Polymer gel dosimetry

A view of a 3D polymer gel dosimeter phantom after irradiation (left)
and after scanning: visualisation of 3D dose distribution (middle and right)

What is polymer gel dosimetry?

Polymer gel dosimetry is a technique of measuring ionizing radiation absorbed dose distributions in three-dimensions. The technique utilizes a gel dosimeter that consists of water, polymer like gelatin to form physical gel matrix and some active ingredients like vinyl monomers. Additionally, a gel dosimeter composition may be saturated with argon, nitrogen monoxide or nitrogen as well as some other substances serving as oxygen scavengers can be added to a composition. It may also contain protective substances reducing polymerization of vinyl ingredients induced by UV light or temperature changes. Before formation of physical gel matrix of gelatine, a gel dosimeter is a solution and therefore any jar or vial can be used to pour the solution into it in order for a gel to take a shape of a vial. There are a few polymer gel dosimeters described in literature such as BANG, MAGIC or PABIG (poly(ethylene glycol) diacrylate, N,N'-methylenebisacrylamide, gelatine, water, Ar-saturated), VIPAR (N-vinylpyrrolidone, N,N'-methylenebisacrylamide, gelatine, water, Ar-saturated), VIPAR^nd (N-vinylpyrrolidone, N,N'-methylenebisacrylamide, gelatine, water, CuSO₄, Ascorbic Acid), PABIG^nx (poly(ethylene glycol) diacrylate, N,N'-methylenebisacrylamide, gelatine, water, CuSO₄, Ascorbic Acid); etc.

Exposure of a polymer gel onto ionizing radiation causes polymerization and crosslinking of its ingredients only in the irradiated part. This effect is naked eye visible since the irradiated part becomes opaque after absorbing a threshold dose.

Irradiated polymer gel can be measured with different techniques and magnetic resonance imaging is the most common one. The main goal is to calculate radiation dose distribution in three dimensions.

Calculation of adsorbed dose distribution and comparison of the results with a dose distribution calculated with the aid of a commercial treatment planning systems is laborious if not facilitated by adequate software (see about polyGeVero®).

What is application of polymer gel dosimetry?

Polymer gel dosimetry serves for dosimetry in radiotherapy. It can be used for measurements of radiation dose distribution in 3D and verification of calculated with the aid of commercial treatment planning systems radiation dose distributions.

If you need more information about polymer gel dosimetry application in radiotherapy please refer to Publications Section.

Below you will find data on the characteristic of two polymer gel dosimeters that were used in radiotherapy dosimetry. Application study results on those systems were extensively published.

A comparative study of PABIG and VIPAR polymer gel dosimeters*

(draft manuscript, not reviewed; any part of this manuscript can be copied for personal use only with reference to the copyright owners)

Abstract

Introduction
Experimental
1. Gel preparation
2. Irradiation
3. NMR measurement
Results and discussion
1. Influence of the ingredients' concentration on PABIG and VIPAR dose range and sensitivity
2. Long-term stability of PABIG and VIPAR
3. PABIG and VIPAR R2net dependence on monomer ratio and measurement temperature
4. Influence of monomer ratio and measurement temperature on PABIG and VIPAR sensitivity
5. Threshold dose
Summary

References

Abstract

The purpose of this work was to evaluate the influence of the concentration of components, measurement temperature and post effect on the T₂ NMR relaxation time dose response of the two polymer gel dosimeters, which are meant for three-dimensional dose distribution measurements in radiotherapy. They are based on N,N-methylenebisacrylamide and either poly(ethylene glycol) diacrylate (PABIG gel dosimeter) or N-vinylpyrrolidone (VIPAR gel dosimeter). The samples of both gel dosimeters were ⁶⁰Co irradiated and T₂ NMR relaxation times were measured using CPMG multiecho pulse sequence. Two specific parameters for the polymer gel dosimeters were assessed: the maximal reciprocal T₂ relaxation time that corresponds to a gel dosimeter dose range, and the dose sensitivity. These parameters were found to increase when the measurement temperature decreases and the total monomer concentration in the gel compositions increases. Additionally, the maximal reciprocal T₂ relaxation time increases for higher ratio of N,N'- methylenebisacrylamide to the second co-monomer. Dose sensitivity peaks at equal weight ratios of the co-monomers in PABIG and VIPAR. The post effect boosts the dose sensitivity of these gel dosimeters. It was found that the dose threshold of the PABIG polymer gel dosimeter is smaller than VIPAR. Possible explanation for this difference is provided.

