Multiple "slow" CT scans for incorporating lung tumor mobility in radiotherapy planning.

PURPOSE: The high local recurrence rates after radiotherapy in early-stage lung cancer may be due to geometric errors that arise when target volumes are generated using fast spiral CT scanners. A "slow" CT technique that generates more representative target volumes was evaluated. METHODS AND MATERIALS: Planning CT scans (slice thickness 3 mm, reconstruction index 2.5 mm) were performed during quiet respiration in 10 patients with peripheral lung lesions. Planning CT scans were repeated twice, followed by three slow CT scans (slice thickness 4 mm, index 3 mm, revolution time 4 s/slice). All, except the first scan, were limited to the tumor region. Three-dimensional registration of all scans was performed. The reproducibility of the imaged volumes was evaluated with each technique using (1) the common overlapping volume (COM), the component of the clinical target volume (CTV) covered by all three CT scans, and (2) the encompassing volume (SUM), which is the volume enveloped by all CTVs. RESULTS: In all patients, the target volumes generated using slow CT scans were larger than those derived using planning scans (mean ratio of planning-CTV:slow-CTV of 88.8% +/- 5.6%), and also more reproducible. The mean ratio of the respective COM:SUM volumes was 62.6% +/- 10.8% and 54.9% +/- 12.9%. CONCLUSIONS: Larger, and more reproducible, target volumes are generated for peripheral lung tumors with the use of slow CT scans, thereby indicating that slow scans can better capture tumor movement.

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. MATERIALS 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 4mm, index 3mm, revolution time 4s/slice) were then performed, followed by three-dimensional image registration. Planning target volumes (PTV) were generated using these GTV-PTV margins: (a) 1cm (PTV1.0); (b) 1.5 cm (PTV1.5); 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, PTV1.5 achieved comparable coverage to that achieved using PTVclinical, 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 PTVclinical. PTV1.0 resulted in inadequate target coverage in all the patients. Reductions in potential lung toxicity (V20) were achievable in six patients despite the larger GTVclinical 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.