Dose evaluation of an NIPAM polymer gel dosimeter using gamma index Yuan-Jen Chang a,b , Jing-Quan Lin b , Bor-Tsung Hsieh c , Chun-Hsu Yao d,n , Chin-Hsing Chen a,n a Department of Management Information Systems, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROC b Institute of Biomedical Engineering and Materials Science, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROC c Department of Biomedical Imaging and Radiological Sciences, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROC d Department of Biomedical Imaging and Radiological Science, China Medical University, Taichung, Taiwan, ROC HIGHLIGHTS  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%/3 mm was performed.  The largest number of rejected points in the gamma map was obtained in the penumbra region. article info Article history: Received 24 June 2013 Accepted 25 November 2013 Keywords: NIPAM gel dosimeter Polymer gel Optical CT Gamma index abstract 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 10 cm (diameter) by 10 cm (height) by 3 mm (thickness) was irradiated by a 4  4 cm 2 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 1 mm 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. & 2013 Published by Elsevier Ltd. 1. Introduction Over the past several decades, 3D dosimetry research has increased rapidly as a result of the development of modern radiation therapy techniques, such as intensity-modulated radiotherapy, tomotherapy, cyber knife, proton therapy, and brachytherapy treatment (Baldock et al., 2010). The development of 3D gel dosimetry is challenged in two major aspects: (1) the development of better gel materials that can provide spatial and dose accuracies and precisions that satisfy the requirements of 3D dose verification for radiation treatments and (2) the development of low-cost readout tools with higher resolution and fast scanning (Baldock et al., 2010; Nelms et al., 2011). Four common types of gel dosimeters have been developed, namely, Fricke gel dosimeters, polymer gel dosimeters, micelle gel dosimeters, and genipin gel dosimeters (McAuley, 2006). The characteristics of each of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/radphyschem Radiation Physics and Chemistry 0969-806X/$ - see front matter & 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.radphyschem.2013.11.031 n Corresponding authors. E-mail addresses: chyao@mail.cmu.edu.tw (C.-H. Yao), chchen@ctust.edu.tw (C.-H. Chen). Please cite this article as: Chang, Y.-J., et al., Dose evaluation of an NIPAM polymer gel dosimeter using gamma index. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2013.11.031i Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