Sci—Fri PM: Delivery — 11: Lung Radiotherapy Using Megavoltage Cone-Beam Computed Tomography Imaging Guidance

PURPOSE: To evaluate the accuracy of patient set-up in lung cancer radiotherapy using daily megavoltage cone-beam computed tomography (MV-CBCT) imaging guidance and couch position adjustments. METHODS: Set-up accuracy using MV-CBCT has been analyzed in five lung cancer patients (108 fractions) to date, with ongoing patient accrual. The MV-CBCT and CT planning data set were matched on-line by a radiation oncologist using manual registration of any two independent structures, (the spinal canal, carina and/or gross tumour volume), to determine the couch shift required for the correct treatment setup. For each patient, the couch was adjusted when positional discrepancies exceeded ±2mm in any direction. To estimate the intrafraction motion, a repeat MV-CBCT was taken after treatment for the first four treatment fractions, or if the initial required shift was larger than 10mm. RESULTS: The average systematic shift was 4.0mm, and 31% of all delivered fractions exceeded significant 3D positioning errors of 5mm. The MV-CBCT demonstrated that initial set-up errors larger than 2mm in any direction can occur in 53% of all fractions. The intrafraction average systematic shift, including the residual error after initial shift to correct the set-up, was 2.5mm, and never exceeded 5mm. Set-up errors larger than 2mm in any direction were measured for 42% of all delivered fractions on the repeat MV-CBCT. CONCLUSION: The analysis indicated that good geometric accuracy is achievable using daily MV-CBCT imaging guidance in lung cancer patients treated with radiotherapy. This can potentially lead to reducing PTV margins in these patients.