【摘要】 目的 调强放射治疗（IMRT）能较好的保护危及器官并给予肿瘤足够的致死剂量，基于多叶准直器（MLC）分步照射的IMRT技术对复杂病例需要更多子野。研究对直肠癌术后放射治疗使用不同子野数目的IMRT计划进行比对，选择合理的子野数。 方法 选取2010年4-8月入院的直肠癌术后患者10例，保持射野入射角度及优化目标参数相同，仅改变MLC子野数目，设计不同IMRT对每一患者治疗计划的靶区适形指数（CI）、均匀性指数、最大剂量、最小剂量、平均剂量，危及器官关注体积的受照剂量，机器跳数及治疗时间进行分析。 结果 所有治疗计划中靶区及危及器官的剂量学评估指标无统计学意义（Pgt;0.05），只有亚临床计划靶区（PTV）CI在15个子野的方案中（0.74±0.06）明显差于25个子野方案（0.82±0.03）、40个子野方案（0.81±0.06）及60个子野方案（0.84±0.03），有统计学意义（Plt;0.05）；治疗机器跳数（MU）随子野数目增多明显增大，15、20、40及60个子野方案所需MU分别为（458±56）、（559±62）、（614±74）、（622±82），有统计学意义（Plt;0.05），但40个子野方案与60个子野方案间无统计学意义。治疗时间明显随子野数增加而增大。 结论 直肠癌术后IMRT计划使用25个子野能满足临床剂量要求，同时能有效降低治疗时间，可作为临床应用参考值。【Abstract】 Objective The intensity modulated radiotherapy (IMRT) can deliver tumor enough doses and protect risk organs as much as possible at the same time. The MLC-based step and shoot IMRT(sIMRT) plan needs much more segment member to meet clinical aims. In this study, several sIMRT plans using different segment number for postoperative rectal cancer were compared to find out the most reasonable segment number setting. Methods Ten patients with rectal carcinoma underwent postoperative adjuvant radiotherapy for rectal cancer from April to August 2010 were selected. For each patient, the angle of field, the prescription expected and the physical parameters optimized were kept the same, while only the number of segments was changed in sIMRT plans. The dose volume histogram-based parameters [conformity index (CI) and homogeneous index (HI)] and other parameters concerned were compared and analyzed. Results The indexes of dosimetry associated with the targets and risk organs showed no significant statistical difference among the 4 sIMRT plans with different segment numbers. The index CI of PTV in the sIMRT plan with 15 segments (CI 0.74±0.06) was less than that in the sIMRT plan with 25 segments (CI 0.82±0.03), the sIMRT plan with 40 segments plan (CI 0.81±0.06), and the sIMRT plan with 60 segments (CI 0.84±0.03) (Plt;0.05). There were significant differences in MU among the sIMRT plans with 15 segments (average MU: 458±56) , with 25 segments (average MU: 559±62 ), and with 40 segments (average MU: 614±74)or with the 60 segments (average MU: 622±82 (Plt;0.05). The more segments meant more MU and more irradiation time. Conclusion The sIMRT plan for patients of rectal cancer to receive postoperative adjuvant radiotherapy may require at least 25 segments to balance the accepted dose results and efficient delivering.