Journal of Medical Physics
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BOOK REVIEW
Year : 2007  |  Volume : 32  |  Issue : 3  |  Page : 133-134
 

A Ph.D. thesis by S. Brindha entitled 'Investigation of the characteristics of fricke and normoxic polymer gel dosimeters with optical, magnetic resonance and computed tomography readout systems'


Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW 2006, Australia

Correspondence Address:
Clive Baldock
Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW 2006
Australia
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How to cite this article:
Baldock C. A Ph.D. thesis by S. Brindha entitled 'Investigation of the characteristics of fricke and normoxic polymer gel dosimeters with optical, magnetic resonance and computed tomography readout systems'. J Med Phys 2007;32:133-4

How to cite this URL:
Baldock C. A Ph.D. thesis by S. Brindha entitled 'Investigation of the characteristics of fricke and normoxic polymer gel dosimeters with optical, magnetic resonance and computed tomography readout systems'. J Med Phys [serial online] 2007 [cited 2019 Oct 17];32:133-4. Available from: http://www.jmp.org.in/text.asp?2007/32/3/133/35726


S. Brindha

Department of Radiotherapy, Christian Medical College, Vellore and Tamil Nadu Dr. M. G. R. Medical, University, Chennai, India


Ever since Gore's seminal 1984 paper on the subject, gel dosimetry continues to show promise as the only true 3D radiotherapy dosimeter for stereotactic radiotherapy, conformal radiotherapy and intensity-modulated radiotherapy (IMRT). Traditionally, magnetic resonance imaging (MRI) was used for the evaluation of absorbed dose distributions in gel dosimeters. However, more recently, other evaluation techniques have also been used including optical and X-ray computed tomography (CT).

S. Brindha is to be commended on writing a significant doctoral thesis on the various aspects of the radiotherapy gel dosimetry technique. In this research project, the feasibility of using the ferrous-xylenol orange-gelatin (FXG) gel dosimeter and the normoxic polymer gel dosimeter for radiotherapy applications has been investigated with MRI, X-ray CT and laser CT.

In order to establish gel dosimetry as a verification tool for radiotherapy, the dose, dose rate and energy dependence of the aqueous modified FBX (MFBX) chemical dosimeter were initially studied and compared with those of the thermoluminescent dosimeter (TLD). It was concluded that the MFBX dosimeter was energy-, dose- and dose rate-independent with a good comparison found with the TLD and the ion chamber. The reagents used in the preparation of the MFBX dosimeter were available locally and with relatively straightforward preparation, the technique could be used for both in vivo measurements and data collection in small radiotherapy centers. The dosimeter in its aqueous state could not however be used for measurements of dose distributions. In order to verify dose distributions of complex treatment plans, the gelatin-based FXG Fricke gel dosimeter was studied.

The dose response of the FGX Fricke gel dosimeter was found to be linear irrespective of the acid concentration for both 0.1 mM and 0.2 mM xylenol orange concentrations. The optical density response was found to be the highest for a sulphuric acid concentration of 50 mM for doses above 1 Gray (Gy). Similarly, the maximum optical density-dose sensitivity was observed for a concentration of 0.1 mM xylenol orange, 0.1 mM ferrous ammonium sulphate and 5% gelatin. The optimum combination for the preparation of Fricke gel was thus concluded to be 5% gelatin, 0.1 mM ferrous ammonium sulphate, 0.1 mM xylenol orange and 50 mM sulphuric acid. A major limitation however in studying the application of the FXG Fricke gel dosimeter in radiotherapy dosimetry is diffusion, which results in a loss of spatial information.

Due to the diffusion in the Fricke gel dosimeter, work was subsequently undertaken with normoxic polymer gel dosimeters. Initially, radiological attenuation properties of the normoxic polymer gel dosimeters were investigated in order to establish the feasibility of using X-ray CT as an evaluation tool for normoxic polymer gel dosimeters. The imaging protocol of a Siemens Somatom diagnostic X-ray CT scanner was optimized to evaluate the Polyacrylamide And Gelatin And Tetrakis (PAGAT) normoxic polymer gel dosimeter at the highest available tube voltage of 130 kV and highest available tube current of 150 mA to enable maximum number of photons to reach the detector, thereby reducing the noise in the acquired image. The optimal slice thickness was determined to be 3 mm while evaluating small, irradiated fields and 5 mm for larger fields. Optimal scan parameters may vary with different X-ray CT scanners so that each scanner used for evaluating the normoxic polymer gels would require individual optimization for the purpose of gel dosimetry evaluation. The total concentration of monomers in the polymer gel dosimeter formulation was optimized at 7% with 10 mM tetrakis(hydroxymethyl)phosphonium (THP) to obtain a maximum irradiated radiation-induced change in the CT number. The dose response curve of the PAGAT normoxic polymer gel dosimeters obtained with X-ray CT was compared with that obtained with MRI. The gel had a linear dose response up to 6 Gy using MRI and a linear dose response up to 10 Gy using X-ray CT. These results compared favorably with previous studies of the hypoxic PAG.

Due to the limited access of MRI and X-ray CT facilities, a laser-based optical CT scanner was constructed and evaluated for the investigation of the normoxic polymer PAGAT gel dosimeter. The spatial resolution, determined to be 2 mm, could be further improved using a smaller diameter laser. The feasibility of using the PAGAT normoxic polymer gel dosimeter was evaluated with a laser CT scanner for verification of IMRT treatment plans. All the three cases investigated showed a good comparison between the treatment planning system (TPS) and gel dosimeter. The gamma distribution for the comparison of TPS and gel dosimeter dose distributions for all the three cases passed the gamma criteria (3 mm spatial and 3% dose deviation) in all regions except the low dose region and the region near the walls of the polymer gel dosimeter container which could be overcome by using larger gel phantoms. To obtain a dose distribution, an absolute measurement of radiation dose calibration of each batch of gel was required. The time required to verify the dose distribution for an IMRT plan was 20 minutes including the time taken for the postirradiation scan. It was concluded that an accurately calibrated normoxic polymer PAGAT gel dosimeter evaluated with optical CT could be used as a quality assurance tool for dose distribution verification in complex treatment plans with high dose gradients.

S. Brindha's doctoral thesis has made a significant contribution to the literature in this continually evolving area of 3D dosimetry.




 

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