【摘要】 背景与目的:目前调强放疗已广泛应用于鼻咽癌,与传统放疗相比,调强放疗对摆位精确度要求更高。本研究通过移动等中心模拟系统误差,研究系统误差在鼻咽癌放疗中对剂量分布的影响,比较调强放疗计划及适形放疗计划中靶区和正常组织的剂量受摆位误差影响的敏感度。方法:分别制定8例初治鼻咽癌患者的调强及适形放疗计划,在治疗计划系统中移动等中心,假设在每次治疗时所有的射野都向同一个方向偏移的情况,分别模拟三维6个方向上2mm和5mm的系统摆位误差。在不改变射野分布和权重的情况下,重新计算剂量分布。分析移动等中心后所有的剂量评估指标与正式计划相应指标的比值,从而比较适形和调强计划受系统误差影响的敏感度。结果:频数分布显示:调强计划当摆位误差为5mm时,4.2%的GTVD98及12.5%的CTVD95剂量减少大于3%,当摆位误差为2mm时,GTVD98、CTVD95均未出现剂量减少大于3%的情况;而适形放疗计划无论摆位误差为2mm还是5mm,均未发生剂量减少大于3%的情况。当系统摆位误差为5mm时,调强计划正常组织的剂量明显增加,脊髓、脑干、左右腮腺剂量增加超过了正式计划10%的机会分别为10.4%、14.6%、31.2%、25%。当系统摆位误差为2mm时,脊髓和脑干剂量增加超过原计划5%的机会仅为6.3%及4.2%,左右腮腺剂量增加超过了正式计划10%的机会分别为12.5%、8.3%。适形放疗计划系统摆位误差为5mm时,仅10.4%脊髓、6.3%脑干剂量增加超过了正式计划的5%,而腮腺剂量增加不明显。无论是靶区还是正常组织,调强计划与适形计划相比,剂量变化百分值的均数受摆位误差的影响更大(P<0.05);误差越大,靶区和正常组织的剂量变化百分值也越大(P<0.05)。结论:误差越大对剂量分布的影响越明显。无论是靶区还是正常组织,与适形放疗相比调强计划的剂量分布受摆位误差的影响更大。当调强放疗时,系统误差对靶区CTV剂量的影响较GTV明显,而腮腺的剂量与脊髓和脑干相比受系统摆位误差的影响更大。更多还原

【Abstract】 Background and purpose:Intensity modulated radiation therapy(IMRT) is now widely used in the radiation therapy of nasopharyngeal carcinoma, but more accurate positioning of patients is required as compared to conventional radiotherapy. The purpose of this study was to determine the impact of dose delivery errors that could result from systematic setup errors for IMRT of nasopharyngeal carcinoma on the dose distribution affecting Dimensional conformal radiotherapy(DCRT) treatments according to simulated systematic setup errors by moving the isocenter. Methods:Eight patients with IMRT and DCRT plan were evaluated to assess the impact of systematic setup errors. The dosimetric effect of systematic error was simulated by randomly shifting the patient isocenter along each of the three Cartesian axes, with each shift selected from a normal distribution. Systematic setup error of 2 mm and 5 mm were simulated. For each dose calculation, we analyzed the ratio of each dose evaluation index to the normal plan used for real time treatment and compared the sensitivity of DCRT and IMRT to setup errors. Results:Frequency distribution showed that simulating systematic setup error with 5 mm resulted in 4.2% of GTV98 and 12.5% of CTV95 with the dose reduction of more than 3%, whereas simulating systematic setup error with 2 mm resulted in having no more than 3% dose error in IMRT. For DCRT, neither simulated systematic error with 5 mm nor with 2 mm resulted in having more than 3% dose error. With 5 mm systematic setup error, the dose of critical organs increased with 10.4% for spinal cord, 14.6% for brain stem, 31.2% for left parotid and 25% for right parotid with more than 10% dose distribution different from real time treatment planning, and with 2 mm systematic setup error, the dose of critical organs increased only with 6.3% for spinal cord, 4.2% for brain stem, there were more than 5% dose distribution difference, 12.5% for left parotid and 8.3% right parotid with more than 10% dose distribution different from real time treatment. However, in DCRT with 5mm systematic setup error, only 10.4% for spinal cord, 6.3% for brain stem were documented with more than 5% dose distribution different from real time treatment, there was no significant increase in the dose of left parotid and right parotid. For both targets and critical organs, comparing IMRT with DCRT, the mean percent dose change of the simulated structure dose in terms of the normal plan was more sensitive to IMRT(P<0.05). the higher the systematic setup error, the more significant the impact on dose distribution(P<0.05). Conclusions:The higher systematic setup error induced more significant dose error. For both targets and critical organs, IMRT was more sensitive to systematic setup error than DCRT. For IMRT, GTV was more sensitive to patient positioning than CTV and parotids were more sensitive to patient positioning than spinal cord and brain stem.更多还原