SIMULATION TO TREATMENT ERROR IN COMPUTED TOMOGRAPHY BASED TREATMENT PLANNING

Purpose: To measure simulation to treatment error (STE) in radiation therapy and to evaluate factors which influence STE in computed tomography (CT) simulation. Materials & Methods: Off-line treatment verification using conventional portal films and CT-simulated digitally reconstructed radiographs (DRRs) was performed to measure the treatment accuracy in radiation therapy. Isocenter displacement of the portal films in relation to the DRRs was measured as a simulation to treatment error (STE). Correlation between STE and several treatment factors were examined statistically for 77 patients. Results: Factors that significantly affected STE were treatment site and use of immobilization devices. The mean STE of the patients with head and neck cancer was 3.9•}2.7 mm whereas that of patients with thoracic or abdominal tumor was 6.4•}6.1 mm (p=0.028). The mean STE in patients with and without immobilization device was 3.7•}2.6 mm and 5.7•}4.7 mm, respectively (p=0.027). Distance between isocenter and reference point at the time of planning CT scan did not correlate with STE. None of other factors influenced the accuracy of isocenter position. Conclusion: Although the position of isocenter was not marked at the time of CT-simulation, STE was within an acceptable range. Distance between the isocenter and reference point at the time of planning CT scan had no impact on STE.

目的：测量放射治疗中模拟与治疗的误差（STE），并评估影响计算机断层扫描（CT）模拟中STE的因素。 材料与方法：使用常规射野片和CT模拟的数字重建放射影像（DRR）进行离线治疗验证，以测量放射治疗的治疗准确性。将射野片相对于DRR的等中心位移作为模拟与治疗的误差（STE）进行测量。对77例患者的STE与若干治疗因素之间的相关性进行统计学检验。 结果：显著影响STE的因素是治疗部位和固定装置的使用。头颈部癌症患者的平均STE为3.9±2.7毫米，而胸部或腹部肿瘤患者的平均STE为6.4±6.1毫米（p = 0.028）。使用和未使用固定装置的患者的平均STE分别为3.7±2.6毫米和5.7±4.7毫米（p = 0.027）。计划CT扫描时等中心与参考点之间的距离与STE无关。其他因素均未影响等中心位置的准确性。 结论：尽管在CT模拟时未标记等中心位置，但STE在可接受范围内。计划CT扫描时等中心与参考点之间的距离对STE没有影响。