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: t0 Gy at the time of planning and t40 Gy at 40 Gy of treatment. We transferred all the planned beam data at t0 Gy to each t40 Gy CT image. The isodose distribution was recalculated at time t40 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 t0 Gy and t40 Gy. A paired t-test was performed to compare the DVH between t0 Gy and t40 Gy.

Results: The mean minimum doses for t40 y GTV, and PTV were lower than t0 y. However, there was no significant difference between t0 Gy and t40 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 t40 Gy was larger than to 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.