Evaluation of MRI-based MAGIC polymer gel dosimeter in small photon fields

Background: Accurate small radiation field dosimetry is essential in modern radiotherapy techniques such as stereotactic radiosurgery [SRS] and intensity modulated radiotherapy [IMRT]. Precise measurement of dosimetric parameters such as beam profile, percentage depth doses and output factor of these beams are complicated due to the electron disequilibrium and the steep dose gradients. In the present work the MAGIC polymer gel was used for dosimetry of small circular photon beams. The results of MAGIC were compared with EBT2 measurements and Monte Carlo [MC] calculations. Materials and Methods: Experimental measurements were made by mentioned dosimeters in four small field sizes 5, 10, 20 and 30 mm. The BEAMnrc code based on EGSnrc was used for simulation to calculate dosimetric parameters at these small fields. The phantoms were irradiated in a 6 MV photon beam Varian 2100C linear accelerator at SSD=100 cm. gel readout performed by 3 Tesla MRI scanner. Results: The results showed that the Percent depth dose [PDD] values measured and calculated by EBT2 film and MC had maximum local differences 4% and 5% with PDD values measured by MAGIC for field size of 5mm respectively. These differences decreased for larger field sizes. The measurements of output factor and penumbra [80%-20%] and [90%-10%] showed good agreement between the measurements and MC calculation. Conclusion: This study showed that the MAGIC polymer gel based on high resolution MRI images is useful detector for small field dosimetry but its agreement with MC is less than agreement of EBT2 film with MC

Three-dimensional gel dosimetry for dose volume histogram verification in compensator-based IMRT

Some tissues in human body are radiobiologically different from water and these inhomogeneity must be considered in dose calculation in order to achieve an accurate dose delivery. Dose verification in complex radiation therapy techniques, such as intensity-modulated radiation therapy [IMRT] calls for volumetric, tissue equivalent and energy independent dosimeter. The purpose of this study is to verify a compensator-based IMRT plan in anthropomorphic inhomogeneous phantom by Dose Volume Histograms [DVH] using polymer gel dosimetry. An anthropomorphic pelvic phantom was constructed with places for gel inserts. Two attached cubic inserts for prostate and bladder and a cylindrical insert for rectum. A prostate treatment case was simulated in the phantom and the treatment was delivered by a five field compensator-based IMRT. Gel dosimeters were scanned by a 1.5 Tesla magnetic resonance imaging [MRI]. Results were analyzed by DVH and difference of differential DVH. Results showed for 3D compensator-based IMRT treatment plan for prostate cancer, there was overall good agreement between calculated dose distributions and the corresponding gel measured especially in planning target volume [PTV] region. Our measurements showed that the used treatment plan configuration has had clinically acceptable accuracy and gel dosimetry can be considered as a useful tool for measuring DVH. It may also be used for quality assurance and compensator-based IMRT treatment verification

Verifying the accuracy of dose distribution in gamma Knife unit in presence of inhomogeneities using PAGAT polymer gel dosimeter and MC simulation

Polymer gel dosimetry is still the only dosimetry method for direct measuring of threedimensional dose distributions. MRI Polymer gel dosimeters are tissue equivalent and can act as a phantom material. In this study the obtained isodose maps with PAGAT polymer gel dosimeter were compared to those calculated with EGSnrs for singleshot irradiations of 8 and 18 mm collimators of Gamma Knife [GK] unit in homogeneous and inhomogeneous phantoms. A custom-built, 16 cm diameter spherical Plexiglas head phantom was. Inside the phantom, there was one cubic cutout for insertion of gel phantoms, and another cutout for inserting the inhomogeneities. The phantoms were scanned with a Siemens clinical 1.5 T MRI scanner. The multiple spin-echo sequence with 32 echoes was used for the MRI scans. The results of measurement and simulation in homogeneous and inhomogeneous phantoms showed that the presence of inhomogeneities in head phantom could cause spatial uncertainty higher than +/- 2 mm and dose uncertainty higher than 7%. the presence of inhomogeneities could cause dose differences which were not in accordance with accuracy in treatment with GK radiosurgery. Moreover, the findings of Monte Carlo calculation revealed that the applied simulation code [EGSnrc] was a proper tool for evaluation of 3D dose distribution in GK unit

Verification of the PAGAT polymer gel dosimeter by photon beams using magnetic resonance imaging

