Study on energy dependence of PAGAT polymer gel dosimeter evaluated using X-Ray CT
Introduction
Modern radiotherapy centers realize the highly localized conformal techniques such as Intensity Modulated Radiation Therapy (IMRT), Streotactic Radiosurgery (SRS) and Stereotactic Radiotherapy (SRT) are highly desirable to minimize the dose to nearby critical structures while maximizing the tumor control probability with high uniformity and precision. Since these techniques provide complex three dimensional conformal dose distributions, the precise measurement of the dose delivered to the patient is one of the main goals in clinical dosimetry for treatment plan validation. So the verification of treatment plans is of substantial interest and these techniques have created an urgent need for three dimensional dosimetry with high resolution and accuracy. Dosimetry has been performed conventionally with small volume ion chamber, diode detector, diamond detector and radiographic/radiochromic films etc. However, most current standard dosimetry methods do not meet modern requirements because they allow for dose measurement in one or two dimensions only.
In recent years certain gel dosimeters like Fricke gel and polymer gel are promising dosimetry tools because of their high sensitivity, superior tissue equivalence and the ability to simulate the response of real tissue to radiation exposure (Gore et al., 1984, Maryanski et al., 1993, Maryanski et al., 1994, Olsson et al., 1989, Olsson et al., 1990). Gel dosimetry offers advantages over the conventional planar and point dosimeters in that the dose distributions can be measured three dimensionally with high spatial resolution and accuracy (Ibbott et al., 1997, De Deene et al., 1998, De Deene et al., 2000). It has been proved that new PAGAT (Polyacrylamide, Gelatin and Tetrakis) polymer gel dosimeters fulfill the requirements of conformal radiotherapy accurately and are suitable for the verification of complex 3D dose distribution (Venning et al., 2005a). The PAGAT gel utilizes the principle of radiation induced polymerization and cross-linking of monomers dispersed in a water based matrix like gelatin (Maryanski et al., 1993, Maryanski et al., 1994, Maryanski et al., 1996a). It has been shown that the rate of polymerization is directly proportional to the amount of dose deposited to the polymer gel and causes the change in relaxation rate, optical properties and attenuation of X-Rays etc. So, one can calculate the dose deposition of the gel by measuring change in the relaxation time with magnetic resonance imaging (MRI) (Maryanski et al., 1993, Maryanski et al., 1994, Maryanski et al., 1996a), by measuring optical properties with optical computed tomography (CT) scanner (Gore et al., 1996, Oldham et al., 2001, Maryanski et al., 1996b), by measuring changes in dosimeter X-Ray linear attenuation coefficient with X-Ray CT scanner (Hilts et al., 2000, Trapp et al., 2001, Audet et al., 2002), by measuring ultrasonic speed of propagation and attenuation (Mather et al., 2002, Mather and Baldock, 2003) and also with vibrational spectroscopy (Baldock et al., 1998, Jirasek et al., 2001).
Recently it has been reported that PAGAT polymer gels evaluated with an X-Ray CT scanner provides a sensitive and stable dosimetry system for the verification of dose distributions in radiotherapy (Jirasek et al., 2006). The CT number or Hounsfield units (HU) obtained from the CT images related to the X-Ray linear attenuation coefficient is proportional to change in physical density of polymer gel upon radiation (Trapp et al., 2001, Trapp et al., 2002, Brindha et al., 2006).
The most important dosimetry properties of polymer gel are a linear dose response and a constant sensitivity irrespective of energy and dose rate. For clinical use, one need to evaluate the response of the polymer gel dosimeter to monomer concentration, energy of radiation, beam modality, dose rate, temperature during evaluation, time between irradiation and evaluation, and the imaging parameters like tube voltage, tube current, and exposure time. Since, these factors affect the response of the dosimeter, it is necessary to study the influence of each factor independently (Hill et al., 2005, Hilts et al., 2000, Hilts and Duzenli, 2004a, Hilts and Duzenli, 2004b).
