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.

BackgroundAn 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). MethodsSeventeen 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 (3mm and 5mm). ResultsAs 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_3mm, and 91.3% for PTVs_5mm), 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_3mm, and 0.2% between PTVs_5mm. Compared with the plan based on all phases and a 3mm margin, the average lung-dose differences found for the PTV based on the two extreme phases and a 5mm margin were 0.41 Gy for the mean lung dose and 0.93% for V20. ConclusionsRegardless of tumor characteristics, PTV construction based only on the two extreme phases and a 5mm setup margin may be a useful tool for reducing the clinical workload involved in target delineation in SBRT for lung cancer.