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SU-F-BRD-09: Is It Sufficient to Use Only Low Density Tissue-Margin to Compensate Inter-Fractionation Setup Uncertainties in Lung Treatment?

PURPOSE
In lung radiation treatment, PTV is formed with a margin around GTV (or CTV/ITV). Although GTV is most likely of water equivalent density, the PTV margin may be formed with the surrounding low-density tissues, which may lead to unreal dosimetric plan. This study is to evaluate whether the concern of dose calculation inside the PTV with only low density margin could be justified in lung treatment.


METHODS
Three SBRT cases were analyzed. The PTV from the original plan (Plan-O) was created with a 5-10 mm margin outside the ITV to incorporate setup errors and all mobility from 10 respiratory phases. Test plans were generated with the GTV shifted to the PTV edge to simulate the extreme situations with maximum setup uncertainties. Two representative positions as the very posterior-superior (Plan-PS) and anterior-inferior (Plan-AI) edge were considered. The virtual GTV was assigned a density of 1.0 g.cm-3 and surrounding lung, including the PTV margin, was defined as 0.25 g.cm-3. Also, additional plan with a 1mm tissue-margin instead of full lung-margin was created to evaluate whether a composite-margin (Plan-Comp) has a better approximation for dose calculation. All plans were generated on the average CT using Analytical Anisotropic Algorithm with heterogeneity correction on and all planning parameters/monitor unites remained unchanged. DVH analyses were performed for comparisons.


RESULTS
Despite the non-static dose distribution, the high-dose region synchronized with tumor positions. This might due to scatter conditions as greater doses were absorbed in the solid-tumor than in the surrounding low-density lungtissue. However, it still showed missing target coverage in general. Certain level of composite-margin might give better approximation for the dosecalculation.


CONCLUSION
Our exploratory results suggest that with the lungmargin only, the planning dose of PTV might overestimate the coverage of the target during treatment. The significance of this overestimation might warrant further investigation.