Dosimetric consequences of tumor mobility in radiotherapy of stage I non-small cell lung cancer - An analysis of data generated using 'slow' CT scans

Background: The target coverage for radiotherapy of early-stage lung cancer was evaluated using two different CT techniques. Material and methods: A conventional planning CT scan and two limited scans of the tumor region were performed in seven patients with peripheral tumors. Three 'slow' scans (slice thickness 4 mm, index 3 mm, revolution time 4 s/slice) were then performed, followed by three-dimensional image registration. Planning target volumes (PTV) were generated using these GTV-PTV margins: (a) 1 cm (PTV_1.0); (b) 1.5 cm (PTV_1.5); and (c) 0.9, 1.0, and 0.9 cm ('PTV_clinical') when set-up errors are avoided. Results: PTVs derived from three 'slow' scans missed 1.9% of the volume derived from three planning scans for an immobile tumor and 9.3% in the case of a mobile tumor. For an immobile tumor, PTV_1.5 achieved comparable coverage to that achieved using PTV_clinical, which was generated from three 'slow' scans and a planning scan. For a mobile tumor, PTV_1.5 covered only 89% of the volume captured by PTV_clinical. PTV_1.0 resulted in inadequate target coverage in all the patients. Reductions in potential lung toxicity (V20) were achievable in six patients despite the larger GTV_clinical when treatment set-up errors were minimized. Conclusions: PTVs derived using 'slow' CT scans consistently produce superior target coverage than that using conventional scans. This may account for the poor local control observed in stage I lung cancer.