肝癌碘油标记图像对锥形束CT图像配准方法及靶区外放的影响

Analysis of CBCT image registration methods and the planning target volume margins for liver cancers using lipiodoi as a direct surrogate for target localization

目的分析碘油标记的肝癌图像对有无图像引导放疗（IGRT）中不同配准方法及靶区外放范围的影响。方法5例经动脉灌注化疗栓塞（TACE）后放疗的肝癌患者，在医科达IGRT联合主动呼吸控制（ABC）系统下，每13治疗前用锥形束CT（CBCT）采集图像。根据靶区及邻近器官进行治疗计划CT图像和CBCT容积图像配准，记录左右、上下、前后方向偏移值（CBCT1），并校正摆位误差；纠正后再次CBCT扫描与CT图像配准（CBCT2）；治疗结束后，再次CBCT扫描与CT图像配准（CBCT3）。用SPSS软件计算CTV-PTV的外放边界，对CBCT，、CBCT：、CBCT，比较结果行配对t检验。结果CBCT，和CBCT：及CBCT，在左右、上下、前后方向偏移值不同，分别为0．254、-0．612、0．314cm和0．020、0．014、-0．064cm及-0．004、0．042、-0．040cm。CTV-PTV外放边界无IGRT时左右、上下、前后方向分别为0．96、0．96、0．83cm，有IGRT时为0．67、0．68、0．58cm。离线分析放疗过程中CBCT下碘油图像形态变化发现，即使在靶区（碘油）精确配准下，肝脏在上下方向、椎体在3个方向上仍有很大偏移值。结论IGRT技术带来的CTV-PTV外放减小约3mm。内靶区无标记物患者，CTV-PTV外放应将肿瘤上下方向误差及椎体误差考虑在内。配准时如果单纯以肝缘图像为参考配准点，图像配准将会有很大不确定性。

Objective To analyze the transitional shifts between with different sets of cone-beam computed tomography （CBCT） and the planning CT for liver cancer patients, and calculate the margins from clinical target volume （CTV） to the planning target volume （PTV） with and without image guided radiotherapy （IGRT）. Methods Five liver cancer patients received radiotherapy after transcatheter arterial chemoembolization （TACE）. The first CBCT images （ CBCT1 ） were obtained with Elekta CBCT plus active breathing control （ABC） system before treatment. The second CBCT images （ CBCT2 ） were obtained after correcting the set-up errors and the third CBCT images （ CBCT3 ） were obtained after treatment. The CBCT images were registered and matched with the planning CT images using lipiodol as a direct surrogate for target localization. The PTV margins were calculated by comparing the shift between planning CT and CBCT according to formula M = 2. 5 （ ∑ doctor^2 ＋ ∑ set-up^2 ＋ ∑ transter^2）^1/2 . Paired t-test was used to compare the differences between the results from CBCTf, CBCT2 and CBCT3. Results The average transition shifts in the left-right （LR）, superior-inferior （SI） and anterior-posterior （AP） directions were 0. 254, -0. 612, 0. 314 cm between planning CT and CBCT1 ;were 0.020, 0. 014, -0. 064 cm between planning CT and CBCT2 ; and they were - 0. 004, 0. 042, - 0. 040 cm between planning CT and CBCT3. The PTV margins were LR 0. 96 cm, SI 0. 96 cm and AP 0. 83 cm without IGRT, and LR 0. 67 cm, SI 0. 68 cm and AP 0. 58 cm with IGRT. Conclusions The PTV margins can be reduced by 3 mm with IGRT for liver cancer using lipiodol as a direct surrogate for target localization.