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Intensity modulated radiation therapy (IMRT) has been widely used in the treatment of lung cancer. The highly conformal dose distribution with steep gradients could miss the target if respiratory motion is not carefully considered during the treatment planning. The issue becomes particularly critical when dose escalation technique is used. To account for this periodical respiratory motion, the common practice is to add an empirical population-based safety margin to the clinical target volume (CTV). However, such a uniform margin does not reflect the fact that respiratory motion is not isotropic. In addition, it is not tailored to each individual patient. Thus, it is not optimal in both tumor targeting and normal tissue sparing. Here, we present our approach to 4D radiation therapy using the Bellows tracking system for targeting mobile lung tumors. The objective was to develop a clinically viable procedure for routine 4D treatment planning.