Four-dimensional ct scans for treatment planning in stereotactic radiotherapy for stage I lung cancer

Purpose: Hypofractionated stereotactic radiotherapy (SRT) for Stage I non-small-cell lung cancer requires that meticulous attention be paid toward ensuring optimal target definition. Two computed tomography (CT) scan techniques for defining internal target volumes (ITV) were evaluated. Methods and Materials: Ten consecutive patients treated with SRT underwent six "standard" rapid multislice CT scans to generate an ITV6 CT and one four-dimensional CT (4DCT) scan that generated volumetric datasets for 10 phases of the respiratory cycle, all of which were used to generate an ITV4DCT. Geometric and dosimetric analyses were performed for (1) PTV4DCT, derived from the ITV4DCT with the addition of a 3-mm margin; (2) PTV6 CT, derived from the ITV6 CT with the addition of a 3-mm margin; and (3) 6 PTV10 mm, derived from each separate GTV(6 CT), to which a three-dimensional margin of 10 mm was added. Results: The ITV4DCT was not significantly different from the ITV6 CT in 8 patients, but was considerably larger in 2 patients whose tumors exhibited the greatest mobility. On average, the ITV6 CT missed on average 22% of the volume encompassing both ITVs, in contrast to a corresponding mean value of only 8.3% for ITV4DCT. Plans based on PTV4DCT resulted in coverage of the PTV6 CT by the 80% isodose in all patients. However, plans based on use of PTV6 CT led to a mean PTV4DCT coverage of only 92.5%, with a minimum of 77.7% and 77.5% for the two most mobile tumors. PTVs derived from a single multislice CT expanded with a margin of 10 rum were on average twice the size of PTVs derived using the other methods, but still led to an underdosing in the two most mobile tumors. Conclusions: Individualized ITVs can improve target definition for SRT of Stage I non-small-cell lung cancer, and use of only a single CT scan with a 10-mm margin is inappropriate. A single 4D scan generates comparable or larger ITVs than are generated using six unmonitored rapid CT scans, a finding related to the ability to account for all respiration-correlated mobility. (C) 2004 Elsevier Inc.