Accounting for target motion during respiration allows for smaller internal target and planning target volumes (ITV and PTV). This in turn facilitates a lower integral dose to the lung, reducing the V20 Gy. This benefit is largest for tumors in the lower lobes as their position tends to vary more with respiration. This can be accomplished with breath hold, abdominal compression, real-time target tracking, or respiratory gating.
The necessary CTV margin reflects the likelihood of microscopic disease extension and should not be arbitrarily reduced to lower the V20 Gy. Induction chemotherapy is commonly used in small cell lung cancer to reduce treatment volumes as these tumors are quite responsive to chemotherapy and this method has been validated to lead to equivalent locoregional control (Liengswangwong et al. JCO 1994). However in non-small cell lung cancer, this is not a preferred method of reducing treatment volumes. Alternatively, the patient could start therapy with the V20 Gy dose constraint exceeded and then be re-planned in 2 weeks, reducing the volumes to account for treatment response. In this situation, patients should be counseled prior to initiation of therapy regarding an elevated risk of radiation pneumonitis. Proton radiotherapy in the treatment of non-small cell lung cancer remains investigational.
