Résumé
The purpose of this study is to develop the optimization method for adjusting the film isocenter shift and to suggest the quantitative acceptable criteria for film dosimetry after optimization in the dynamic conformal arc radiation therapy (DCAR). The DCAR planning was performed in 7 patients with brain metastasis. Both absolute dosimetry with ion chamber and relative film dosimetry were performed throughout the DCAR using BrainLab's micro-multileaf collimator. An optimization method for obtaining the global minimum was used to adjust for the error in the film isocenter shift, which is the largest part of systemic errors. The mean of point dose difference between measured value using ion chamber and calculated value acquired from planning system was 0.51+/-0.43% and maximum was 1.14% with absolute dosimetry. These results were within the AAPM criteria of below 5%. The translation values of film isocenter shift with optimization were within +/-1 mm in all patients. The mean of average dose difference before and after optimization was 1.70+/-0.36% and 1.34+/-0.20%, respectively, and the mean ratios over 5% dose difference was 4.54+/-3.94% and 0.11+/-0.12%, respectively. After optimization, the dose differences decreased dramatically and a ratio over 5% dose difference and average dose difference was less than 2%. This optimization method is effective in adjusting the error of the film isocenter shift, which is the largest part of systemic errors, and the results of this research suggested the quantitative acceptable criteria could be accurate and useful in clinical application of dosimetric verification using film dosimetry as follows; film isocenter shift with optimization should be within +/-1 mm, and a ratio over 5% dose difference and average dose difference were less than 2%.