradiations at scanning room temperature (18 °C).

Fig.8

R₂-dose curves of PASSAG-U gel dosimeter with 5% urea as a function of dose value for various dose rates (with 6 MV photon energy) at 1 day (a), 1 days (b) and 14 days (c) after irradiations at scanning room temperature (18 °C).

Fig.9

R₂-dose curves of PASSAG-U gel dosimeter with 5% urea as a function of dose value for various photon energies (with 200 cGy/min dose rate) at 1 day (a), 1 days (b) and 14 days (c) after irradiations at scanning room temperature (18 °C).

Fig.10

MR images (left) and R₂–maps (right) of PASSAG-U polymer gel dosimeters with 3% urea for dose rate of 300 cGy/min and 6 MV photon energy at 1 day (a), 1 days (b) and 14 days (c) after irradiations at scanning room temperature (18 °C). In all parts, from top to bottom, the vials received absorbed dose values of 0, 2, 4, 6, 8 and 10 Gy, respectively.

The results related to PASSAG-U gel dosimeter with 3% (Fig. 6) showed 0.24–5.90%, 0.04–1.79%, and 0.30–2.28% difference ranges between the R₂ values of gel dosimeters for different dose rates at 1, 10, and 14 days after irradiation process, respectively, and 0.24–3.16%, 0.13–0.66%, and 0.29–2.64% difference ranges were obtained for different photon energies at the corresponding times, respectively (Fig. 7). For the PASSAG-U gel dosimeter with 5% (Fig. 8), there were 0.19–5.43%, 0.05–3.55%, and 0.09–7.12% difference ranges between the R₂ values of gel dosimeters for different dose rates at 1, 10, and 14 days after irradiation process, respectively, and 1.24–6.75%, 0.14–1.87%, and 0.73–9.887% difference ranges were revealed for different photon energies at the corresponding times, respectively (Fig. 9).

By analyzing the data presented in Figs. 6–9, it was found that there were 0.18–2.53%, 0.35–1.60%, and 2.69–5.59% difference ranges between the R₂-dose sensitivities of PASSAG-U gel dosimeter with 3% urea for different dose rates at 1, 10, and 14 days after irradiation process, respectively, and 5.68%, 1.05%, and 5.61% differences were found for different photon energies, respectively. There were 0.24–8.44%, 1.12–6.03%, and 1.77–14.92% difference ranges between the R₂-dose sensitivities of PASSAG-U gel dosimeter with 5% urea for different dose rates at 1, 10, and 14 days after irradiation process, respectively, and 13.34%, 0.61%, and 19.13% differences were obtained for different photon energies, respectively.

The results mentioned in this section revealed that the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on dose rate and photon energy can vary over post irradiation time; for example, it was found that there is a small R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters on dose rate and photon energy at 10-days post irradiation time. It should be noted that only three post irradiation times (1, 10, and 14 days) were evaluated in this study and assessment of other post irradiation times to obtain the best time to analyze the results of polymer gel dosimeters could be considered as a complementary study on the results presented in this section. For practical point of view, it is necessary that phantom of gel dosimeter (used for 3D dose measurement) and calibration gel vials be scanned at the same time, obtaining an accurate dose map.

4 Conclusion

In this study, the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters with 3% and 5% urea on dose rate and photon energy were evaluated. According to the obtained data, it was found that the R₂-dose responses of PASSAG-U gel dosimeters with 3% and 5% urea do not vary on different dose rates (100, 200, and 300 cGy/min) and photon energies (6 and 15 MV). The R₂-dose sensitivity of PASSAG-U polymer gel dosimeter with 3% urea does not change under different dose rates and photon energies, but it changes for the PASSAG-U polymer gel dosimeter with 5% urea. The dose resolution values of PASSAG-U gel dosimeter with 3% urea for different evaluated dose rates and photon energies ranged from 0.20 Gy to 0.86 Gy and for the PASSAG-U gel dosimeter with 5% urea ranged from 0.27 to 2.20 Gy. Furthermore, it was revealed that the R₂-dose response and sensitivity dependence of PASSAG-U gel dosimeters on dose rate and photon energy can vary over post irradiation time. According to findings of the present study, it can be concluded that dosimetric characteristics of PASSAG-U gel dosimeter with 3% were better than that of 5% urea.

As a future study, the use of X-ray CT for evaluation of the response of PASSAG-U polymer gel dosimeters is suggested. Moreover, the dosimetric evaluation of PASSAG-U polymer gel dosimeter for use in clinical situations can be valuable study.

Conflict of interest

None.

Footnotes

Acknowledgments

This experimental study was supported by Kashan University of Medical Sciences (Kashan, Iran) under grant number 97118.

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