肩膀位置移动对头颈部肿瘤放疗的影响

Effect of shoulder position shift on radiotherapy for head and neck cancer

分析头颈部肿瘤放射治疗摆位过程中肩膀位置移动对靶区和危及器官的剂量学影响。选取14例不同类型头颈部肿瘤患者,通过放疗计划系统在原始定位CTs(o-CTs)的基础上,使肩膀位置向头-脚(SI)、腹-背(AP)、左-右(LR)6个方向均分别移动3 mm、6 mm和15 mm,产生252(14×6×3=252)个新的CTs(n-CTs),设定移动后新的组织密度为1 g/cm^(3),同时考虑骨组织的移动。将治疗用的放疗计划(o-plan)复制到n-CTs上重新计算,得到252个新计划(n-plan)。研究固定野调强放疗(Intensity modulated radiation therapy,IMRT)、容积弧形调强(Volume modulated arc therapy,VMAT)方式下o-plan与n-plan靶区D_(95%)和危及器官D_(max)的差异,对比肩膀位置移动对靶区和脊髓的剂量变化。肩膀位置移动对头颈部肿瘤靶区和危及器官影响客观存在,特别是VMAT方式下肩膀朝头(S)方向移动15 mm影响最大。靶区位置越靠近肩膀,影响越严重;VMAT方式下肩膀朝脚(I)方向移动3 mm、6 mm、15 mm脊髓剂量增加较大,最大分别达到了72.8 cGy、100 cGy和212 cGy;n-plan与o-plan做3Dγ通过率(3%/3 mm)分析,结果除向头方向移动15 mm外均在95%以上。放疗技师在头颈部肿瘤放疗摆位时应当注意肩膀轻贴热塑模具,防止病人耸肩,物理师在计划设计时应考虑危及器官剂量限量留有足够余量,防止摆位带来的剂量超量。

To analyze the dosimetric effects of shoulder position shift on target volumes and organs at risk(OARs)during radiotherapy for head and neck cancer,14 patients with different types of head and neck cancer were selected.Based on original positioning CTs(o-CTs),the shoulder position was moved toward the superior inferior(SI)direction,anterior posterior(AP)direction,and the left right(LR)direction by 3 mm,6 mm,and 15 mm,then 252 new CTs(n-CTs)were produced.After setting the movement,the new adipose tissue density was set to 1 g/cm^(3),and the movement of the bone tissue was also considered.The original plan(o-plan)on the o-CT was copied to the n-CTs and recalculated to obtain 252 new plans(n-plans).The differences between the o-plan and n-plan target volumes and OARs in field-fixed intensity modulated radiotherapy and the volumetric modulated arc intensity(VMAT)were studied,and the dose changes of the target volume D_(95%) and spinal cord D_(max) were compared with the movement of the shoulder position.It was proved that the effects of the shoulder position shift on the target volume and OARs for head and neck cancers are substantial,especially a 15 mm shift toward the superior direction had the greatest influence,reaching 4.16%(VMAT).The closer the target area is to the shoulder,the more serious the impact.The spinal cord dose increased greatly,reaching a maximum of 72.8 cGy,100 cGy and 212 cGy(VMAT),respectively,for shift of 3 mm,6 mm,and 15 mm towards the inferior direction.The o-plan and p-plan were analyzed using a 3D γ passing rate(3%/3 mm),that was more than 95%except for the 15 mm shift towards the superior direction.Radiotherapy technicians should pay attention to ensure that the shoulder is lightly touching the thermoplastic mold during radiotherapy positioning for head and neck tumors to prevent patients from shrugging their shoulders.The OAR dose limit should be considered in the planning design,and a sufficient margin should be left to prevent the excessive dose caused by the positioning.