The investigation of dose rate and photon beam energy dependence of optimized PASSAG polymer gel dosimeter using magnetic resonance imaging

Abstract

OBJECTIVE:

It seems that dose rate (DR) and photon beam energy (PBE) may influence the sensitivity and response of polymer gel dosimeters. In the current project, the sensitivity and response dependence of optimized PASSAG gel dosimeter (OPGD) on DR and PBE were assessed.

MATERIALS AND METHODS:

We fabricated the OPGD and the gel samples were irradiated with various DRs and PBEs. Then, the sensitivity and response (R₂) of OPGD were obtained by MRI at various doses and post-irradiation times.

RESULTS:

Our analysis showed that the sensitivity and response of OPGD are not affected by the evaluated DRs and PBEs. It was also found that the dose resolution values of OPGD ranged from 9 to 33 cGy and 12 to 34 cGy for the evaluated DRs and PBEs, respectively. Additionally, the data demonstrated that the sensitivity and response dependence of OPGD on DR and PBE do not vary over various times after the irradiation.

CONCLUSIONS:

The findings of this research project revealed that the sensitivity and response dependence of OPGD are independent of DR and PBE.

Keywords

Dose rate optimized PASSAG magnetic resonance imaging photon beam energy polymer gel dosimeter

1 Introduction

In advanced radiotherapy techniques, a precise and conformal radiation dose distribution can be delivered to the tumor, while the dose received to the healthy tissues around the tumor can be at its lowest value [1, 2]. Since the radiation dose distribution generated by these modern radiotherapeutic techniques can be complex (due to the creation of a higher dose gradient between the tumor and its surrounding normal tissues), it seems to be essential to validate three-dimensional (3D) dose distribution before radiation therapy [3 –5].

In radiation therapy, different dosimetric systems can be used to verify radiation dose verification which include electron spin resonance dosimeters, film dosimeters, diodes, thermoluminescent dosimeters, ionization chambers, etc. Nevertheless, these conventional dosimetric systems are capable of measurement of the radiation doses in 1D or 2D [6 –15]. To overcome this problem and measure the radiation dose in 3D, the use of 3D dosimetric systems has been suggested. Maryanski and coworkers introduced a polymer gel dosimetric system in 1993 [16]. In addition to measuring the radiation dose distribution in 3D, they have beam energy independency, high accuracy of radiation dose measurement, good spatial resolution, and so on [17 –19]. These 3D dosimeters are fabricated from the chemical materials sensitive to radiation [20]; as the monomers dissolved in the gelatin hydrogel matrix of these dosimeters are polymerized following the irradiation [21, 22]. The radiation-generated polymers induce a number of structural alterations in the dosimeters [23, 24] and various imaging systems are able to measure these changes [21, 25]. One of these imaging systems is magnetic resonance imaging (MRI) that the radiation-induced proton nuclear magnetic resonance relaxation times of polymer gel dosimeters are measured by this imaging technique [16 , 26].

So far, the researchers recommend different polymer dosimeters and they in these projects were able to improve and/or optimize these dosimeters [25 –31]. In this regard, a research group introduced PASSAG gel dosimeter in 2018 [32]. This polymer gel dosimeter has very low toxicity, no carcinogenic and genotoxic effects, and eco-friendliness [32]. They also found that this dosimeter is a soft tissue and water-equivalent material, has an excellent linear response, and its response is independent of beam energy (photon and electron) and dose rate [19 , 33]. In another project, the researchers reported that the sensitivity of PASSAG gel dosimeter could improve by addition of urea to its structure [1, 34].

Recently, in a study on PASSAG gel dosimeter, it was demonstrated that its sensitivity could increase by optimizing the amounts of fabricating components [35]. Nevertheless, the dosimetric specifications (the sensitivity and response) of optimized PASSAG gel dosimeter (OPGD) over different dose rates (DRs) and photon beam energies (PBEs) have been not investigated. Notably, for using the OPGD in clinical applications, its dosimetric specifications need to be assessed in different physical conditions. Hence, in this project, the sensitivity and response dependence of the OPGD on DR and PBE were investigated.

