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