子野小数机器跳数取整对鼻咽癌静态调强放射治疗计划剂量学的影响

目的:探讨Prowess5.1计划系统在鼻咽癌静态调强放疗（IMRT）计划设计时子野小数机器跳数（MU）的四舍五入取整对计划剂量学的影响。 方法:选择33例病理诊断明确的鼻咽癌患者的IMRT计划纳入研究，分为两组计划:计划系统优化完成的原始计划组为Porig；对原始计划组的子野小数机器跳数四舍五入取整，做最终剂量计算后生成新计划组Pnew。两组计划所有子野形状未改变，比较两组计划的MU变化及剂量学差异。 结果:两组计划所有靶区的CI和HI相近，差异均无统计学意义；Pnew组靶区覆盖率、PGTVnd的D98、D95、D2、Dmean及PTV2的D98、D95，Pnew组均低于Porig组（P<0.05），PGTVnd、PTVnx和PTV1的V100%分别降低了1.807%、0.655%和1.258%；危及器官仅左颞叶的Dmean差异具有统计学意义（P<0.05）；对于P1计划，单次Pnew比Porig跳数增加了1.600，差异具有统计学意义（519.758±46.410 vs 518.158±46.693, P<0.05)。 结论:Prowess5.1计划系统在制定鼻咽癌静态调强IMRT计划时，子野小数MU的微小变化会引起剂量学的变化，临床实践中应引起重视。

Dosimetric effects of rounding subfield monitor units up for step-and-shoot IMRT plan of nasopharyngeal carcinoma

Abstract: Objective To investigate the dosimetric effects of rounding subfield monitor units (MU) up to their nearest integers in step-and-shoot intensity-modulated radiotherapy (IMRT) plans for nasopharyngeal carcinoma (NPC) which was designed by Prowess5.1 planning system. Methods The IMRT plans for 33 NPC patients with definite pathological diagnosis were enrolled into this study. These IMRT plans were divided into two groups, namely Porig and Pnew. Porig was composed of all the original plans which had been optimized completely by the treatment planning system itself. Pnew was a new group of which all the subfield monitor unit were rounded up to their nearest integers based on Porig, and the shapes of segments in Pnew were all the same as those in Porig. The dosimetric characteristics and MU differences were recorded and compared between Porig and Pnew. Results The conformity index and homogeneity index of target areas in two groups were similar, without statistical significance. The target coverage, the D98, D95, D2, Dmean of PGTVnd, and the D98, D95 of PTV2 in Pnew were lower than those in Porig (P<0.05). The V100% of PGTVnd, PTVnx and PTV1 were decreased by 1.807%, 0.655% and 1.258%, respectively. For organs-at-risk, only the Dmean of left temporal lobe showed statistical differences between two groups (P<0.05). For P1 plans, the MU per fraction in Pnew was increased by 1.600 MU as compared with Porig (519.758±46.410 vs 518.158±46.693, P<0.05). Conclusion When Prowess5.1 planning system is used to design step-and-shoot IMRT plan for NPC, the tiny changes of subfield monitor units will cause dosimetric differences. Hence the calculation of monitor units should be paid attention to in clinical practice.