PURPOSE: To evaluate the accuracy of patient set‐up in lungcancerradiotherapy 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 lungcancer 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 lungcancer patients treated with radiotherapy. This can potentially lead to reducing PTV margins in these patients.
