MLC运动误差对不同射野面积的鼻咽癌容积调强计划的影响

Impacts of MLC_(error) on the Different Beam Area of Volumetric Arc Therapy for Nasopharyngeal Carcinoma

目的 研究多叶准直器运动误差(Multi-Leaf Collimator Error,MLC_(error))对不同射野平均面积(Mean Aperture Area,MAA)的鼻咽癌容积调强的影响。方法 选取30例鼻咽癌患者为研究对象,每例患者的原始计划导出后使用Python编译,依据不同的MLC_(error)制作6组模拟计划(±0.2 mm、±0.5 mm和±1.0 mm)。对比原始计划与模拟计划的剂量学参数和验证通过率,分析MLC_(error)和MAA对计划验证的影响。其中,将鼻咽肿瘤计划靶区(Planning Target Volume,PTV)与颈部转移淋巴结肿瘤靶区通过布尔运算整合为PTV69。结果 随着MLC_(error)的变化(-1~1 mm),PTV69、原发灶高危亚临床靶区和中危亚临床靶区的剂量梯度分别为6.1、6.0、6.3%/mm(P<0.05);危及器官(Organ At Risk,OAR)的剂量梯度均高于靶区剂量梯度。脊髓、脑干、左侧腮腺、右侧腮腺、口腔和喉的剂量梯度分别为10.6、10.5、12.3、12.0、8.1、11.2%/mm(P<0.05)。在对射野面积、MLC_(error)及验证通过率关系的研究中发现,不同MAA计划的验证通过率差异随MLC_(error)的增加而增大;当MLC_(error)误差达到±1.0 mm时,不同MAA计划的验证通过率差异最大(51.7%~90.2%)。结论 相较于靶区,控制MLC_(error)更有利于OAR的保护。对于鼻咽癌等复杂的放疗计划,建议将MLC_(error)精度控制在0.5 mm以内并降低小子野使用,以保护危及器官,保证计划执行的精确度。

Objective To analyze the influence between the mean aperture area(MAA)and the multi-leaf collimator error(MLC_(error))for nasopharyngeal carcinoma volumetric modulated arc therapy.Methods 30 patients diagnosed with postoperative nasopharyngeal carcinoma were selected as the study objects.Every original plan for each patient was exported and compiled using Python.Six modified plans were generated by different MLC_(error)(±0.2 mm,±0.5 mm,±1.0 mm).The dosimetry parameters and passing rate of the original and modified plan were compared and the relationship among MLC_(error),and MAA was analyzed.Among them,the planned target volume(PTV)of nasopharyngeal tumor and the target area of cervical metastatic lymph node tumor were integrated into PTV69 by Boolean operation.Results The corresponding variation of PTV69,planning clinical target volume 1 and planning clinical target volume 2 due to MLC_(error)(-1-1 mm)were 6.1%/mm,6.0%/mm and 6.3%/mm(P<0.05).The dose gradient of the organs at risk(OAR)was higher than that of the target area.The dose fall-off were 10.6%/mm,10.5%/mm,12.3%/mm,12.0%/mm,8.1%/mm and 11.2%/mm in spinal-cord,brainstem,parotid-L,parotid-R,oral and larynx(P<0.05).With the increase of MLC_(error),the passing rate difference of different MAA plan showed an increasing trend.When the MLC_(error) reached±1.0 mm,the discrepancy of different MAA plan was largest(51.7%-90.2%).Conclusion It is more beneficial for limiting MLC_(error) value to protect OAR than target.For the complicated nasopharyngeal carcinoma,less than 0.5 mm MLC_(error) is suggested to be the operation tolerance and decreasing the the small segment is recommended to protect OAR and ensure the dose accuracy.