SU-E-I-20: Megavoltage and Kilovoltage Cone-Beam Computed Tomography Image Guidance for Lung Radiotherapy

PURPOSE: To evaluate the geometric accuracy of patient set‐up in lungcancerradiotherapy using daily megavoltage cone‐beam computed tomography (MV‐CBCT) image guidance, and compare this with kilovoltage (kV)‐CBCT image guidance performed under a similar protocol. Methods: Set‐up accuracy using MV‐CBCT was analyzed in eleven lungcancer patients (246 fractions). The MV‐CBCT and CT planning data sets were matched on‐line by a radiation oncologist and/or radiation therapist using manual registration of any two independent structures (spinal canal, carina and gross tumour volume) to determine the couch shift required for the correct treatment set‐up. For each patient, the couch was adjusted when positional discrepancies exceeded ±2mm in any direction. To estimate the intra‐fraction motion, a repeat MV‐CBCT was taken after treatment, for the first four treatment fractions. Lungcancer patients to be treated under kV‐CBCT image guidance will be recruited in the coming months. Results: The average systematic error was 4.0mm and 37% of all delivered fractions exceeded significant 3D‐positioning errors of 5mm on the initial MV‐CBCT. The MV‐CBCT demonstrated that initial set‐up errors larger than 2mm in any direction occurred in 72% of all fractions. After couch shift to correct the set‐up, the average intra‐fraction systematic error, including the residual error, was reduced to 2.5mm, and never exceeded 5mm. Errors larger than 2mm, in any direction, were measured for 43% of all delivered fractions on the repeat MV‐CBCT. A margin of 5 mm between CTV and PTV was estimated to account for set‐up errors including the intra‐fraction motion for lungcancerradiotherapy using MV‐CBCT image guidance. Conclusion: The analysis indicated that using daily MV‐CBCT image guidance in lungcancer patients treated with radiotherapy results in reducing the set‐up errors, therefore allowing for reduced PTV margins. This is similar with data reported in literature for lung patients treated under kV‐CBCT image guidance.