Dosimetric effects study of set-up error in postoperative intensity modulated radiotherapy of rectal cancer

Abstract

Objective To measure set-up errors in intensity modulated radiotherapy of postoperative rectal cancer (IMRT-PRC), and analyze the effects of target volumes and normal tissues by set-up errors in IMRTPRC.

Methods electrical field device was used to measure the set-up errors of 30 patients in IMRTPRC; we randomly selected 6 patients, and simulated their radiotherapy set-up errors in TPS. Then were counted the dose distribution and analyze relative dosimetrical parameters of target volumes, normal tissues and got the set-up errors effecting on dosimetry. Statistical analysis was performed using an SPSS statistical package(Version 19.0).

Results The set-up errors on X, Y and Z direction are (-0.82 ± 2.67) mm, (0.42 ± 2.91) mm and (0.47 ± 1.64) mm respectively. In regard to set-up errors of L5, R7, G8, T5, B6 and A4, most of statistical properties of PTV’s D _98%, D _95%, D _50%, D _2%, Hi and Ci are smaller than 0.05, so the differences have statistical values; In L5- Hi, G8- D _2%, T5- D _50% and B6- D _2%, so these differences have not statistical value ( P > 0.05). In L5, R7, G8, T5, B6 and A4, about half of the statistical properties of normal tissue like bowl, bladder and femur heads, are smaller than 0.05 and have statistical value, and the rest of them are negative.

Conclusion The dose of PTV is decreased due toset-up error. Uniformity and conformity is also gone bad. So the dosimetry quantity of PTV can not get reach to the initial design level. However, the dose of normal tissues like bowl, bladder and femur heads, are increased unexpectedly.

Abstract

摘要： 目的 测量直肠癌在调强放疗中的摆位误差, 探讨直肠癌术后调强放疗摆位误差对靶区和正常组织剂量学的 影响。 方法 利用电子射野影像系统测量30例直肠癌患者在调强放疗中的摆位误差; 随机选择6例, 在治疗计划系 统中模拟患者治疗时体位的三维误差, 重新计算剂量分布, 分析相关靶区和正常组织的剂量参数, 明确摆位误差对剂 量学的影响。采用SPSS19.0软件进行统计分析。 结果直肠癌术后调强放疗摆位误差在X、Y和Z 3个方向分别是 (-0.82 ± 2.67) mm、(0.42 ± 2.91)mm 和 (0.47 ± 1.64) mm。摆位误差 L5、R7, G8、T5, B6、A4 对 PTV 的 D _98%、 D _95%、 D _50%、 D _2%、 Hi及 CI , 大部分差异有统计学意义 ( P < 0.05); L5时 HI、G8时 D _2%、T5时 D _50%、B6 时 D _2% 的差异无统计学 意义 ( P > 0.05)。摆位误差L5、R7, G8、T5, B6、A4对正常组织小肠的D _max和 D _2%, 膀胱、左股骨头和右股骨头的 D _mean和的 V ₅₀ 影响, 一半左右差异有统计学意义 ( P < 0.05)。 结论 摆位误差降低了 PTV的受照射剂量, 均匀性、适形 度变差, 使靶区剂量达不到原计划水平;增加了正常组织小肠、膀胱、左股骨头和右股骨头的受照射剂量。