Choosing the optimal gated window for defining target volume in lung stereotactic ablative radiotherapy

Background: This study aims to compare retrospectively generated gated internal target volumes (ITVs) and to evaluate whether gated ITVs can reduce planned target volumes (PTVs) compared with standard ITV expansions. Materials and Methods: In this study, we retrospectively generated respiratory-gated ITVs and PTVs for our cohort of patients who underwent four-dimensional computed tomography for thoracic radiotherapy in our department between August 2018 and February 2019. We calculated the standard ITVs and two gated ITVs to analyze the volumetric reduction. Further, we considered a PTV reduction of >10% to be significant, and we analyzed the role of the localization and the size of the gross tumor volumes. Results: We included 38 patients with a median age of 70 years (mean = 68, SD ± 13,4, range =43–89), of whom 18 (47%) were females and 20 (52%) were males. The two gated PTVs (PTV 30%–70% and PTV 80%–20%) were significantly smaller than the standard PTVs (p-value < 0,001 for both PTVs). Considering the volume of the gross target volume (GTV), we found a significant correlation between GTV30cc and ITV30%–70% (chi-square analysis, p:0,006) and between GTV5cc and ITV80%–20% (p:0,003). We also found a correlation between the localization of the target lesion (mediastinal/ central/peripheral lesion) for both the gated ITVs (p: 0,030 for ITV 30%–70% and p:0,018 for ITV 80%–20%). Conclusion: Gated ITV plans could be useful for the sparing of normal tissues. Our results show that this approach could be useful for small lesions and for certain localizations (island tumors).