Benefit of respiration-gated stereotactic radiotherapy for stage I lung cancer: An analysis of 4DCT datasets

Purpose: High local control rates have been reported with stereotactic radiotherapy (SRT) for Stage I non-small-cell lung cancer. Because high-dose fractions are used, reduction in treatment portals will reduce the risk of toxicity to adjacent structures. Respiratory gating can allow reduced field sizes and planning four-dimensional computed tomography scans were retrospectively analyzed to study the benefits for gated SRT and identify patients who derive significant benefit from this approach. Methods and Materials: A total of 31 consecutive patients underwent a four-dimensional computed tomography scan, in which three-dimensional computed tomography datasets for 10 phase bins of the respiratory cycle were acquired during free breathing. For a total of 34 tumors, the three planning target volumes (PTVs) were analyzed, namely (1) PTV_10bins, derived from an internal target volume (ITV) that incorporated all observed mobility (ITV_10bins), with the addition of a 3-mm isotropic setup margin; (2) PTV_gating, derived from an ITV generated from mobility observed in three consecutive phases ("bins") during tidal-expiration, plus addition of a 3-mm isotropic margin; and (3) PTV_{10 mm}, derived from the addition of a 10-mm isotropic margin to the most central gross tumor volumes in the three bins selected for gating. Results: The PTV_10bins and PTV_gating were, on average, 48.2% and 33.3% of the PTV_{10 mm}, and respective mean volumes of normal tissue (outside the PTV) receiving the prescribed doses were 57.1% and 39.1%, respectively, of that of PTV_{10 mm}. A significant correlation was seen between the extent of tumor mobility (i.e., a three-dimensional mobility vector of at least 1 cm) and reduction in normal tissue irradiation achieved with gating. The ratio of the intersecting and the encompassing volumes of GTVs at extreme phases of tidal respiration predicted for the benefits of gated respiration. Conclusion: The use of "standard population-based" margins for SRT leads to unnecessary normal tissue irradiation. The risk of toxicity is further reduced if respiration-gated radiotherapy is used to treat mobile tumors. These findings suggest that gated SRT will be of clinical relevance in selected patients with mobile tumors.