The response of the PAG (Polyacrylamide and Gelatin) gel has been reported to be independent of energy for a wide range of electron and photon beams by several authors (Baldock et al., 1996, Maryanski et al., 1996a, Maryanski et al., 1996b). Based on these results Ibbott et al. (1997) determined the dose response curve with a 6 MV photon beam and applied it to the gel irradiated with Co-60 beam from Gamma knife unit without any corrections. But Novotny et al., 2001a, Novotny et al., 2001b has observed a trend in polymer gel sensitivity dependence on both photon and electron energy. Similarly Farajollahi et al. (1999) and Baldock et al. (1996) irradiated the polymer gel with different photon energies of 300 kV X-Rays, 660 keV, 1.25 MeV, 6 MV, 8 MV and 16 MV and with electron energies of 5, 7, 9, 12, 15, 17 and 20 MeV and they concluded that polymer gels dose response is independent of photon and electron energies. Wuu et al. (2003) has studied the dose response of BANG (Bis Acrylic acid Nitrogen Gelatin) polymer gel for 6 MV photon, 6 MeV electron and 662 keV photon beams and found to be same and linear from 0 to 30 Gy and used the same calibration curve to evaluate the dose distribution around a Re-188 brachytherapy source. Radiological transport properties studied by Pantelis et al. (2004) and Venning et al. (2005b) have reported that normoxic polymer gel dosimeters are independent of energy over the wide range of 100 keV–10 MeV. Based on these results Hurley et al. (2005) have used the calibration curve determined with 6 MV photons for the evaluation of MAGIC (Methacrylic acid, Ascorbic acid in Gelatin Initiated by Copper) gel irradiated with Ir-192 source. De Deene et al., 2001, De Deene et al., 2006 have reported that no change was observed in the dose response curve for both PAG and nPAG (normoxic PolyAcrylamid) gel dosimeters and a small increase with photon energy was observed in the nMAG (normoxic methacrylic based gel) gel dosimeter for the photon beams of 6 MV and 25 MV. Several other workers have reported a decrease in gel dosimeter response when irradiated with high linear energy transfer (LET) particles (Ramm et al., 2000, Jirasek and Duzenli, 2002, Heufelder et al., 2003) and shown that there is a energy dependence in the polymer gel dosimeter response.
The aim of this study was to evaluate the dependence of the PAGAT polymer gel dosimeter response for different photon and electron energies. The response of the polymer gel dosimeter was determined by calculating the slope of the linear portion of dose response curve for each beam energy. The dose response curve was obtained from the percentage depth dose (PDD) curve of each beam energy.
Section snippets
Gel preparation
The polymer gel dosimeter used in this study was the normoxic “PAGAT” gel described elsewhere (Venning et al., 2005a, Brindha et al., 2006) and was prepared under normal atmospheric conditions. The PAGAT polymer gel consisted of 3.5% (w/w) N, N′-methylene-bis-acrylamide (BIS), 3.5% (w/w) acrylamide, 5% (w/w) gelatin, 89% triple distilled water and 10 mM tetrakis hydroxymethyl phosphonium chloride (THP) as antioxidant. Details of elemental composition of PAGAT gel and water are summarized in
Results and discussion
For each photon and electron energy central axis depth dose (PDD) curves were obtained by measuring the Hounsfield (HU) number as a function of depth and values were normalized at a depth of maximum Hounsfield (HU) number. All gel dosimeters data were compared with diode and film measurements. From Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9 it is clear that the gel dosimeter measurements were found to be good agreement with that of diode and film measurements with
Conclusion
This study shows the feasibility of preparing the PAGAT polymer gel under normal atmospheric conditions and the evaluation of dose distribution using the X-Ray CT scanner. Our results shows PAGAT polymer gel has a linear dose response up to 10 Gy with satisfactory correlation between the polymer gel data, diode and film measurements. From the study it was clear that dependence of PAGAT gel dosimeter sensitivity to different modality of the beam used was insignificant and comparable with results
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