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Objective To quantify the setup errors for the different anatomical sites of patients who received intensity-modulated radiotherapy (IMRT) with linear accelerator on-board kilovolt fan beam CT(kV-FBCT) as non-isocenter IGRT and megavolt cone beam CT (MV-CBCT) as isocenter IGRT. Methods A retrospective analysis was performedon 70 patients who underwent radiotherapy, kV-FBCT, and/or MV-CBCT scans after each routine setup prior to IMRT. The average displacement (M), systematic error (Σ), and random error (б) at different treatment sites in the left-right, anterior-posterior, and cranial-caudal directions were calculated according to the individual displacements. The formula 2.5Σ+0.7б was used to estimate the PTV margin in respective direction. For each single patient, the root mean square in three directions was used as 3D displacement. Results A total of 1130 displacements were recorded in the 70 patients. The PTV margin was estimated to be 1.9-3.1 mm in head and neck cancer, 2.8-5.1 mm in thoracic cancer, 4.6-5.1 mm in breast cancer, 3.0-5.5 mm in upper abdominal cancer, and 3.5-6.8 mm in pelvic tumor. For the 3D mean displacements, the head and neck, thoracic, breast, upper abdominal, and pelvic cancer were 2.4±1.0, 4.0±1.6, 4.1±2.0, 4.6±2.1, and 4.6±2.1 mm, respectively. The average 3D displacement obtained by kV-FBCT and MV-CBCT were 4.1 and 3.4 mm, respectively (P=0.212). Conclusion The quantitative setup-error data can be obtained using linear accelerator on-board FBCT, and the non-isocenter IGRT induced set-up error cannot be negligible.
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Objective:To compare the comprehensive performance of three Varian on-board image (OBI) image systems (KV-CBCT, KV-planar and MV-EPID) and to explore the value of the combined application of these three systems in daily image-guided radiotherapy for nasopharyngeal cancer.Methods:KV-CBCT, KV-planar and MV-EPID scanning and registration were carried out in the left and right/abdominal and back/head and foot direction on human head and neck phantom. The set-up error, registration time, additional radiation dose and image quality of the three systems were compared by F-test.Results:KV-CBCT, kV-planar and MV-EPID were scanned for 55 times, respectively, and the set-up errors in the left and right/abdominal and back/head and foot direction of the three image-guided systems were (0.00±5.43)/(-0.02±5.49)/(0.02±5.58) mm, (0.04±5.49)/(0.02±5.56)/(0.02±5.54) mm, (0.02±5.22)/(0.11±5.34)/(-0.04±5.33) mm, respectively ( P=0.999, 1.000, 0.989). The average time consuming was (200±45) s, (120±36) s and (115±42) s; the additional radiation dose from low to high was kV-planar, KV-CBCT and MV-EPID; the image quality from low to high was MV-EPID, kV-planar and KV-CBCT. Conclusions:Three image-guided systems can meet the requirements of image-guided radiotherapy for nasopharyngeal cancer. Based on the overall performance of the three systems, 1 CBCT+ 4 kV planar per week is recommended and EPID should be used as a backup system in daily image-guided radiotherapy for nasopharyngeal cancer. This scheme makes full use of the high image quality of CBCT and the low radiation of kV planar to realize the regular detection of nasopharyngeal cancer volume change and the implementation of high-precision radiotherapy.
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Objective To study the method of using phantom test for daily quality assurance of on board image(OBI) system. Methods The routine procedures of radiotherapy, including CT simulation, planning,setup,cone beam CT(CBCT) scan and kilovolt X-ray orthogonal film were carried out on a head phantom. The procedures repeated once a day in the following 10 days. The geometric errors of the phantom were recorded. Results The geometric errors of the phantom by CBCT were [0.06±0.11] cm, [0.03±0.05] cm, [0.07±0.07] cm and [0.03±0.10] cm in longitudinal, vertical, lateral and rotation directions, respectively. The geometric errors of the phantom by kilovolt X-ray orthogonal film were [ 0.04±0.10] cm, [0.03±0.05] cm, [0.08±0.06] cm and [0.05±0.05] cm, respectively. The differences of geometric errors of the phantom by CBCT and kilovolt X-ray orthogonal film were not significant(t = 0.44,P=0.667 in longitudinal direction ; t=0.00, P=1.060 in vertical direction ; t=0.34, P=0.735 in lateral direction; t=0.58,P=0.568 in rotation direction). Conclusions The OBI system of our accelerator is reliable and at excellent performance status. The method by using phantom test for the daily quality assurance of OBI system is easy and reliable.