CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响。方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy)。两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(D_max)、平均剂量(D_mean)及最小剂量(D_min)。将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cm EFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率。结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积。较小体积的晶体最大剂量D_max常规FOV与EFOV图像之间差异有统计学意义(t =-3.14, P<0.007),其余靶区及危及器官的最大剂量D_max差异无统计学意义。CTV和GTV平均剂量D_mean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65, P< 0.001),危及器官的平均剂量D_mean和靶区及危及器官最小剂量D_min差异均无统计学意义。两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义。结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通量图通过率,两者之间的差异并不显著。

Volumetric and dosimetric variations in radiation treatment planning using CT images based on different reconstructed field-of-view

Objective To analyze the volumetric and dosimetric variations in radiation treatment planning (RTP) using CT images based on normal and extended reconstructed field-of-view (FOV).Methods Original data of CT scans from 16 cases of nasopharyngeal carcinomas were reconstructed to form 2 sets of CT images with normal (45 cm) and EFOV (65 cm), which were then exported to RTP. Contouring of targets/OAR including GTV (gross tumor volume), CTV (clinical target volume,CTV), brain stem, lens, parotids and cord was made on normal FOV CT set. A 7-field equi-angular IMRT (intensity modulated radiation Therapy) plan was generated with prescribed GTV dose of 70 Gy. Two sets of CT images were fused in DICOM coordinate system and targets/OARs on normal FOV CT were copied to EFOV CT. IMRT plans were then transplanted from normal FOV to EFOV CT, with the same isocenter on DICOM coordinates. Volumetric and dosimetric variations including GTV, CTV brain stem, lens, parotids and cord were calculated on dose-volume-histogram (DVH) . For dosimetric verification, IMRT plans were input into fluence maps of Mapcheck 1175 phantom based on normal FOV and EFOV, and DTA (distance to agreement) was used to analyze the passing rate of calculated/measured absolute doses at 5 cm depth. Paired-t test was used to compare the passing rate of field 1-7 of IMRT plans based on 2 CT sets.Results Volumes of targets and OARs on 2 CT sets of different FOVs were statistically different, with larger calculated volume on normal FOV in all cases. There was no statistic difference in the maximal (D_max) doses received by all targets and OARs except the small-volume lens, in which the dose was higher on normal CT than that on EFOV CT (t =-3.14, P<0.007). The mean doses (D_mean) to the CTV (clinical target volume) and GTV (gross tumor volume) were higher on EFOV than normal FOV CT (t =-6.45,-5.65, P< 0.001).There was no statistic difference in D_mean received by OARs and the minimal dose (D_min) by all targets and OARs (P >0.05). There was also no statistic difference in the passing rate of field 1-7 of IMRT plans based on 2 CT sets.Conclusions There were volumetric and dosimetric variations as evaluated on DVH using different reconstructed FOV during CT simulation, though the difference between the passing rates as verified in 2 dimensional fluence map was not significant.