Correlation of gross tumor volume excursion with potential benefits of respiratory gating

Purpose: To test the hypothesis that the magnitude of thoracic tumor motion can be used to determine the desirability of respiratory gating. Methods and materials: Twenty patients to be treated for lung tumors had computed tomography image data sets acquired under assisted breath hold at normal inspiration (100% tidal volume), at full expiration (0% tidal volume), and under free breathing. A radiation oncologist outlined gross tumor volumes (GTVs) on the breath-hold computed tomographic images. These data sets were registered to the free-breathing image data set. Two sets of treatment plans were generated: one based on an internal target volume explicitly formed from assessment of the excursion of the clinical target volume (CTV) through the respiratory cycle, representing an ungated treatment, and the other based on the 0% tidal volume CTV, representing a gated treatment with little margin for residual motion. Dose-volume statistics were correlated to the magnitude of the motion of the center of the GTV during respiration. Results: Patients whose GTVs were >100 cm3 showed little decrease in lung dose under gating. The other patients showed a correlation between the excursion of the center of the GTV and a reduction in potential lung toxicity. As residual motion increased, the benefits of respiratory gating increased. Conclusion: Gating seems to be advantageous for patients whose GTVs are <100 cm3 and for whom the center of the GTV exhibits significant motion, provided residual motion under gating is kept small