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

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. Ten consecutive patients treated with SRT underwent six "standard" rapid multislice CT scans to generate an ITV _{6 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 ITV _4DCT. Geometric and dosimetric analyses were performed for (1) PTV _4DCT, derived from the ITV _4DCT with the addition of a 3-mm margin; (2) PTV _{6 CT}, derived from the ITV _{6 CT} with the addition of a 3-mm margin; and (3) 6 PTV _{10 mm}, derived from each separate GTV _{6 CT}, to which a three-dimensional margin of 10 mm was added. The ITV _4DCT was not significantly different from the ITV _{6 CT} in 8 patients, but was considerably larger in 2 patients whose tumors exhibited the greatest mobility. On average, the ITV _{6 CT} missed on average 22% of the volume encompassing both ITVs, in contrast to a corresponding mean value of only 8.3% for ITV _4DCT. Plans based on PTV _4DCT resulted in coverage of the PTV _{6 CT} by the 80% isodose in all patients. However, plans based on use of PTV _{6 CT} led to a mean PTV _4DCT 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 mm 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. 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.