2 Materials and methods

At first, we prepared the OPGD and then, the gel samples were exposed to different DRs and PBEs. In the next stage, the sensitivity and response of OPGD were obtained by using the MRI technique at various times after the irradiation.

2.1 Preparation process of OPGD

We prepared the gel samples in accordance with the OPGD recipe reported in ref. [35]. A list of the chemical materials (along with other information) applied in the fabrication of OPGD is mentioned in Table 1.

Table 1
A number of information regarding the chemical components applied in the preparation process of OPGD

Chemical component type Chemical concentration

(% by weight)

Water (Merck, Germany) 87.0±1.0 %

Gelatin (type A, 300 Bloom, Sigma Aldrich, USA) 5.0±0.5 %

AMPS (Sigma Aldrich, USA,≤% 100) plus NaOH (Sigma Aldrich, USA, ≤% 100) 4.0±0.5 %

Bis (≥99.5%, Sigma Aldrich, USA) 4.0±0.5 %

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

Chemical component type	Chemical concentration
Water (Merck, Germany)	87.0±1.0 %
Gelatin (type A, 300 Bloom, Sigma Aldrich, USA)	5.0±0.5 %
AMPS (Sigma Aldrich, USA,≤% 100) plus NaOH (Sigma Aldrich, USA, ≤% 100)	4.0±0.5 %
Bis (≥99.5%, Sigma Aldrich, USA)	4.0±0.5 %
THPC (80% solution in water, Sigma Aldrich, USA)	10 mM

The OPGD was prepared in the following five successive stages: a) fabrication of monomer: for this aim, we added adequate sodium hydroxide pellets to a 4.0% concentration of 2-Acrylamido-2-Methy-1-PropaneSulfonic acid (AMPS) at 10% of HPLC grade water and finally, a pH 7 salt solution was attained; b) the gelatin was swelled in 80% of HPLC grade water for about 10 min and during this time, we increased the solution temperature slowly; c) when the temperature of the solution reached to 50 °C, a 4.0% concentration of N-Methylene-Bis-Acrylamide (Bis) was added to the swelled gelatin; d) after that, we reduced the solution temperature to 37 °C and in this temperature, the salt solution fabricated in “stage a” was added to the mixture produced in “stage c”; e) in the last stage of OPGD fabrication, we blended 10 mM of Tetrakis Hydroxymethyl Phosphonium Chloride with 10% of HPLC grade water and then, we added the obtained solution to the mixture produced in “stage d” at a temperature of about 35 °C. After the OPGD preparation, it we transferred into specific glass tubes and then, we closed the gel-filled tubes with their caps and these tubes were stored for about 24 h a at 4°C temperature.

2.2 Irradiation process of OPGD

One day after the OPGD fabrication, the gel-filled tubes were irradiated by a Siemens Primus LINAC (Siemens AG, Erlangen, Germany). For the irradiation of OPGD, we located the gel-filled tubes at a 5 cm distance from the wall of a 45 cm³ water phantom. Moreover, a thermometer was used to measure the temperature of water phantom and the findings obtained from the temperature measurements demonstrated that the temperature of water phantom was the same before and after the irradiation (the room temperature was 22°C.). Table 2 shows the specifications of the radiation field used for irradiating the gel tubes.

