Dose evaluation of an NIPAM polymer gel dosimeter using gamma index

An N-isopropylacrylamide (NIPAM) polymer gel dosimeter has great potential in clinical applications. However, its three-dimensional dose distribution must be assessed. In this work, a quantitative evaluation of dose distributions was performed to evaluate the NIPAM polymer gel dosimeter using gamma...

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Veröffentlicht in:Radiation physics and chemistry (Oxford, England : 1993) England : 1993), 2014-11, Vol.104, p.180-187
Hauptverfasser: Chang, Yuan-Jen, Lin, Jing-Quan, Hsieh, Bor-Tsung, Yao, Chun-Hsu, Chen, Chin-Hsing
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Sprache:eng
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Zusammenfassung:An N-isopropylacrylamide (NIPAM) polymer gel dosimeter has great potential in clinical applications. However, its three-dimensional dose distribution must be assessed. In this work, a quantitative evaluation of dose distributions was performed to evaluate the NIPAM polymer gel dosimeter using gamma analysis. A cylindrical acrylic phantom filled with NIPAM gel measuring 10cm (diameter) by 10cm (height) by 3mm (thickness) was irradiated by a 4×4cm2 square light field. The irradiated gel phantom was scanned using an optical computed tomography (optical CT) scanner (OCTOPUS™, MGS Research, Inc., Madison, CT, USA) at 1mm resolution. The projection data were transferred to an image reconstruction program, which was written using MATLAB (The MathWorks, Natick, MA, USA). The program reconstructed the image of the optical density distribution using the algorithm of a filter back-projection. Three batches of replicated gel phantoms were independently measured. The average uncertainty of the measurements was less than 1%. The gel was found to have a high degree of spatial uniformity throughout the dosimeter and good temporal stability. A comparison of the line profiles of the treatment planning system and of the data measured by optical CT showed that the dose was overestimated in the penumbra region because of two factors. The first is light scattering due to changes in the refractive index at the edge of the irradiated field. The second is the edge enhancement caused by free radical diffusion. However, the effect of edge enhancement on the NIPAM gel dosimeter is not as significant as that on the BANG gel dosimeter. Moreover, the dose uncertainty is affected by the inaccuracy of the gel container positioning process. To reduce the uncertainty of 3D dose distribution, improvements in the gel container holder must be developed. •We evaluated the 3D dose characteristics of an NIPAM gel dosimeter.•The NIPAM gel dosimeter has good uniformity and temporal stability.•We used optical CT equipped with an oscillating mirror and Fresnel lens as readout tool.•Gamma evaluation with a criterion of 3%/3mm was performed.•The largest number of rejected points in the gamma map was obtained in the penumbra region.
ISSN:0969-806X
1879-0895
DOI:10.1016/j.radphyschem.2013.11.031