兆伏级锥形束CT图像引导放疗所致鼻咽癌患者剂量的探讨

Evaluation of the patient doses from megavoltage cone-beam CT imaging in the image.guided radiation therapy for nasopharyngeal carcinoma.

目的评价和估算兆伏级锥形束CT（MV CBCT）成像系统在图像引导放疗中所致鼻咽癌患者的辐射剂量。方法选择MV CBCT系统头颈部8MU扫描预案，利用0．65cm^1指型电离室和CT头部剂量体模测量出体模不同位置的吸收剂量。并利用XiO治疗计划系统模拟MV CBCT扫描过程，计算体模电离室测量点的吸收剂量和鼻咽癌患者肿瘤靶区及危及器官的吸收剂量。结果体模不同位置吸收剂量的测量值和计算值具有很好的一致性，相对误差均小于3．5％。MV CBCT图像引导放疗所致鼻咽癌患者肿瘤靶区平均剂量为6．43cGy，脑干、脊髓和视交叉的平均剂量分别为6．36、6．83和6．90cGy，左、右视神经平均剂量分别为7．70和7．53cGy，左、右腮腺平均剂量分别为6．86和6．43cGy。结论使用治疗计划系统模拟MV CBCT图像采集过程估算剂量准确、可靠。在设计患者治疗计划时，要充分考虑MV CBCT图像采集过程所致患者剂量。

Objective To evaluate and estimate the patient doses from megavohage cone-beam CT imaging system in the image-guided radiation therapy for nasopharyngeal carcinoma. Methods 8 MU protocol of the MV CBCT system was selected for the head-and-neck region. The absorbed doses at the different positions in the phantom were measured using a 0.65 cm^3 ion chamber and the cylindrical acrylic phantom. The absorbed doses at the measurement positions of the phantom were calculated and the patient doses to the tumor and critical organs were derived from dose-volume histogram by the TPS mimicking the MV CBCT scanning with 8 MU protocol. Results The error between the measured dose and the calculated dose was less than 3.5%. The average doses to the tumor target, brain stem, spinal cord and chiasm were 6.43, 6.36, 6.83 and 6.90 cGy, respectively, while those to left and right of both optic nerve and parotid were 7.70 and 7.53 cGy, 7.70 and 7.53 cGy, respectively. Conclusion The patient doses estimated using the TPS mimicking the MV CBCT image acquiring procedure are accurate and reliable. The patient doses from the MV CBCT imaging must be considered when treatment plan of the patient is designed.