Measurement of interfraction variations in position and size of target volumes in stereotactic body radiotherapy for lung cancer.

PURPOSE: To investigate the interfraction variations in volume, motion range, and position of the gross tumor volume (GTV) in hypofractionated stereotactic body radiotherapy (SBRT) for lung cancer using four-dimensional computed tomography.

METHODS AND MATERIALS: Four-dimensional computed tomography scans were acquired for 8 patients once at treatment planning and twice during the SBRT period using a stereotactic body frame. The image registration was performed to correct setup errors for clinical four-dimensional computed tomography. The interfraction variations in volume, motion range, and position of GTV were computed at end-inhalation (EI) and end-exhalation (EE).

RESULTS: The random variations in the GTV were 0.59 cm(3) at EI and 0.53 cm(3) at EE, and the systematic variations were 3.04 cm(3) at EI and 3.21 cm(3) at EE. No significant variations in GTV were found during the SBRT sessions (p = .301 at EI and p = .081 at EE). The random variations in GTV motion range for the upper lobe in the craniocaudal direction were within 1.0 mm and for the lower lobe was 3.4 mm. The interfraction variations in the GTV centroid position in the anteroposterior and craniocaudal directions were mostly larger than in the right-left direction; however, no significant displacement was observed among the sessions in any direction.

CONCLUSION: For patients undergoing hypofractionated SBRT, interfraction variations in GTV, motion range, and position mainly remained small. An additional approach is needed to assess the margin size.