• Jenghwa Chang, Gig S Mageras, Ellen Yorke, Fernando De Arruda, Jussi Sillanpaa, Kenneth E Rosenzweig, Agung Hertanto, Hai Pham, Edward Seppi, Alex Pevsner, C Clifton Ling, and Howard Amols.
• Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY 10021, USA. changj@mskcc.org
• Int. J. Radiat. Oncol. Biol. Phys. 2007 Apr 1; 67 (5): 1548-58.

PurposeTo evaluate the use of megavoltage cone-beam computed tomography (MV CBCT) to measure interfractional variation in lung tumor position.Methods And MaterialsEight non-small-cell lung cancer patients participated in the study, 4 with respiratory gating and 4 without. All patients underwent MV CBCT scanning at weekly intervals. Contoured planning CT and MV CBCT images were spatially registered based on vertebral anatomy, and displacements of the tumor centroid determined. Setup error was assessed by comparing weekly portal orthogonal radiographs with digitally reconstructed radiographs generated from planning CT images. Hypothesis testing was performed to test the statistical significance of the volume difference, centroid displacement, and setup uncertainty.ResultsThe vertebral bodies and soft tissue portions of tumor within lung were visible on the MV CBCT scans. Statistically significant systematic volume decrease over the course of treatment was observed for 1 patient. The average centroid displacement between simulation CT and MV CBCT scans were 2.5 mm, -2.0 mm, and -1.5 mm with standard deviations of 2.7 mm, 2.7 mm, and 2.6 mm in the right-left, anterior-posterior and superior-inferior directions. The mean setup errors were smaller than the centroid shifts, while the standard deviations were comparable. In most cases, the gross tumor volume (GTV) defined on the MV CBCT was located on average at least 5 mm inside a 10 mm expansion of the GTV defined on the planning CT scan.ConclusionsThe MV CBCT technique can be used to image lung tumors and may prove valuable for image-guided radiotherapy. Our conclusions must be verified in view of the small patient number.

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