调强放疗靶区邻近体表时皮肤受量过高解决方案的探讨

Resolutions of high superficial dose in intensity-modulated radiation therapy when the target area is close to the surface

目的研究头颈部调强放疗在靶区邻近体表时不同计划靶区(PTV)边界的设置对浅表剂量分布的影响,探讨靶区靠近体表皮时皮肤受量过高的解决方案。方法设计圆柱形的颈部浅表肿瘤模型,采用8MVX线分别作两组逆向调强放疗试验计划。(1)计划组A:设置PTV在体表侧边界为0～5mm,其余方向边界为5mm。(2)计划B:建立取消剂量建成的调强计划优化专用的虚拟治疗机剂量学数据,用虚拟机进行逆向优化设计,再用实际的临床治疗机剂量学数据重新计算计划剂量分布。比较上述各计划浅表区域靶区与皮肤附近的剂量分布随摆位误差变化和临床靶区(CTV)剂量覆盖程度,分析当CTV靠近体表时该侧PTV安全边界的合理设置和依据。结果(1)计划组A在PTV与皮肤距离d<3mm时,皮肤附近出现明显超出处方剂量的高剂量区,且3mm摆位误差可令高剂量显著增加;适当缩小体表侧PTV边界可以在保证CTV得到处方剂量的同时有效降低浅表高剂量现象。为避免CTV不出现<95%处方剂量的低剂量,体表侧PTV安全边界至少为1mm。(2)计划B方法可以在保证CTV剂量条件下有效消除皮肤附近的高剂量分布。结论利用适当的设计技巧进行逆向调强计划优化可得到更好的浅表剂量分布。

Objective To study the impact of different planning target volume （PTV） margin settings on surface and superficial dose distribution and explore the resolution of high superficial dose when the target area is close to the surface during head and neck intensity-modulated radiotherapy （IMRT）.Methods A typical superficial target volume was designed in an circular neck phantom.Two experimental inverse IMRT plans were conducted with 8MV X ray,and in plan A,the superficial side of PTV margin ranged from 0 to 5 mm,while other side margins were 5 mm;in plan B,an suppositional machine dosimetry data for IMRT optimization was established in which the build-up dose was eliminated,and this machine data was used to optimize the inverse IMRT plan followed by recalculation of the planned dose distribution with the actual clinical machine dosimetry data.The variation of the surface and superficial dose resulting from set-up error and the dose distribution to CTV were compared.The adequate PTV margin was analyzed when the CTV approached the surface.Results In plan A,the high dose greater than the prescribed dose was found in the surface and superficial region when the superficial sparing distance was between the surface and PTV d3 mm,and the 3 mm set-up margin increased this superficial dose remarkably.Reducing the superficial side of PTV margin lowered the high superficial dose effectively and allowed maintenance of the prescribed dose to the CTV.To avoid reduction of the dose to the CTV to below 95% of the prescribed dose,the superficial side of PTV margin should be greater than 1 mm.Plan B effectively lowered the surface doses and maintained the prescribed dose to the CTV.Conclusions With appropriate techniques for optimizing inverse IMRT,more homogeneous superficial dose can be achieved.