Effect of the modification of CT scanner calibration curves on dose using density correction methods for chest cancer

International audiencePurposeThis work sought to establish whether the choice of CT scanner calibration curve has a significant effect on dose computation using density correction methods for chest cancer.Material and methodsCIRS®062 phantom was used to calculate the Hounsfield Unit using 80, 120 and 140 kV. Four CT calibration curves were implanted in the Eclipse® TPS. Forty-two irradiation fields for 4 patients with lung cancer were included and analysed. The patients were treated with 3-dimensional radiation therapy. For each patient, 3 treatment plans were generated using exactly the same beam configuration. In plan 1, the dose was calculated using the Modified Batho (MB) method. In plan 2, the dose was calculated using the Batho power law (BPL) method. In plan 3, the dose was calculated using the Equivalent Tissue Air Ratio (ETAR) method. To evaluate the treatment plans computed by the three methods, the monitor units, dose volume histograms, conformity index, homogeneity index, planning target volumes conformity index, geometrical index and 2D gamma index were compared. The statistical analysis was carried out using Wilcoxon signed rank test.ResultsThe three density correction methods in plans 1, 2 and 3 using tested curves produced a difference less than 1% for MUs and DVH. Wilcoxon test showed a statically significant difference for MUs using ETAR method with calibration curves based on 80 and 120 kV. There was no significant difference for the quality indices between plan 1, 2 and 3, (P > 0.05), but a significant difference for the planning target volumes conformity index between plans 1, 2 and 3 (P < 0.05) was observed. The 2D gamma analysis showed that 100% of pixels had gamma ≤ 1.ConclusionThe impact of the modification of CT calibration curves on dose is negligible for chest cancer using density correction methods. One calibration curve can be used to take into account the density correction for lung