Purpose: The technique known as 'Gamma Evaluation Method' was an effective measure for dose distribution comparison; however, it could not be used for accurate real-time verification because of the unstable systematic factors. To reduce the errors induced by systematic factors and there was a requirement for real-time dose verification in Dose-guided Radiotherapy (DGRT), the formula of Gamma Method was improved, and four-dimensional(4D) Gamma Method was proposed.
Methods: The gamma index, showing the deviation of two sets of dose distributions, should remove the error caused by different treatment delivery time to realize accurate real-time verification in Dose-guided Radiotherapy. Besides dose and distance, the variable of time, which was proportional to monitor unit (MU), was added to the formula. The criterion for acceptable distribution was generally defined as a tolerance of the dose, the distance and monitor unit. The algorithm was designed and realized using the tool of Microsoft Visual C++ 6.0.
Results: Two typical five beam treatment plans of Conformal Radiotherapy and Intensity-Modulated Radiotherapy were adopted to verify the algorithm. The two-dimensional (2D) ionization chamber MatriXX was used to measure the dose distribution. The verification for each beam was as follows: (1) analyze the evaluation results of gamma method and the improved gamma method. (2)Analyze the evaluation results of measured dose distributions at different time with the same dosage .The result showed that there existed errors caused by systematic factors before formula was improved. Improved method could obviously reduce the error induced by systematic factors, meeting DGRT's requirement for accurate real-time verification. The result could also be used to judge the stability of the accelerator.
Conclusions: The 4-dimensional gamma method could reduce errors caused by systematic factors, and it could be used to reflect the stability of different accelerators. Supported by the National Natural Science Foundation under grant No.30900386 and the National Science Foundation of Anhui Province (Nos.090413095 and111040606Q55).
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