Objective: 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. Methods: 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. Results: 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. Conclusions: 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.
目的： 探讨不同CT值赋值法对鼻咽癌调强放射治疗(IMRT)计划剂量计算的影响，以及基于磁共振(MR)图像制定鼻咽癌IMRT计划的赋值方法。 方法： 选取2018年3月至2018年11月在山东省肿瘤医院行IMRT的32例鼻咽癌患者，在其定位CT图像上勾画并统计骨骼、空腔、大脑、眼球、视神经、晶状体、腮腺、咬肌、皮肤等组织器官的群体化CT值。通过不同CT值赋值法生成伪CT：(1)CT1：对全组织整体赋予0 HU；(2)CT2：在CT1的基础上对空腔赋予群体化CT值；(3)CT3：在CT2的基础上对骨骼赋予群体化CT值；(4)CT4：在CT3的基础上对各个软组织分别赋予群体化CT值。在定位CT上制定鼻咽癌IMRT计划Plan0，再将Plan0分别移植到4个伪CT上重新进行剂量计算，得到Plan1、Plan2、Plan3和Plan4，并分别与Plan0进行剂量学比较。在定位MR图像上对各组织器官分别赋予群体化CT值并制定鼻咽癌IMRT计划。 结果： 鼻咽癌患者的头颈部CT图像中，骨骼群体化CT值平均为621 HU，空腔群体化CT值平均为-720 HU，其余软组织群体化CT值范围为-20～70 HU。Plan1、Plan2、Plan3、Plan4与Plan0的剂量指标差异依次减少，其中Plan4与Plan0的剂量学参数差异最小，PTV D(95)、PTV D(99)、等中心点剂量、全组织D(mean)、左右眼球D(max)、左右晶状体D(max)、左右视神经D(max)、左右腮腺D(mean)、左右腮腺V(20)、左右腮腺D(50)、脊髓D(max)、脑干D(max)、脑干D(5)差异<1%，左右腮腺V(30)差异<1.5%。像素点剂量分布比较，剂量分布差异>1%的区域主要分布于空腔间隙、骨骼周边以及皮肤。基于MR图像制定的鼻咽癌IMRT计划靶区满足95%处方剂量，各危及器官的受量均在剂量限制条件内。 结论： 将所有器官和组织分别赋予群体化CT值的赋值方法较其他方法对鼻咽癌IMRT计划剂量计算影响最小，可以基本满足临床要求，应作为基于MR图像制定鼻咽癌IMRT计划时的首选赋值方法。.
Keywords: Computed tomography; Intensity-modulated radiotherapy; Magnetic resonance imaging; Nasopharyngeal neoplasms; Radiotherapy plan.