• Nuclear Engineering and Technology
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• v.42 no.6
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• pp.670-679
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• 2010

In this paper, a new approach using a pixel-based correction method was developed to fix the non-uniform responses of flat-bed type scanners used for radiochromic film dosimetry. In order to validate the method's performance, two cases were tested: the first consisted of simple dose distributions delivered by a single port; the second was a complicated dose distribution composed of multiple beams. In the case of the simple individual dose condition, ten different doses, from 8.3 cGy to 307.1 cGy, were measured, horizontal profiles were analyzed using the pixel-based correcton method and compared with results measured by an ionization chamber and results corrected using the existing correction method. A complicated inverse pyramid dose distribution was made by piling up four different field shapes, which were measured with GAFCHROMIC$^{(R)}$EBT film and compared with the Monte Carlo calculation; as well as the dose distribution corrected using a conventional method. The results showed that a pixel-based correction method reduced dose difference from the reference measurement down to 1% in the flat dose distribution region or 2 mm in a steep dose gradient region compared to the reference data, which were ionization chamber measurement data for simple cases and the MC computed data for the complicated case, with an exception for very low doses of less than about 10 cGy in the simple case. Therefore, the pixel-based scanner correction method is expected to enhance the accuracy of GAFCHROMIC$^{(R)}$EBT film dosimetry, which is a widely used tool for two-dimensional dosimetry.

• The Journal of Korean Society for Radiation Therapy
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• v.21 no.1
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• pp.17-23
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• 2009

Purpose: In treating head and neck cancer, it is very important to irradiate uniform dose on the junction of the bilateral irradiation field of the upper head and neck and the anterior irradiation field of the lower neck. In order to improve dose distribution on the junction, this study attempted to correct non uniform dose resulting from under dose and over dose using the field-in-field technique in treating the anterior irradiation field of the lower neck and to apply the technique to the treatment of head and neck cancer through comparison with conventional treatment. Materials and Methods: In order to examine dose difference between the entry point and the exit point where beam diffusion happens in bilateral irradiation on the upper head and neck, we used an anthropomorphic phantom. Computer Tomography was applied to the anthropomorphic phantom, the dose of interest points was compared in radiation treatment planning, and it was corrected by calculating the dose ratio at the junction of the lower neck. Dose distribution on the junction of the irradiated field was determined by placing low-sensitivity film on the junction of the lower neck and measuring dose distribution on the conventional bilateral irradiation of the upper head and neck and on the anterior irradiation of the lower neck. In addition, using the field-in-field technique, which takes into account beam diffusion resulting from the bilateral irradiation of the upper head and neck, we measured difference in dose distribution on the junction in the anterior irradiation of the lower neck. In order to examine the dose at interest points on the junction, we compared and analyzed the change of dose at the interest points on the anthropomorphic phantom using a thermoluminescence dosimeter. Results: In case of dose sum with the bilateral irradiation of the upper head and neck when the field-in-field technique is applied to the junction of the lower neck in radiation treatment planning, The dose of under dose areas increased by 4.7~8.65%. The dose of over dose areas also decreased by 2.75~10.45%. Moreover, in the measurement using low-sensitivity film, the dose of under dose areas increased by 11.3%, and that of over dose areas decreased by 5.3%. In the measurement of interest point dose using a thermoluminescence dosimeter, the application of the field-in-field technique corrected under dose by minimum 7.5% and maximum 17.6%. Thus, with the technique, we could improve non.uniform dose distribution. Conclusion: By applying the field-in-field technique, which takes into account beam divergence in radiation treatment planning, we could reduce cold spots and hot spots through the correction of dose on the junction and, in particular, we could correct under dose at the entry point resulting from beam divergence. This study suggests that the clinical application of the field-in-field technique may reduce the risk of lymph node metastasis caused by under dose on the cervical lymph node.