Table 2
The radiation field specifications used for the OPGD irradiation

Features Properties

field size 30×30 cm²

Gantry angle 90°

Source to surface distance 95 cm

Source to axis distance 100 cm

Radiation doses delivered to the gel tubes 0, 1, 2, 3, 4, 6, 8, 10 Gy

Features	Properties
field size	30×30 cm²
Gantry angle	90°
Source to surface distance	95 cm
Source to axis distance	100 cm
Radiation doses delivered to the gel tubes	0, 1, 2, 3, 4, 6, 8, 10 Gy

2.3 Investigation of DR and PBE dependence of OPGD

To assess the sensitivity and response dependence of OPGD on DR, the gel tubes were exposed to 6 MV X-rays at three DRs of 100, 200, and 300 cGy/min. Moreover, we evaluated the PBE dependence of OPGD and for this aim, the gel tubes were exposed to 6 and 15 MV MV X-rays at the same DR (200 cGy/min).

For each DR/PBE, we used eight gel tubes and they were exposed to a radiation dose range of 0–10 Gy.

2.4 Reading out process of OPGD response

We obtained the responses (R₂ values) of OPGD at 1, 10, 14, and 30 days after the irradiation. In this project, we used a 1.5 T MRI scanner (Siemens Avanto, Germany). As well, for recording the signals, a standard radio frequency head coil was used. Additionally, we used the pulse sequence proposed in the previous study to measure the R₂ values. A list of MRI scanning parameters used in this project is presented in Table 3 [35].

Table 3
A number of MRI scanning parameters used to obtain the OPGD response

Features Properties

Feld of view (FOV) 24×26 cm²

First echo time 22 ms

Repetition time 4000 ms

Echoes spacing 22 ms

Matrix size 256×232

Pixel band-width 100 Hz

Slice thickness 2 mm

Features	Properties
Feld of view (FOV)	24×26 cm²
First echo time	22 ms
Repetition time	4000 ms
Echoes spacing	22 ms
Matrix size	256×232
Pixel band-width	100 Hz
Slice thickness	2 mm

2.5 Calculation of the sensitivity and dose resolution quantities

An important quantity for comparison of various polymer gel dosimeters is the “R₂-dose sensitivity (α) ” which is expressed as the linear region slope of response-dose curve [36]. So, we obtained the sensitivities of OPGD for various DRs and PBEs by taking the derivative of their R₂-dose curves.

Another useful quantity for comparison of polymer gel dosimeters is the “dose resolution ( $D_{Δ}^{p}$ )” which is expressed as the minimal separation of two measurable dose values with a certain level of confidence (p) [37]. So, we calculated the dose resolution values of OPGD for various DRs and PBEs by using equation 1 (k_p = coverage factor and σ_R₂ = standard deviation of R₂ values): $D_{Δ}^{p} = k_{p} \sqrt{2} \frac{σ_{R_{2}}}{α}$ (1)

The kp applied in the present project was equal to 1 and it was selected based on the previous studies [1 , 32–34].

3 Results and discussion

3.1 DR dependence of OPGD

The findings obtained from the effect of different DRs (100, 200, and 300 cGy/min) on the response and sensitivity dependence of OPGD are illustrated in Fig. 1. We found a difference range of 0.31 to 4.97% between the R₂ values of OPGD at the above-mentioned DRs. Our data proved that the R₂-dose response of OPGD is independent of DR; in other words, the differences between the R₂ values of OPGD at the assessed DRs were less than 5%.

Fig. 1

The R₂-dose curves of OPGD irradiated with 6 MV X-rays for various dose rates. These findings are related to one day after the irradiation.

It is noteworthy that the R₂ variations of OPGD irradiated to the DRs of 100, 200, and 300 cGy/min, as a function of dose value, can be calculated by equations 2, 3, and 4, respectively. According to these equations, it is also mentioned that there are exact linear fittings for the OPGD at three DRs (Table 4). $R_{2} = 0.223 \times Dose + 1.453$ (2) $R_{2} = 0.225 \times Dose + 1.390$ (3) $R_{2} = 0.234 \times Dose + 1.422$ (4)

Table 4

A list of parameters regarding the goodness of the linear fits of the\\ response-dose curves of OPGD irradiated to the various dose rates

