Pre-clinical study of reducing the low-dose-area of tomotherapy on lung cancer planning

Objective: To seek a optimization method for lung cancer planning with Helical TomoTherapy for reducing the low dose area of total lung. Methods: CT images of thirty patients with unilateral lung cancer were selected. Seven plans (Groups A, B, C, D, E, F and G) were generated for each patient using an identical optimization procedure with the conditions that implemented contralateral lung with unblocked (control group), 1/4 directional block, 1/2 directional block, directional block, 1/4 complete block, 1/2 complete block and complete block, respectively. The benefits in different schemes of reducing the low dose area of normal lung tissue were estimated, in order to provide a reference treatment plan scheme in clinical. Results: Groups B, C, D and E had less influence on the target than that of group A. And there were no statistical difference between the target dosimetric parameters. The median dose and average dose of group F were increased within 0.5 Gy. The conformal index of group G had great influence on the target. The low dose area of total lung were reduced effectively in Groups C, D, E, F and G, the average decrease of V₅ and V₁₀ was 8.06%-45.26% and 6.21%-33.95%, respectively. The V₂₀ decreased by 1.71%-3.78% in directional block group, while V₂₀ increased in complete block group (2.07%-5.07%). The single treatment time was increased by 8.51%-79.22%. Conclusions: The results showed that the low dose area of total lung was higher for the plan without any block limitation. It could reduce the low dose area of total lung with directional block. We should lengthen the blocking arc of contralateral lung with directional block based on the fractional treatment time and the patient's physical condition. A certain arc of contralateral lung with complete block could effectively reduce low dose area. When complete block was used, it is suggested that the arc was no more than half of the contralateral lung. (authors)