Statistical methods to evaluate the correlation between measured and calculated dose using quality assurance method in IMRT
Purpose: 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).
Patients and methods: 10 patients including 56 fields for head and neck cancer treatment were analyzed. For each patient, one treatment plan was generated using Eclipse TPS®. To compare the calculated dose with measured dose a CT-scan of solid water slabs (30 × 30 × 15 cm3) was used. The measurements were done for absolute dose by a pinpoint ionization chamber and 2D dose distributions using electronic portal imaging device dosimetry. Six criteria levels were applied for each 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-Wilks test and Levene’s test, respectively. Wilcoxon signed-rank paired test was used to calculate p-value. 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 do not fulfill normal distribution, p < 0.001 and had a homogenous variance, p = 0.85. The upper and lower limit of agreements for absolute dose measurements were 6.44% and -6.40%, respectively. Wilcoxon test indicated a significance difference between calculated and measured dose with ionization chamber, p = 0.01. Spearman’s test indicated a strong correlation between calculated and absolute measured dose, 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 common acceptance criteria’s using gamma evaluation are not able to predict the dose difference for a global treatment plan or per beam. The current QA method is limited to protect the patient. The described method provides an overall analysis for dosimetric data issued from calculation and measurement and it can be quickly integrated in QA system for IMRT.
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