In this work investigation of the normoxic PAGAT polymer gel dosimeter such as sensitivity, the R2-dose response with post time and the percentage depth dose [PDD] of PAGAT polymer gel dosimeter have been undertaken. Using MRI, the formulation to give the maximum change in the transverse relaxation rate R2 was determined to be 4.5% N,N'-methylenbis- acrylamide [C][bis], 4.5% acrylamid[AA], 5% gelatin, 5 mM tetrakis [hydroxymethyl]phosphonium chloride [THPC], 0.01 mM hydroquinone [HQ] and 86% HPLC [Water]. Irradiation of vials was performed using photon beams by Co-60 therapy unit and an Electa linear accelerator. Gel dosimeters were imaged in a Siemens Symphony 1.5 Tesla clinical MRI scanner using a head coil. Gel dosimeters were irradiated 1 day post-manufacture, and imaged 1 day post irradiation. The R2- dose response was linear up to 30 Gy. The response of the PAGAT gel is very similar in the lower dose region and the R2-dose response for doses less than 2 Gy is not exact. The R2-dose response of the PAGAT polymer gel dosimeters varies between 10 to 30 Gy with R2-dose sensitivities of 0.0905, 0.1037, 0.1023, 0.0907 and 0.123 S-1Gy-1 when imaged at 1, 8, 15, 29 and 38 days post-irradiation respectively. The percentage depth dose [PDD] of PAGAT gel dosimeter was determined and at depth of 21cm, the percentage depth dose for 1.25, 4, 6 and 18 MV photons were determined to be, 48%, 52%, 57.3% and 59.73% respectively. Calibration curve of the PAGAT polymer gel dosimeter, were found to be linear between 2-10 Gy and 10-30 Gy with different slopes of the R2-dose sensitivity also showed stability with post time imaging after 38 days. In this study, the percentage depth dose [PDD] of the PAGAT gel dosimeter was measured. It can be concluded that in case of higher energy of photon beams, higher doses can be delivered to deep-seated tumors

new approach to contrast enhancement in MAGICA gel dosimeter image with MRI technique

Sensitivity and resolution are two main parameters that have to be measured in gel dosimetry. However, the resolution in gel is strongly dependant on gel composition. Selection of optimum method in dose response readout and proper values of parameters can result in noise reduction as well as improvement of contrast, and spatial resolution considerably. MAGICA polymer gel dosimeters were manufactured and irradiated to different doses using a 60Co therapy unit. Imaging was performed in a 0.5T MRI with 8 echoes in air and water as a hydrogenous environment. Imaging condition was kept constant, as much as possible, in both imaging modalities. Images obtained from these two procedures were compared quantitatively. R2- dose curves have three different sections, sensitivity obtained in these three sections were 1.039, 1.671, 1.260 Gy[-1]S[-1] and 1.032, 1.729, 1.37 Gy[-1]S[-1] for water and air respectively. Calibration errors were investigated and graphically were compared in two different methods. Imaging in water medium for doses lower than 17 Gy led to a small reduction in spatial resolution was exchanged to a considerable increase of contrast in R2 map. For doses higher than17 Gy, imaging in water or air was preferred depending on the importance of contrast or spatial resolution

Enhancement of MD-55-2 radiochromic film sensitivity using a multilayer film technique for applications in the low dose range

MD-55-2 is one of the Radiochromic film models with the sensitivity suitable for dose measurements ranging from 5 to 100 Gy. However, this lower limit makes the film impractical for its applications in many areas such as brachytherapy source dosimetry. n this project, the useful range of the film has been extended by using a multilayer film technique. In this technique, single-, double-, and triple- layers of films were exposed to the doses ranging from 0.5 to 10 Gy using a Co-60 photon beam. Calibration curves for corresponding layers of films were obtained with a spectrophotometer using a 680nm wavelength. The results indicated that the sensitivities of double and triple layers were approximately 200% and 300%, respectively, higher than a single-layer film. The impact of multilayer film arrangement on the energy dependence of the MD-55-2 Radiochromic film has also been examined using 100KVp, 80 KVp, and 6 MV X-ray beams. The results indicated an insignificant [within 5%] change in film responses with the beam energy. Therefore, the multilayer technique enhances the Radiochromic film sensitivity and expands its application to the low dose range in field of brachytherapy source dosimetry

Brachytherapy polymer gel dosimetry with xCT

Polymer gels are an emerging new class of dosimeters which are being applied to the challenges of modern radiotherapy modalities. Research on gel dosimetry involves several scientific domains, one of which is the imaging techniques with which dose data is extracted from the dosimeters. In the current work, we present our preliminary results of investigating capability of X-ray CT for extracting brachytherapy dose distributions from a normoxic gel dosimeter. A normoxic radiosensitive polymer gel was fabricated under normal atmospheric conditions and poured into three phantoms. Using Cs137 brachytherapy sources, the phantoms were irradiated with different dose distributions with a LDR Selectron remote after-loader. To improve SNR, 25 images were obtained of each slice for image averaging and an averaged background image of an un-irradiated gel phantom was then subtracted for artifact removal. To further improve the accuracy, a self-consistent normalized method was used for calibration of the dosimeters based on an assumption of a linear dose response between zero and maximum dose regions in the gel. Although results reveal very similar CT-number gradients to that of brachytherapy dose distributions, but the method does not fulfill brachytherapy dosimetry requirements. This might be due to the high prescribed doses in this study which in turn results in a large change in the CT numbers. This change in the CT numbers of the images can not be considered to have a linear relationship with dose which was the basic assumption of our calibration method, so the results are just qualitatively comparable. In this study, the results of using X-ray CT for brachytherapy polymer gel dosimetry is promising but not still satisfying. Improving a proper calibration method for correlating CT numbers to dose will be significantly helpful for performing measurements with CT. The main limitation for CT is still a low signal to noise ratio especially in lower dose areas