Parameters	100 cGy/min	200 cGy/min	300 cGy/min
SSE	0.022	0.043	0.014
R-square	0.995	0.990	0.997
Adjusted R-square	0.994	0.988	0.997
RMSE	0.061	0.085	0.049

Other data revealed that the R₂-dose sensitivities of OPGD for the DRs of 100, 200, and 300 cGy/min were 0.223±0.022, 0.225±0.016, and 0.234±0.013 s^–1 Gy^–1, respectively. We also found a 0.89 to 4.98% difference range between the R₂-dose sensitivities of OPGD irradiated to the various DRs. According to these results, we can state that the R₂-dose sensitivity of OPGD is not affected by the different DRs (the differences between the R₂-dose sensitivities of OPGD were less than 5%).

Based on the results obtained from the current project and other published studies, it can be stated that dependent/independent of the sensitivity and response on DR depends on the type of polymer gel dosimeter [25 , 38–45]. Farhood and coworkers [34] reported that the response of PASSAG-U gel dosimeters (both 3% urea and 5% urea) is not affected by various DRs. They also stated that the sensitivity of PASSAG-U gel dosimeter with 3% urea is independent of DR, but this quantity is dependent on DR for PASSAG-U gel dosimeter with 5% urea [34]. In another research project, it was reported that the sensitivity and response of conventional PASSAG gel dosimeter are independent of the DR [33]. Waldenberg and coworkers assessed the effect of DR on the response of NIPAM gel dosimeter and reported that its response varies on various DRs [38]. Abtahi and coworkers reported that the sensitivity of U-NIPAM gel dosimeter is independent of DR [45]. Novotny and coworkers demonstrated that the sensitivity of BANG-2 gel dosimeter does not vary on different DRs [39].

3.2 PBE dependence of OPGD

To evaluate the sensitivity and response dependence of OPGD on PBE, the gel tubes were exposed to 6 and 15 MV X-rays, and the obtained results are seen in Fig. 2. Our analysis revealed a difference range of 0.71–4.88% between the R2 values of OPGD irradiated with 6 and 15 MV PBEs. According to these findings, we can state that the R₂-dose response of OPGD is independent of PBE.

Fig. 2

The R₂-dose curves of OPGD irradiated with 200 cGy/min for different PBEs. These findings are related to 24 h after the irradiation.

Equations 5 and 6 belong to the OPGD irradiated with 6 and 15 MV X-rays, respectively; so that by using these equations, we can calculate the R₂ variations of OPGD over radiation dose. Table 5 demonstrates the exact linear fittings for the OPGD irradiated with the evaluated BPE. $R_{2} = 0.225 \times Dose + 1.390$ (5) $R_{2} = 0.232 \times Dose + 1.495$ (6)

Table 5

A list of parameters regarding the goodness of the linear fits of\\ the response-dose curves of OPGD irradiated to the various PBEs.

Parameters	6 MV	15 MV
SSE	0.043	0.039
R-square	0.990	0.992
Adjusted R-square	0.988	0.990
RMSE	0.085	0.080

The findings disclosed the R₂-dose sensitivities of 0.225±0.016 and 0.232±0.011 s^–1 Gy^–1 for the OPGD irradiated with 6 and 15 MV X-rays, respectively. A difference of 3.96% between the R₂-dose sensitivities of OPGD irradiated with the various DRs was found. According to these results, we can report that the R₂-dose sensitivity of OPGD is not affected by different DRs (the differences between the R₂-dose sensitivities of OPGD were less than 5%).

