A 3D quantitative evaluation for assessing the changes of treatment planning system and irradiation techniques in radiotherapy

Purpose: This work proposes and compares two 3D global evaluation methods for assessing the alteration of calculated dose distributions when treatment planning system algorithms or irradiation techniques is modified in radiation therapy. Methods: The global analysis is based on gamma index (γ) proposed by Low et al.1 and Chi (χ) index proposed by Bakai et al.2. The γ and χ values are signed in order to identify the over and under estimating dosage. The 3D maps, the cumulative Gamma Voxels Histograms (GVHs) and Chi Voxels Histograms (CVHs) were generated using two software. The γ and χ criteria were set to 3 mm for the distance to agreement and 3% for dose. Pearson's Chi-squared test was applied to assess the statistically significance between GVHs and CVHs. We illustrated this method for the change of dose calculation algorithms for lung cancer, and the change of irradiation techniques for breast cancer. For each patient, 2 treatment plans were generated. For the example of change of dose calculation algorithms, a plan 1 was calculated using Pencil Beam Convolution (PBC) algorithm and a plan 2 was calculated using Modified Batho method (PBC-MB). For the example of change of irradiation technique, a plan 1 was calculated using Source Skin Distance SSD technique and a plan 2 was calculated using a single isocenter technique. Results: The 3D analysis based on γ and χ indexes showed a significant effect on the dosimetric representation in the lung cancer when we change the PBC algorithm to PBC-MB method. The comparison between the two irradiation techniques showed that the single isocenter technique produces a better dose distribution for the treatment of breast cancer. Pearson's Chi-squared test showed that there was no statistically significance between GVHs and CVHs generated by γ and χ indexes, (p > 0.05). The global analysis using 3D for γ and χ indexes confirmed the results obtained from dosimetric analysis. Conclusion: The methods proposed in this study provide useful tools for radiotherapy to compare two dose distributions obtained using different algorithms or different irradiation techniques. The χ-index was (~190) times faster than γ-index. The χ-index is thus a valuable and more convenient method for 3D global analysis compared with γ-index.