SU‐E‐T‐790: The Effect of Pulmonary Emphysema on Lung SBRT Dosimetry

Abstract

Purpose: The purpose of this study is to systematically analyze the effect of pulmonary emphysema (PE) on the dosimetric results for intensity modulated arc therapy (IMAT). Methods: One patient diagnosed with non‐small cell lung cancer was chosen as the model patient. Hypothetic tumors with volumes of 2cc, 48cc and 169cc were contoured in the right middle lung. GPU based helical tomotherapy IMRT planning system is used in the study. 50 Gy in 5 fractions were prescribed to planning target volume (PTV). The planning parameters include a jaw size of 2.5 cm and 1cm, a pitch of 0.287 and 0.1, and dosimetric constrains according to RTOG 0813. To simulate emphysema lung density was overwritten from 0.05 to 0.4 g/cc. The dosimetric results such as tumor coverage, R50, and lung V20 were analyzed. Results: All plans satisfy 95% of PTV receiving 50Gy prescription. Lung density has a significant effect on the dosimetric results for all tumor sizes studied. For plans with jaw size of 2.5 cm and pitch of 0.287, R50 increases by as much as 92%, 55% and 35% for PTV of 2cc, 48cc and 169cc, when the lung density was reduced from 0.3 to 0.05 g/cc. Similarly, lung V20 increases by 32%, 20% and 18% for PTV of 2cc, 48cc and 169cc, respectively. Interestingly, for plans with jaw size of 1cm and pitch of 0.1, although the R50 for different tumor sizes follows similar trend as 2.5cm plans, the effect of lung density on V20 is non‐linear. For PTV of 2cc and 48cc, V20 plot shows a peak with varying lung densities. Conclusions: Systematic planning studies show that lower lung densities generally result in poorer dosimetry due to the greater difficulty to achieve electron equilibrium. Therefore, the lung density should be taken into consideration in the lung SBRT protocol.