Keywords: polymer gel dosimetry, radiotherapy dosimetry, poly(ethylene glycol) diacrylate, PABIG, VIPAR, 3D dose measurements

1. Introduction

Three-dimensional conformal radiotherapy techniques employ non-uniform beam intensities and high dose gradients to shape dose distributions which match the tumor accurately, thereby reducing the dose to adjacent healthy tissue and the likelihood of harmful repercussions. The complexity of current radiotherapy techniques (e.g. gamma-knife, Intensity Modulated Radiation Therapy, brachytherapy) calls for quality assurance measures (patient- and a radiotherapy technique-specific) able to verify the planned-calculated dose distribution that should correspond to the dose delivered. Besides other limitations of the dosimetry systems, typically used in radiotherapy clinical practice (such as ionization chambers, diodes, films, etc.), the need to acquire high resolution, complete three-dimensional dose distribution measurements in each single irradiation commenced studies towards new methods. A great deal of data on the new technique of gel-based dosimetry for 3-D dose distribution measurements in radiotherapy has appeared, confirming significant interest of various research groups in this scientific area (DosGel, 1999, 2001, 2004). The key tools of this technique are detectors, so-called polymer gel dosimeters, initiated inter alia by Maryanski (Maryanski et al 1993). The radical polymerisation and crosslinking of component ingredients of the gel dosimeters occurs if they are exposed to ionizing radiation. The yield of polymerisation products depends on the absorbed dose of radiation. The spatial changes in a gel dosimeter were proposed to be measured with the aid of magnetic resonance imaging (Gore et al 1984) as well as other techniques: optical scanning (Gore et al 1996, Oldham et al 2003), ultrasound measurements (Mather et al 2002) or Raman spectroscopy (Baldock et al 1998). Methodology of preparation of various polymer gel compositions were described by a few research groups (e.g. Maryanski et al 1993, Maryanski et al 1994, Pappas et al 1999, Baldock et al 1998, Kozicki et al, 2005b, 2007).

In this work, the characteristics of the PABIG polymer gel dosimeter are discussed. This composition is manufactured from poly(ethylene glycol) diacrylate macromonomer (PEGDA) and N,N'-methylenebisacrylamide. Both compounds exhibit high sensitivity to ionizing radiation and details on their reactions with transient species of water radiolysis (ˇH, ˇOH, e_aq^-) were discussed in previous papers (Kozicki et al 2002, Kozicki et al 2003). In addition, the compounds were embedded in a gelatine matrix. A comparison of some properties of the PABIG gel with a VIPAR gel (Pappas et al 1999) of similar composition is also provided here. The only difference between these two gels is their basic monomer constituent, since, instead of poly(ethylene glycol) diacrylate in PABIG, N-vinylpyrrolidone is used in VIPAR gel. This alteration substantially influences their properties which has implications when medical application of the gel compositions is taken into account. The outcomes of application studies on both polymer gel dosimeters were published (i.e.: Kipouros et al 2001, 2003, Karaiskos et al 2005, Kozicki et al 2005a,b, Pantelis et al 2004, 2005, Papagiannis et al 2005a,b, Pappas et al 2005, Petrokokkinos et al 2005a,b). In this paper results on the influence of absorbed dose, concentration of components, measurement temperature and long-term storage on the R2 response of PABIG and VIPAR gel dosimeters are presented.

