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目的:利用不均匀等效模体,探讨不同CT扫描条件对CT值及照射跳数(MU)的影响。方法:采用大小2种CT孔径(80和70cm)、2种模体几何摆放顺序及2种扫描电压(120和140KV),测量不同组合,比较各自的CT值,建立相应的CT-电子密度(ED)转换曲线,选取盆腔、胸部、头颈部各10例患者的CT图像模拟适形计划(CRT)和调强计划(IMRT),分析照射MU数值的偏差。结果:对于小孔径CT,无论扫描电压、模体几何位置如何变化,其MU数值相差均≤0.1%;大孔径CT扫描电压改变对MU值无影响,模体几何摆放顺序的改变有影响,但<0.3%;CRT计划和IMRT计划各自偏差值相同。结论:对于精确放疗计划系统,CT扫描条件改变和测量模体位置改变均可引起照射MU数值误差。若使用大孔径CT进行模拟定位,需根据其特性建立合理的电子密度曲线,并应用在计划制定中。
OBJECTIVE: To investigate the effect of different CT scanning conditions on CT values and irradiation hops (MU) with an unequal equivalent phantom. Methods: Two kinds of CT aperture (80 and 70cm), two kinds of phantom order and two kinds of scanning voltage (120 and 140KV) were used to measure different CT values. CT values of the two CTs were compared to establish corresponding CT-electron density (ED) conversion curve. CT, CRT and IMRT of 10 patients in pelvic, thoracic and head and neck were selected to analyze the deviation of MU values. Results: For the small aperture CT, the difference of the MU values is ≤0.1% regardless of the change of the geometric position of the phantom regardless of the scan voltage. The change of the large aperture CT scan voltage has no effect on the MU value, But <0.3%; the respective differences between CRT and IMRT are the same. CONCLUSIONS: For accurate radiotherapy planning systems, changes in CT scan conditions and changes in the position of the measurement phantom can all cause numerical errors in irradiated MU. If the use of large aperture CT for simulation positioning, according to its characteristics to establish a reasonable electron density curve, and used in planning.