Quantitative comparison of dose distribution in radiotherapy plans using 2D gamma maps and X-ray computed tomography

Abstract

Background: The advanced dose calculation algorithms implemented in treatment planning system (TPS) have remarkably improved the accuracy of dose calculation especially the modeling of electrons transport in the low density medium. The purpose of this study is to evaluate the use of 2D gamma (γ) index to quantify and evaluate the impact of the calculation of electrons transport on dose distribution for lung radiotherapy.

Methods: X-ray computed tomography images were used to calculate the dose for twelve radiotherapy treatment plans. The doses were originally calculated with Modified Batho (MB) 1D density correction method, and recalculated with anisotropic analytical algorithm (AAA), using the same prescribed dose. Dose parameters derived from dose volume histograms (DVH) and target coverage indices were compared. To compare dose distribution, 2D γ-index was applied, ranging from 1%/1 mm to 6%/6 mm. The results were displayed using γ-maps in 2D. Correlation between DVH metrics and γ passing rates was tested using Spearman's rank test and Wilcoxon paired test to calculate P values.

Results: the plans generated with AAA predicted more heterogeneous dose distribution inside the target, with P<0.05. However, MB overestimated the dose predicting more coverage of the target by the prescribed dose. The γ analysis showed that the difference between MB and AAA could reach up to ±10%. The 2D γ-maps illustrated that AAA predicted more dose to organs at risks, as well as lower dose to the target compared to MB.

Conclusions: Taking into account of the electrons transport on radiotherapy plans showed a significant impact on delivered dose and dose distribution. When considering the AAA represent the true cumulative dose, a readjusting of the prescribed dose and an optimization to protect the organs at risks should be taken in consideration in order to obtain the better clinical outcome.

Keywords: Electrons transport; bootstrap; computed tomography; gamma maps; radiotherapy dosage.

Figure 1

Flowchart to compare radiotherapy plans, using γ-index. The fluency in the heterogeneity (het) was calculated using MB and AAA for each patient with the same CT-scan. MB, Modified Batho; AAA, anisotropic analytical algorithm.

Figure 2

Results of 2D gamma index including γ_max, γ_mean, and γ_SD for all plans.

Figure 3

Dose distribution from MB and AAA for the same patient and same CT-scan. (A) Shows 2D dose maps, the target is presented in violet and the reference isodose curve presented in green; (B) shows 2D γ-map plotted in the axial plan. MB, Modified Batho; AAA, anisotropic analytical algorithm.

Figure 4

Cumulative and differential dose volume histograms, respectively on left and right panels calculated using MB-1D density correction and AAA. MB, Modified Batho; AAA, anisotropic analytical algorithm.

Figure 5

Correlation matrix for the DVH metrics from MB and AAA. The bar represent the Spearman’s rank correlation coefficient value (ρ). DVH, dose volume histogram; MB, Modified Batho; AAA, anisotropic analytical algorithm.

Figure 6

Correlation between γ_rates or γ_mean and ΔDVH metrics from Eq. [1]. (A) Correlation between γ_rates and ΔDVH metrics; (B) correlation between γ_mean and ΔDVH metrics. DVH, dose volume histogram.