2. Experimental

2.1. Gel preparation

Aqueous PABIG gels were manufactured from poly(ethylene glycol) diacrylate (4% w/v) of M_n = 700 g mol^-1 (PEGDA), N,N'-methylenebisacrylamide (4% w/v) (MBA) and gelatine type A, 300 Bloom (5% w/v). All the mentioned compounds were delivered by Aldrich. For the preparation of VIPAR composition, N-vinylpyrrolidone (4% w/v) (NVP) (Fluka), N,N'-methylenebisacrylamide (4% w/v) and gelatine type A, 300 Bloom (5% w/v) were used. In order to examine the influence of the components concentration and the ratio of the monomers on T₂ NMR relaxation time of these gels, the concentrations given above were adequately changed, which is indicated in the corresponding graphs. VIPAR and PABIG components were dissolved in de-ionised water of the resistivity of 18.0 M? cm.

Prior to preparation, components of PABIG and VIPAR were purified as follows: NVP was distilled under a reduced pressure (~80oC, ~8 mbar), PEGDA was passed through a disposable column to remove inhibitors (Aldrich). The MBA was purified by triple recrystallisation from hot acetone and it was vacuum-dried (24 hours, 30°C). Gelatine was used as supplied. Pure compounds were stored in a fridge (10°C) hidden from daylight.

The solutions of PABIG and VIPAR were prepared by dissolving MBA in water of the temperature of approx. 50°C. When the solutions were transparent, appropriate amount of gelatine was dissolved. Afterwards, the mixture was cooled down to the temperature of around 25°C, and subsequently PEGDA or NVP was added.

In order to remove oxygen from the VIPAR or PABIG solutions, the cylindrical glass tubes designed for irradiation and NMR measurement (wall thickness 0.63 mm, inner diameter 8.75 mm, length 100 mm), were filled with 1.2 cm³ of solution and were placed in a water bath (~33°C). They were bubbled with argon for 30 min. Then the tubes were tightly closed with rubber septa (Aldrich) and additionally covered with Parafilm® foil. Afterwards, the solutions were hidden from daylight at a constant temperature of 23°C for 20 hours for solidification.

The densities of the gels measured at 23°C were: ?_PABIG = 1.020 ± 0.004 g cm^-3 and ?_VIPAR = 1.018 ± 0.004 g cm^-3. The elemental composition of PABIG and VIPAR can be found in Table 1 and 2 as well as in Pantelis et at 2004.

Table 1.Comparison of elemental composition of VIPAR and PABIG gels (weight fractions denoted as w_x, x - single element)
Gel	w_C	w_H	w_N	w_O
VIPAR	0.0718	0.1074	0.0206	0.8001
PABIG	0.0678	0.1075	0.0156	0.8091

Table 2.Composition of VIPAR and PABIG gels
1. Compound (VIPAR)	Formula
Gelatin	(C₁₇H₃₂N₅O₆)_n
1-vinyl-2-pyrrolidone	C₆H₉NO
N,N'-methylenebisacrylamide	(CH₂CHCONH)₂CH₂
Water	H₂O
2. Compound (PABIG)	Formula
Gelatin	(C₁₇H₃₂N₅O₆)_n
Poly(ethylene glycol) diacrylate	CH₂CHCOO(CH₂CH₂O)₁₃COCHCH₂
N,N'-methylenebisacrylamide	(CH₂CHCONH)₂CH₂
Water	H₂O

2.2. Irradiation

The samples were irradiated with gamma rays from ⁶⁰Co (irradiation unit, BK-10000, Poland) (dose rate 0.023 Gy s^-1, measured using Fricke dosimeter). The samples absorbed doses ranging from ~0.5 up to ~450 Gy.

2.3. NMR measurements

For the relaxation times T₂ measurements a minispectrometer NMR (Minispec 20 MHz, Bruker) was used. The relaxation times were gained by a CPMG sequence with every second echo sampled, using 65 data points. The time delay between consecutive 180° pulses was 40 ms (first echo at 20 ms). T₂ values were calculated on the basis of the first order exponential decay of free induction ( FID). T₂ NMR measurements were performed at constant temperature provided by a heating-cooling system (Haake, Germany; heating-cooling liquid - Fluorinert™ FC-43, 3M Company). The relaxation times T₂ were measured at 0, 4.5, 5, 20, 21, 24, 45, 48, 69 hours after irradiation at a temperature of 25°C. For temperature dependence measurements, an appropriate temperature for the samples was set and this is indicated in the corresponding graphs. Between the measurements, the samples were stored in a dark place at a temperature of ~25°C. R_2net values were derived by subtracting the R₂ values (R₂ = 1/T₂) of non-irradiated gel samples from the irradiated ones.