The PBE dependence of some polymer gel dosimeters has been investigated in previous studies [3 , 46–53]. Farhood and coworkers evaluated the dosimetric specifications of a gel dosimeter (with very low toxicity) exposed by two PBEs (6 and 18 MV X-rays), and their results revealed that the sensitivity and response of this gel dosimeter (PASSAG) are independent of the evaluated PBEs [33]. Abtahi and coworkers studied the PBE dependence of U-NIPAM gel dosimeter, and their data showed that various PBEs of 6 and 15 MV had a significant effect on the sensitivity of the assessed gel tubes [45]. Farhood and coworkers reported that the response of PASSAG-U gel dosimeters (both 3% urea and 5% urea) is not affected by different PBEs (6 and 15 MV). They also stated that the sensitivity of PASSAG-U gel dosimeter with 3% urea is independent of PBE, but this quantity is dependent on the PBE for PASSAG-U gel dosimeter with 5% urea [34]. The findings of other research projects revealed that the responses of PAG and nPAG gel dosimeters are independent of PBE (6 and 25 MV); however, these studies reported a small response dependent on PBE for nMAG gel dosimeter [36, 52]. The results obtained from two research projects on BANG gel dosimeter revealed that its response is independent of PBE [46, 47].

According to the data represented in this section, we can state that depending on the gel dosimeter type, its sensitivity and response can alter over various PBEs.

3.3 Dose resolution values of OPGD irradiated with various DRs and PBEs

The findings presented in Fig. 3 demonstrated that the dose resolutions of OPGD exposed by various DRs (100, 200, and 300 cGy/min), as a function of dose, vary from 0.09 to 0.33 Gy. Moreover, the dose resolutions of OPGD exposed by various PBEs (6 and 15 MV), as a function of dose, vary from 0.12 to 0.34 Gy (Fig. 4).

Fig. 3

The dose resolutions of OPGD irradiated with the dose rates of 100, 200, and 300 cGy/min. These findings are related to 24 h after the irradiation.

Fig. 4

The dose resolutions of OPGD exposed by 6 and 15 MV X-rays. These findings are related to 24 h after the irradiation.

One of the important parameters applied to evaluate the polymer gel dosimeters suggested by researchers is “dose resolution” [17]. The results of current project demonstrated that the dose resolution of OPGD is comparable to those reported by other studies such as the gel dosimeters of MAGIC (> 50 cGy) [54], MAGIC-f (20 cGy) [55], PVABAT (8–19 cGy) [56], and NIPAM (20–30 cGy) [57].

3.4 Post irradiation time effect on DR and PBE dependence of OPGD

We investigated the dosimetric characteristics of OPGD irradiated with the different DRs and PBEs at 1, 10, 14 and 30 days after the irradiation; as these findings are listed in Figs. 5 and 6, respectively.

We obtained the difference ranges of 0.31 to 4.97%, 0.17 to 4.99%, 0.04 to 4.96%, and 0.31 to 4.60% between the R₂ values of OPGD exposed to the different DRs at 1, 10, 14 and 30 days after the irradiation, respectively. Our analysis also revealed the difference ranges of 0.89 to 4.98%, 2.19 to 4.88%, 1.31 to 2.69%, and 0.91 to 2.05% between the sensitivities of OPGD exposed to the different DRs at 1, 10, 14 and 30 days after the irradiation, respectively.

Fig. 5

The R₂-dose curves of OPGD irradiated with various DRs at 24 h (a), 10 days (b), 14 days (c), and 30 days (d) after the irradiation.

Fig. 6

The R₂-dose curves of OPGD irradiated with various PBEs at 24 h (a), 10 days (b), 14 days (c), and 30 days (d) after the irradiation.

Other findings demonstrated the difference ranges of 0.71 to 4.88%, 1.38 to 4.58%, 2.32 to 4.91%, and 0.98 to 4.98% between the R₂ values of OPGD exposed to the different PBEs at 1, 10, 14 and 30 days after the irradiation, respectively. Moreover, the 3.96%, 4.75%, 4.01%, and 3.46% differences between the R₂ values of OPGD exposed to the various PBEs at 1, 10, 14 and 30 days after the irradiation, respectively.

Our results (Figs. 5 and 6 and Tables 6 and 7) showed that the sensitivity and response dependence of OPGD on DR and PBE are not affected by the various times after the irradiation.