3D MRI gel dosimetry bases on image intensity: a new approach

Since 1984 MRI gel dosimetry has been introduced as a potential technique for 3D dosimetry. Most of the studies have measured R1 [1/T1] or R2 [1/T2] properties of the gel depending on the gel type. We have studied image intensity change in the Fricke gel by different MRI protocols. Materials and Gel Dosimeters contain 0.4 mM ferrous sulphate, 1 mM NaCl, 50 mM H2SO4 and 1% by weight agarose in distilled water. Prepared gels were poured in Plastic tube phantom and irradiated to a beam of Co-60 gamma rays. Imaging was performed by a 0.5T MRI system in the head coil with SE and GRE techniques. Our results showed that linear response exists between the variations of image intensity with absorbed dose [1-15 Gy]. Optimal imaging parameters should be defined locally according to the type of MRI scanner and exact composition of the gel. Gradient echo [GRE] imaging technique also have been studied in comparison with classic spin echo [SE] imaging technique which will be discussed in detail. Conclusions: Linearity of absorbed dose with intensity exists up to 15 Gy and can be used for MRI gel dosimetry. Reduction of imaging time is achievable in image intensity technique so that it's possible to image the gel in less than 20 minutes, which is critical to over-come the adverse ion-diffusion properties of the Fricke gel

Diffusion measurement in ferrous infused gel dosimeters

The compositions of Ferrous sulphate, Agarose and Xylenol orange dye [FAX] and Ferrous sulphate, Gelatin and Xylenol orange dye [FGX] in solution of distilled water and sulphuric acid are two tissue-equivalent gel dosimeters. Ionizing radiation causes oxidation of Fe2+ ion to Fe3+ ions which diffuse through the gel matrix and blur the image of absorbed dose over a period of hours after irradiation. Materials and methods: 25 mM sulphuric acid, 0.4 mM ferrous ammonium sulphate, 0.2 mM xylenol orange dye and 1% by weight agarose in distilled water named FAX and 0.1 mM ferrous ammonium sulphate, 0.1 mM xylenol orange dye, 50 mM sulphuric acid and 5% by weight gelatin in distilled water named FGX are used as two gel dosimeters. All chemicals were supplied by Sigma Aldridge Company, Germany. The gels were poured in Perspex casts and were irradiated to a beam of X ray from linear accelerators or x ray machine. In this study diffusion coefficients of FAX and FGX dosimeters have been measured through a computer program for different temperatures. The ferric ion diffusion coefficient [D] for the FAX and FGX dosimeters were measured as [1.19 +/- 0.03] x 10-2 cm2.hr-1 and [0.83 +/- 0.03] x 10-2 cm2.hr-1 respectively at room temperature. For both dosimeters the diffusion coefficients decreased with gel storage temperatures down to 6oC. FGX dosimeters have advantage of lower diffusion coefficient for a specified temperature

Dental tissue as a TLD dosimeter

Thermoluminescence dosimetry is one of the dosimetry procedures used widely as routine and personal dosimeters. In order to extend this kind of dosimeters, dental tissue has been examined and was found promising as a TLD dosimeter. Materials and In this study, 70 health teeth were collected. The only criterion, which was considered for selection of the teeth, was the healthiness of them regardless of age and gender of the donors. All collected samples were washed and cleaned and milled uniformly. The final powder had a uniform grain size between 100 - 300 micrometer. The sample was divided into four groups. Group A and B were used for measurement of density and investigation of variation of thermoluminescent characteristics with temperature respectively. Groups C and D were used for investigation of variation of thermoluminescent intensity with dose and fading of this intensity with time. In all cases the results obtained with dental tissue were compared to a standard LiF, TLD dosimeter. It was found that, average density of the dental tissue was 1.570 g/cm3, which is comparable to density of LiF, which is 1.612 g/cm3. It was also concluded that the range of 0-300°C, dental tissue has a simple curve with two specific peaks at 140 and 250°C respectively. The experiment also showed that, the variation of relative intensity versus dose is linear in the range of 0.04 - 0.1Gy. The fading rate of dental tissue is higher than LiF but still in the acceptable range [14% per month in compare to 5.2% per month] Dental tissue as a natural dosimeter is comparable with TLD and can be used in accidental events with a good approximation