3. Results and discussion

3.1. Influence of the ingredients' concentration on the PABIG and VIPAR dose range and sensitivity

In order to study the influence of the concentration of an ingredient on PABIG and VIPAR R₂ dose response, the concentration of three compounds was fixed whereas one, either gelatine, MBA, NVP or PEGDA, was varied. A chosen composition was irradiated and T₂ relaxation times were measured. It was found that the concentration of gelatine has no influence on R_2net = f(dose) dependence in the range of 3 - 7% (w/v). However, the amount of MBA, NVP and PEGDA in a gel mixture does affect the before-mentioned relation. The increase in MBA, NVP or PEGDA concentration corresponds to the reinforcement of the dose range and sensitivity of PABIG and VIPAR gel dosimeters. Typical characteristics of the selected gel compositions are given in Table 3. Based on the achieved results, the two compositions of PABIG and VIPAR of the 8% total concentration of co-monomers and 5% gelatine were selected for further studies due to their enhanced sensitivity and range of measurable doses.

Table 3.Comparison of the elemental properties of the selected VIPAR (1a - 1f) and PABIG (2a - 2c) formulae
Composition	Formula	Dose range [Gy]	Linear dose range [Gy]	Dose sensitivity [Gy^-1 s^-1]
(1) a	2% NVP, 4% MBA, 5% gelatine	~10 - >280	~15 - 60	0.036
(1) b	4% NVP, 4% MBA, 5% gelatine	~10 - >280	~15 - 60	0.044
(1) c	8% NVP, 4% MBA, 5% gelatine	~5 - 200	~8 - 60	0.049
(1) d	4% NVP, 4% MBA, 3% gelatine	~10 - >280	~15 - 60	0.044
(1) e	4% NVP, 4% MBA, 7% gelatine	~10 - >280	~15 - 60	0.044
(1) f	4% NVP, 2% MBA, 5% gelatine	~10 - 120	~15 - 50	0.027
(2) a	2% PEGDA, 4% MBA, 5% gelatine	~0 - 25	~0 - 15	0.036
(2) b	4% PEGDA, 4% MBA, 5% gelatine	~0 - 100	~0 - 60	0.041
(2) c	4% PEGDA, 2% MBA, 5% gelatine	~0 - 15	~0 - 8	0.026

3.2. Long-term stability of PABIG and VIPAR

Post-effect in various polymeric systems were investigated since the early studies (Chapiro, 1962). In polymer gel dosimeters, these reactions may continue for a significant period (McJury et al 1999). Therefore, time dependent changes of the irradiated PABIG and VIPAR gels were examined. In Figures 1 and 2 R_2net dependencies on absorbed doses for PABIG and VIPAR compositions, respectively are presented. Additionally, in Figure 3, the dose sensitivity vs. time of storage of PABIG and VIPAR after irradiation is shown.

Fig. 1. The dependence of the relaxation rate on the dose absorbed by PABIG composition - a); linear range of the dependence- b). Measurements were registered 0 - 48 hours after irradiation at 25°C.

Fig. 2. Dependence of the relaxation rate on the dose absorbed by VIPAR composition- a); linear regions of the dependence - b); registered 0 - 69 hours after irradiation at 25°C. Inset: the comparison of R_2net dose response for PABIG and VIPAR compositions. Measurements were taken at 25°C directly after irradiation.

It may be discerned that the dynamic dose response of PABIG composition ranges between 0 and 100 Gy for the composition consisting of equal parts per weight of co-monomers (Fig. 1). However, after irradiation, the shape of R_2net curve for PABIG alters, affecting its sensitivity (Fig. 3).