Table 6

The sensitivities of OPGD exposed to 100, 200 and 300 cGy/min dose rates during various post-irradiation times

Post irradiation time (day)	R₂-dose sensitivity (Gy^-1s^-1)
	100 cG/min	200 cG/min	300 cG/min
1	0.223±0.022	0.225±0.016	0.234±0.013
10	0.231±0.016	0.266±0.023	0.237±0.013
14	0.238±0.013	0.235±0.020	0.241±0.020
30	0.244±0.011	0.242±0.022	0.247±0.017

Table 7

The sensitivities of OPGD exposed to 6 and 15 MV X-rays during various post-irradiation times

Post irradiation time (day)	R₂-dose sensitivity (Gy^–1s^–1)
	6 MV	15 MV
1	0.225±0.016	0.232±0.011
10	0.266±0.023	0.237±0.024
14	0.235±0.020	0.244±0.022
30	0.242±0.022	0.250±0.020

4 Conclusion

In this research project, the dosimetric characteristics of OPGD on the different DRs and PBEs were investigated. Based on the obtained data, we can state that the sensitivity and response of OPGD are not affected by different DRs (100, 200, and 300 cGy/min) and PBEs (6 and 15 MV). In addition, the dose resolution values of OPGD irradiated with the various DRs and PBEs ranged from 9 to 33 cGy and 12 to 34 cGy, respectively. We also found that the sensitivity and response dependence of OPGD on DR and PBE do not vary over the various times after the irradiation.

As a future research project, it is recommended to evaluate the dosimetric characteristics of OPGD on the different DRs and PBEs using a 3 T MRI scanner.

Footnotes

Acknowledgments

None.

Conflict of interest

None.

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Chemical component type	Chemical concentration
	(% by weight)
Water (Merck, Germany)	87.0±1.0 %
Gelatin (type A, 300 Bloom, Sigma Aldrich, USA)	5.0±0.5 %
AMPS (Sigma Aldrich, USA,≤% 100) plus NaOH (Sigma Aldrich, USA, ≤% 100)	4.0±0.5 %
Bis (≥99.5%, Sigma Aldrich, USA)	4.0±0.5 %
THPC (80% solution in water, Sigma Aldrich, USA)	10 mM

The investigation of dose rate and photon beam energy dependence of optimized PASSAG polymer gel dosimeter using magnetic resonance imaging

Abstract

OBJECTIVE:

MATERIALS AND METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1 Introduction

2 Materials and methods

2.1 Preparation process of OPGD

Table 2 The radiation field specifications used for the OPGD irradiation Features Properties field size 30×30 cm2 Gantry angle 90° Source to surface distance 95 cm Source to axis distance 100 cm Radiation doses delivered to the gel tubes 0, 1, 2, 3, 4, 6, 8, 10 Gy

2.4 Reading out process of OPGD response

Table 3 A number of MRI scanning parameters used to obtain the OPGD response Features Properties Feld of view (FOV) 24×26 cm2 First echo time 22 ms Repetition time 4000 ms Echoes spacing 22 ms Matrix size 256×232 Pixel band-width 100 Hz Slice thickness 2 mm

3.1 DR dependence of OPGD

Footnotes

Acknowledgments

Conflict of interest

References

Table 2
The radiation field specifications used for the OPGD irradiation

Features Properties

field size 30×30 cm²

Gantry angle 90°

Source to surface distance 95 cm

Source to axis distance 100 cm

Radiation doses delivered to the gel tubes 0, 1, 2, 3, 4, 6, 8, 10 Gy

Table 3
A number of MRI scanning parameters used to obtain the OPGD response

Features Properties

Feld of view (FOV) 24×26 cm²

First echo time 22 ms

Repetition time 4000 ms

Echoes spacing 22 ms

Matrix size 256×232

Pixel band-width 100 Hz

Slice thickness 2 mm