Yao X S, Gong G Z, Ren J X, Zuo G P, Yin Y
Radiotherapy Center, Chenzhou First People's Hospital, Chenzhou 423000, China.
Department of Radiation Physics, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan 250117, China.
Zhonghua Zhong Liu Za Zhi. 2021 Aug 23;43(8):850-855. doi: 10.3760/cma.j.cn112152-20190605-00355.
To discuss the influence of different computed tomography (CT) value assignment methods on dose calculation of intensity modulated radiotherapy (IMRT) plan which designed for nasopharyngeal carcinoma (NPC) and the value assignment methods of IMRT plan for NPC based on magnetic resonance (MR) images. Simulation CT and MR image of 32 NPC patients in Shandong Cancer Hospital from March 2018 to November 2018 were selected for this study. Populate CT values were obtained by contouring and analyzing the simulation CT of patients' tissue, including bone, air, brain, eyeball, optic-nerve, lens, parotid, masseter, skin. Pseudo-CT were generated by different CT value assignment methods: CT1: CT value of all tissues was set to 0HU; CT2: CT value of air cavity was set to populate CT value based on CT1; CT3: CT value of Bone was set to populate CT value based on CT2; CT4: CT value of each soft tissue were set to populate CT value based on CT3. The IMRT plan for NPC as Plan0 was designed base on simulation CT. Then Plan0 was transplanted to four pseudo-CT to recalculate the dose and obtain Plan1, Plan2, Plan3 and Plan4, the differences of dosimetric parameters were compared with Plan0. NPC-IMRT plan was designed base on MR images by using the assignment method with CT value of each tissue were set to populate CT value. In the head and neck CT images, the average populate CT values of bone and cavity were 621 HU and -720 HU, respectively. The populate CT values of other soft tissue ranges from -20 HU to 70 HU. The differences of dosimetric indexes of Plan1, Plan2, Plan3, Plan4 decreased sequentially compare to Plan0, the difference of the dosimetry parameters of Plan4 and Plan0 was the smallest. The differences of PTV D(99), PTV D(95), isocenter dose, D(mean) of all tissues, D(max) of bilateral eye balls, D(max) of bilateral lens, D(max) of bilateral optic nerves, D(mean) of bilateral parotid, V(20) of bilateral parotid, D(50) of bilateral parotid, D(max) of spinal cord, D(max) of brainstem, D(5) of brainstem between Plan4 and Plan0 were all less than 1%. The difference of V(30) in bilateral parotid between Plan4 and Plan0 was less than 1.5%. In the comparison of the pixel dose distribution, the regions of dose distribution difference greater than 1% mainly distributed in the air cavity, bone periphery and the skin. The target area of the IMRT plan for NPC based on MR images met 95% of the prescribed dose, and the dose of each organ at risk was within the dose limit. The assignment method of each tissue and organs set to populate CT value compared with other methods has the least influence on the dose calculation of NPC-IMRT plan, which could meet the clinical requirements. Therefore, it should be the first choice of assignment method when designing NPC-IMRT plan based on MR image.
探讨不同计算机断层扫描（CT）值赋值方法对鼻咽癌（NPC）调强放疗（IMRT）计划剂量计算的影响以及基于磁共振（MR）图像的NPC的IMRT计划的赋值方法。选取2018年3月至2018年11月在山东省肿瘤医院的32例NPC患者的模拟CT和MR图像进行本研究。通过勾画和分析患者组织的模拟CT来获取填充CT值，包括骨骼、空气、脑、眼球、视神经、晶状体、腮腺、咬肌、皮肤。通过不同的CT值赋值方法生成伪CT：CT1：所有组织的CT值设为0HU；CT2：基于CT1将气腔的CT值设为填充CT值；CT3：基于CT2将骨骼的CT值设为填充CT值；CT4：基于CT3将每个软组织的CT值设为填充CT值。以模拟CT为基础设计NPC的IMRT计划作为Plan0。然后将Plan0移植到四个伪CT上重新计算剂量，得到Plan1、Plan2、Plan3和Plan4，将剂量学参数的差异与Plan0进行比较。采用将每个组织的CT值设为填充CT值的赋值方法基于MR图像设计NPC-IMRT计划。在头颈部CT图像中，骨骼和气腔的平均填充CT值分别为621HU和-720HU。其他软组织的填充CT值范围为-20HU至70HU。与Plan0相比，Plan1、Plan2、Plan3、Plan4的剂量学指标差异依次减小，Plan4与Plan0的剂量学参数差异最小。Plan4与Plan0之间PTV D(99)、PTV D(95)、等中心剂量、所有组织的D(mean)、双侧眼球的D(max)、双侧晶状体的D(max)、双侧视神经的D(max)、双侧腮腺的D(mean)、双侧腮腺的V(20)、双侧腮腺的D(50)、脊髓的D(max)、脑干的D(max)、脑干的D(5)的差异均小于1%。Plan4与Plan0之间双侧腮腺V(30)的差异小于1.5%。在像素剂量分布比较中，剂量分布差异大于1%的区域主要分布在气腔、骨骼周边和皮肤。基于MR图像的NPC的IMRT计划的靶区达到规定剂量的95%，且各危及器官的剂量在剂量限值内。与其他方法相比，将每个组织和器官的CT值设为填充CT值的赋值方法对NPC-IMRT计划剂量计算的影响最小，能够满足临床要求。因此，在基于MR图像设计NPC-IMRT计划时，应作为赋值方法的首选。