Fig. 3. The dependence of the dose sensitivity on the time of storage after irradiation for a) PABIG, b) VIPAR.

The gel is stable after approximately 20 hours post irradiation. An interesting behaviour of the R2net curve recorded 24 hours post irradiation was noticed (Fig. 1). The formerly observed plateau, for the curve right after irradiation, no longer exists and R_2net slightly decreases for doses above 60 Gy. Such behaviour might be caused by a degradation of either the structures that were formed by polymerisation and crosslinking or the gelatine matrix or gelatine chains themselves since this process occurs after the gel reaches the maximum of R_2net corresponding to the total monomer conversion. It is likely that degraded structures become more flexible and the T₂ relaxation time of the water protons increases and thus R_2net decreases. A similar phenomenon was observed elsewhere for BANG-type composition that exhibits low dose range response. However, the BANG-type gel was non-homogenously irradiated with ¹⁹²Ir source. It was implied, the monomer diffusion occurs from the regions of low to high absorbed doses in the gel dosimeter following their polymerisation. It has been suggested, this leads to the dose overestimation in the saturation area (De Deene et al, 2001).

R_2net dynamic changes of VIPAR composition during the first 69 hours after irradiation are depicted in Fig. 2 and Fig. 3. One may estimate that the equilibrium for this gel is reached after approximately 20 hours post irradiation. The R_2net fast change is observed up to 100 Gy, above which it approaches a plateau, however, up to the dose of 300 Gy it does not plateau (Fig. 2). The key characteristic of this gel is the starting point of the R_2net linear relation that is assessed to be approx. 10 Gy (Fig. 2). This implies, the measurements with VIPAR composition may be constrained if the detection of low-range doses is considered. Similar observations were illustrated elsewhere (Kipouros et al 2001). In the work of Kipouros et al, the alteration of VIPAR sensitivity was found to occur during the post-irradiation time and VIPAR dose response was shown to start at the dose of approximately 5 Gy. The discrepancies in the dose thresholds for VIPAR can be caused by a magnetic field applied in NMR measurements (Maryanski et al 1993) that was 1.5 T in the case of the Kipouros et al experiment and 0.5 T in this work.

3.3. PABIG and VIPAR R_2net dependence on monomer ratio and measurement temperature

In Fig. 4a, R_2net versus absorbed dose for PABIG gel is presented for various ratios of N,N'-methylenebisacrylamide (MBA) to poly(ethylene glycol) diacrylate depicted as weight fraction of MBA. The differences between the curves are significant especially in the case of saturation level of the gel. This level increases with the higher concentration of MBA in PABIG gel. It should be underlined that the highest fraction of MBA in the gel composition was equal to 0.57 since the solubility of the compound in water is hindered for concentrations above 4% (w/v). For higher MBA concentrations, the compound crystallises while the gel is stored. Similar R_2net dependencies were also observed for VIPAR composition (Fig. 4b).

Fig. 4. The dependence of the relaxation rate on the dose absorbed by a) PABIG, b) VIPAR for various ratios of N,N'-methylenebisacrylamide to poly(ethylene glycol) diacrylate or N-vinylpyrrolidone, respectively, given as the weight fractions of N,N'-methylenebisacrylamide (total monomer concentration 8% w/v). Measurements were taken at 25°C directly after irradiation.

The saturation levels of PABIG and VIPAR, represented as R₂^max values, are given in Figs. 5a and 5b, in dependence of MBA weight fraction and measurement temperature. In the case of PABIG dosimeter, R₂^max values signify the relaxation rate at the dose above which no further change of relaxation is observed. However, since in the case of VIPAR gels there was no saturation observed in the range of measured doses for 0.50 and 0.60 weight fraction of MBA, therefore the maximum relaxation rates correspond to the doses indicated by an upper index of R₂. For both compositions, the saturation levels increase with higher MBA weight fraction (Figs. 5a, 5b) and decrease if the temperature rises (Figs. 6a and 6b).

