Change in dose distribution of three-dimensional conformal radiotherapy during treatment for lung tumor

Purpose We investigated the changes in dose distribution of three-dimensional conformal radiotherapy (3D CRT) during lung tumor treatment. Materials and Methods Ten patients with non-small cell lung cancer who had undergone planning for radical radiotherapy were selected for study. Computed tomography (CT) examination was performed at two time intervals during the course of conformal radiotherapy: t_{0 Gy} at the time of planning and t_{40 Gy} at 40 Gy of treatment. We transferred all the planed beam data at t_{0 Gy} to each t_{40 Gy} CT image. The isodose distribution was recalculated at time t_{40 Gy} for the same beam characteristics. Variations in volumes and dose-volume histograms (DVHs) were analyzed and compared for lung, gross target volume (GTV), and planning target volumes (PTV) between t_{0 Gy} and t_{40 Gy}. A paired t-test was performed to compare the DVH between t_{0 Gy} and t_{40 Gy}. Results The mean minimum doses for t_{40 Gy} GTV, and PTV were lower than t_{0 Gy}. However, there was no significant difference between t_{0 Gy} and t_{40 Gy} (p=0.493, 0.378, respectively). There was a patient whose minimum doses of GTV and PTV were decreased and who had notable improvement of lobar atelectasis after 40 Gy of radiotherapy. Comparison of the percent volume of received dose exceeding 20 Gy (V₂₀) and the mean dose for the total lung revealed that t_{40 Gy} was larger than t_{0 Gy} (p=0.013, 0.012). Conclusion Incorporation of the time factor into 3D treatment planning is mandatory for frequent reiteration of treatment planning during treatment periods. Clearly, more work in this area should be considered. Presented at the 62nd annual meeting of the Japan Radiological Society in Yokohama, Japan. Presented at the RSNA 89th scientific assembly and annual meeting in Chicago, USA.