The objective of this study is to validate a procedure based on a statistical method to assess the agreement and the correlation between measured and calculated dose in the process of quality assurance (QA) for intensity-modulated radiation therapy (IMRT). Methods: Fifty-six fields for head and neck cancer treatment from 10 patients were analyzed. For each patient a treatment plan was generated using Eclipse TPS ®. To compare the calculated dose with the measured dose a CT-scan of solid water slabs (30 × 30 × 15 cm 3) was used. Absolute dose was measured by a pinpoint ionization chamber and 2D dose distributions using electronic portal imaging device dosimetry. Six criteria levels were applied for each field case (3%, 3 mm), (4%, 3 mm), (5%, 3 mm), (4%, 4 mm), (5%, 4 mm) and (5%, 5 mm). The normality of the data and the variance homogeneity were tested using Shapiro-Wilk's test and Levene's test, respectively. The Wilcoxon signed-rank paired test was used to calculate p-values. The Bland-Altman method was used to calculate the limit of agreement between calculated and measured doses and to draw a scatter plot. The correlation between calculated and measured doses was assessed using Spearman's rank test. Results: The statistical tests indicate that the data were not normally distributed, p < 0.001, and had a homogenous variance, p = 0.85. The upper and lower limits of agreement for absolute dose measurements were 6.44% and-6.40%, respectively. The Wilcoxon test indicated a significant difference between calculated and dose measured with the ionization chamber, p = 0.01. Spearman's test indicated a strong correlation between calculated and absolute measured dose, with correlation coefficient ρ = 0.99. Therefore, there is a lack of correlation between dose difference for absolute dose measurements and gamma passing rates for 2D dose measurements. Conclusion: The statistical tests showed that the commonly accepted criteria using gamma evaluation are not able to predict the dose difference for a global treatment plan or per beam. The current QA method provides inadequate protection of the patient. The method described here provides an overall analysis for dosimetric data from calculation and measurement, and can be quickly integrated into QA systems for IMRT.

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Purpose: This study investigates the use of gamma indices (γ) criteria and the digital imaging and communications in medicine (DICOM) images in order to compare dose distribution and readjust the prescription dose due to the change in dose calculation engine, to maintain the same clinical results when changing the dose calculation algorithm in a radiation oncology department.Methods: Twelve treatment plans for lung cancer were analyzed. The dose calculation was performed using two different inhomogeneity correction methods using pencil beam convolution (PBC) turning on 1D and 3D density correction, respectively using the same beam arrangements. This analysis was performed using 2D and 3D γ with a variety of dose-difference and distance-to-agreement: 1%/1 mm, 2%/2 mm, 3%/3 mm, 3%/1 mm and 3%/2 mm. The average γ values and percentages of pixels passing the γ criteria were evaluated. For statistical analysis, Spearman's test was used to calculate the correlation coefficient "rho". Results:The comparison between PBC-1D and PBC-3D showed that 95% of pixels have γ ≤ 1 with 2%/2 mm or 3%/3 mm. There is no impact of distance to agreement using global evaluation on γ passing rates leading to the dose difference only related to turning on/off the 3D density correction instead of 1D density correction method. There was a weak correlation between 2D and 3D γ passing rates. Conclusion:The 2D γ-maps and cumulative pixels-γ-histograms can be used to validate a new dose calculation algorithm and to make a medical decision to readjust the prescribed dose. The 3%/3 mm γ criteria confirms that the modification of the prescription is not justified. However, a careful analysis should be taken for γ-maps concerning the organs at risks and of course; this method should be adjusted for different systems to be compared.