Fig. 5. The dependence of the maximal relaxation rate of PABIG composition measured at the saturation dose - a) and the relaxation rate of VIPAR composition measured at the dose of 285 Gy - b), on the weight fraction of N,N'-methylenebisacrylamide (total monomer concentration 8% w/v). Measurements at various temperatures, 5 hours after irradiation (PABIG) and at the temperature of 25°C, 20 hours after irradiation (VIPAR).

The R_2max values are related to the useful range of absorbed doses that can be applied for measurements with the aid of a gel dosimeter. Although this range usually consists of a linear and non-linear part it is possible to apply both in 3-D dose distribution measurements, which was previously shown (Kipouros et al, 2003). In order to increase this range, higher weight fraction of MBA should be used and the T₂ measurements should be performed in reasonably low temperature.

Fig. 6. The temperature dependence of the maximal relaxation rate of PABIG - a), and the relaxation rate of VIPAR measured at the saturation dose and at 340 Gy - b), on the weight fraction of N,N'- methylenebisacrylamide (total monomer concentration 8% w/v).

3.4. Influence of monomer ratio and measurement temperature on PABIG and VIPAR sensitivity

It was shown in the section 3.2 that both PABIG and VIPAR reach equilibrium during the first few hours after irradiation. During this time period, the sensitivities of these compositions change as well. Apart from the post effect following irradiation of gels, two other factors, temperature of T₂ NMR measurements (temperature of samples) and monomer ratio, affect the sensitivities. Figs. 7a and 7b relate to the dose sensitivity dependence versus the temperature of measurement of PABIG and VIPAR, respectively. The increase in the temperature of measurements leads to decreasing the sensitivity of these polymer gels. In the Fig. 8a and 8b, the influence of the monomer ratio on PABIG and VIPAR sensitivity is given. It is clear that the dose sensitivity increases up to 0.5 weight fraction of MBA in every gel composition and at this point it is maximal. For higher concentrations of MBA it decreases. The sensitivity of PABIG composition seems to be slightly lower than VIPAR (Fig. 8b).

Fig. 7. The dependence of the dose sensitivity on the temperature of a) PABIG, b) VIPAR gels for various weight fractions of N,N'-methylenebisacrylamide (total monomer concentration 8% w/v).

It may be concluded that the preparation of the optimal gel composition it is a compromise between the dose range and the dose sensitivity. Increasing the MBA concentration in the gels results in higher dose range and dose sensitivity, however only up to equal concentrations of co-monomers in every gel composition. Further increase in MBA concentration causes the decrease of the dose sensitivity although the dose range increases. Therefore, PABIG and VIPAR compositions of 8% monomer concentration and equal monomer weight fractions were selected for the application studies in clinical dosimetry due to their extended dose range and high sensitivity (i.e.: Kipouros et al, 2001, Kipouros et al, 2003, Sandilos et al, 2004, Karaiskos et al, 2005, Papagiannis et al, 2005a).

Fig. 8. The dose sensitivity dependence versus the weight fraction of N,N'-methylenebisacrylamide for PABIG measured at various temperatures (5 hours after irradiation) - a), and for VIPAR in comparison with PABIG (25°C, directly after irradiation) - b).

Similar results of the influence of monomer ratio, measurement temperature and absorbed dose on R2 NMR were achieved for BANG composition (Maryanski et al, 1997). It was shown that relaxation of BANG gels increases when both temperature of measurement decreases and concentration of MBA increases. Dose sensitivity and maximal relaxation rate of BANG similarly depend on monomer ratio and measurement temperature as in the case of PABIG and VIPAR. However, the difference in composition of BANG and either PABIG or VIPAR, makes this gel more sensitive to radiation, but it can be useful for measurements of narrower ranges of absorbed doses.

3.5. Threshold dose

Following Audet's description of R₂ dose response model for polymer gel dosimeter (Audet, 1999), the relaxation changes of the irradiated gel dosimeter may be described by the equations (1) and (2):

R₂ = a D + R_2.0'

where ? - sensitivity of a gel dosimeter; D - dose of radiation; R_2.0 - relaxation rate of a non-irradiated polymer gel. For the saturation conditions, when R₂ = R₂^max:

D_sat = (R₂^max - R_2.0)/a.

