摘要目的：利用模拟机测定不同体质量指数（BMI）直肠癌患者不同体位的放疗摆位误差，并以此计算计划靶体积（PTV）外扩边界的大小。方法选择36例直肠癌患者，根据体位固定方式不同分为真空垫组（18例）和常规腹板组（18例），计算每例患者的BMI，每例患者治疗前和治疗后每周拍摄正、侧位X线片，并与三维适形放疗系统中的数字重建图像（DRR）进行比较，测定出X、Y、Z轴三个方向的摆位误差，并进一步计算出不同BMI患者三维方向的PTV外扩边界。结果真空垫组和常规腹板组患者随BMI增加在X、Y、Z轴方向的摆位误差均逐渐增大，差异有统计学意义（P＜0.01或＜0.05）。BMI正常患者在X、Y、Z轴方向的PTV外扩边界分别为4.60、5.51、5.29 mm，超重患者分别为5.48、6.81、6.16 mm，肥胖患者分别为8.92、8.59、7.02 mm。结论超重和肥胖直肠癌患者摆位误差更大，确定患者的PTV外扩边界时需考虑BMI的影响。

abstractsObjective To measure the placing position error of different body position by analog positioning machine in patients with different body mass index (BMI) during radiotherapy for rectal cancer, then calculate the clinical target volume (CTV) to plan target volume (PTV) margins (Mptv). Methods Thirty-six patients with rectal cancer were selected, and the patients were divided into vacuum bag group (18 cases) and routine belly board group (18 cases) according to method of placing position. The BMI was calculated. Each patient was treated with positive and lateral position X-ray film before treatment and once a week during the radiotherapy. The data were contrasted with digitally rendered radiographs (DRR) of three-dimensional conformal radiotherapy plan system, then the placing position error on X, Y and Z axis was calculated, and the Mptv on X, Y and Z axis of patients with different BMI was calculated. Results The placing position error on X, Y and Z axis gradually increased with the increase of BMI in vacuum bag group and routine belly board group, and there were statistical differences (P<0.01 or <0.05). The Mptv on X, Y and Z axis in patients with normal BMI were 4.60, 5.51 and 5.29 mm respectively. The Mptv on X, Y and Z axis in patients with overweight were 5.48, 6.81 and 6.16 mm respectively. The Mptv on X, Y and Z axis in patients with obesity were 8.92, 8.59 and 7.02 mm respectively. Conclusions The rectal cancer patients with overweight and obesity have more placing position error. The influence of BMI should be considered when the patient's Mptv is determined.