Usefulness of target delineation based on the two extreme phases of a four-dimensional computed tomography scan in stereotactic body radiation therapy for lung cancer

BACKGROUND: An evaluation of the usefulness of target delineation based only on the two extreme phases of a four-dimensional computed tomography (4D CT) scan in lung stereotactic body radiation therapy (SBRT). METHODS: Seventeen patients treated with SBRT via 4D CT scans for lung cancer were retrospectively enrolled. Volumetric and geometric analyses were performed for the internal target volumes (ITVs) and planning target volumes (PTVs) generated using different respiratory phases (all phases and 2 extreme phases) and setup margins (3 mm and 5 mm). RESULTS: As the setup margins were added to the ITVs, the overlap percentage between the PTVs based on all phases and the two extreme phases increased (85.1% for ITVs, 89.8% for PTVs_3 mm, and 91.3% for PTVs_5 mm), and there were no differences according to the tumor parameters, such as the gross tumor volume and 3D mobility. The missing-volume differences for ITVs derived from cone-beam CT images also decreased, with values of 5.3% between ITVs, 0.5% between PTVs_3 mm, and 0.2% between PTVs_5 mm. Compared with the plan based on all phases and a 3 mm margin, the average lung-dose differences found for the PTV based on the two extreme phases and a 5 mm margin were 0.41 Gy for the mean lung dose and 0.93% for V20. CONCLUSIONS: Regardless of tumor characteristics, PTV construction based only on the two extreme phases and a 5 mm setup margin may be a useful tool for reducing the clinical workload involved in target delineation in SBRT for lung cancer.