The conversion of the co-monomers (x%) initiated by the absorbed unit dose (D_unit) may be given by equation, as follows (on the basis of Maryanski et al, 1997):

100%/D_sat = x%/D_unit'

The results given in Figs. 4a and 4b as well as Eq. 2 were used for the saturation dose calculation for PABIG and VIPAR. Subsequently, applying Eq. 3, the x%/D_unit dependence on the ratio of MBA to PEGDA and MBA to NVP for PABIG and VIPAR, respectively (given as weight fraction of MBA), were achieved (Figs. 9a and 9b).

Fig. 9. The dependence of the per cent conversion of monomers per unit dose on the weight fraction of N,N'-methylenebisacrylamide in a) PABIG, b) VIPAR.

The data shows that for higher MBA concentrations in both gel compositions, the radiation yield of the crosslinked structures decreases. It is worth noting that the conversion of co-monomers in PABIG is more efficient in comparison to VIPAR. For equal concentration of co-monomers in both compositions, 2.7% of co-monomers are converted to crosslinked structures by absorption of unit dose in the case of PABIG, whereas 0.88% in the case of VIPAR. This dissimilarity is caused by the reactivity of the second monomer in the gels (since MBA is used in both compositions). According to Darwis (Darwis et al, 1993) and Rosiak (Rosiak, 1994) 100% conversion of NVP irradiated in aqueous solution is reached after the dose of approximately 1 kGy, however only after 20 Gy is PEGDA completely converted in aqueous solution (2 - 60%) (unpublished data). For comparison, in BANG composition, 8.3% co-monomers are converted after absorption of the unit dose in similar conditions (Maryanski et al, 1997).

The above results based on simplified calculations may give a rough explanation of the R₂ dose response differences of the three gel dosimeters. It may be assumed that the range of measurable doses for the gel dosimeters directly relates to the efficiency of conversion of monomers, which is the highest in the case of VIPAR and the lowest for BANG. The high range of measurable doses for VIPAR was shown to be invaluable in several radiotherapy dosimetry applications (Kipouros et al, 2001, Kipouros et al, 2003, Karaiskos et al, 2004). The lowest co-monomers conversion efficiency of VIPAR impacts on the low dose region and the dose threshold that was shown here to be around 10 Gy. Higher efficiency of co-monomers conversion in BANG and PABIG makes these gels suitable for low dose measurements, however, with the aid of the latter, it is possible to measure doses of extended range (approx. 100 Gy).

4. Summary

In this work, the features of the PABIG polymer gel dosimeter are presented in comparison with VIPAR composition. Results have revealed that the dose sensitivity, dose range and threshold dose are affected by the measurement temperature, monomer ratio, monomer concentration and post irradiation reactions. Substitution of N-vinylpyrrolidone with poly(ethylene glycol) diacrylate in PABIG results in lower dose threshold. Parallel, high doses can be detected with PABIG, however lower than with the aid of VIPAR composition. Due to these unique features, PABIG composition has been used to dose distribution measurements in radiotherapy (Sandilos et al, 2004, Pantelis et al, 2005).

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A few polymer gel compositions are Polish Patent protected:

M. Kozicki, J. M. Rosiak, L. Sakelliou, A. Angelopoulos
"Dozymetr żelowy do pomiarów rozkładów przestrzennych dawek promieniowania jonizującego i promieniowania UV" (eng. "Gel dosimeters for UV light and ionizing radiation dose distribution spatial measurements")
Polish Patent No 208876 (2011.01.14); (submitted in 2007)
M. Kozicki, J. M. Rosiak, L. Sakelliou, A. Angelopoulos
"Dozymetr żelowy do pomiarów rozkładów przestrzennych dawek promieniowania jonizującego i promieniowania UV" (eng. "Gel dosimeters for UV light and ionizing radiation dose distribution spatial measurements")
Polish Patent No 208875 (2011.01.14); (submitted in 2007)