面罩及乳腺托架固定下乳腺癌保乳术后放疗锁骨上下区摆位误差分析

目的 利用锥形束CT分析乳腺癌保乳术后全乳及锁骨上下区照射时使用头罩固定后的摆位误差,与不使用头罩固定时做比较,并计算两种不同固定方式的CTV-PTV外放边界。方法 选取2016-2018年于中国医学科学院肿瘤医院行保乳术后全乳加锁骨上下区放疗的乳腺癌患者,10例采用单纯乳腺托架固定体位,20例在托架基础上联合U型面罩固定。利用CBCT图像比较两组患者在不同方向上的摆位误差,并行独立样本t检验和χ²检验差异。用2.5Σ＋0.7δ计算CTV-PTV外放边界。结果 面罩组共110套图像,对照组共56套图像。对照组和面罩组在x、y、z轴向摆位误差分别为(0.212±0.174) cm和(0.272±0.242) cm (P=0.070)、(0.364±0.246) cm和(0.242±0.171) cm (P=0.001)、(0.423±0.302) cm和(0.364±0.269) cm (P=0.204)。对照组和面罩组在x、y、z轴向摆位误差位移量在0.5 cm以内的累积分布比例分别为91.07%和85.32%(P=0.294)、67.86%和89.91%(P=0.001)、67.86%和74.31%(P=0.381)。x、y、z轴向外放边界对照组分别为0.645、0.981、1.317 cm,面罩组分别为0.873、0.709、0.961 cm。BMI未超重患者在x轴向的摆位误差明显小于超重组患者(P=0.001)。结论 保乳术后照射全乳及锁骨上下区患者在使用乳腺托架的基础上加用面罩固定可显著改善头脚方向上的摆位误差。无论是否使用面罩固定,前后方向上的摆位误差较大,仍需进一步改良体位固定方法和优化摆位操作流程。BMI超重患者的摆位误差增大,在摆位时需特别关注,以保证放疗的精确度。

Breast board combined with a thermoplastic head mask immobilization can improve the reproducibility of the treatment setup for breast cancer patients receiving whole breast and supraclavicular nodal region irradiation

Objective To quantify the setup errors measured with kV cone-beam CT (CBCT) using breast board with or without a thermoplastic head mask in breast cancer patients who received whole breast and supraclavicular nodal region irradiation. Clinical target volume (CTV) to planning target volume (PTV) in 3 directions were also calculated. Methods The study included thirty patient receiving both whole breast and supraclavicular nodal region irradiation on Elekta Versa HD linear accelerators (Elekta Oncology Systems,Crawley,UK) between June 2016 and January 2018.The setup error data were retrospectively analyzed. All the patients were immobilized in the supine position on a breast board system (Carbon fibre breast board,Civco,Iowa,USA) with both arms raised. Twenty of the patients added an extra thermoplastic head mask to immobilize the neck. A CBCT scan was taken before treatment at the first 3 to 5 fractions and then once every week. Registration with the planning CT was performed and adjusted to match the target volume of the supraclavicular nodal region and the cervical vertebra by experienced medical staff,the position of the arms and the breast were also considered. For all patients the couch shifts in left-right (x),superior-inferior (y),anterior-posterior (z) were recorded. Statistical analysis included two-tails significance tests (t-Student and Manne-Whitney test for means, χ² test for variances).Population of the two groups was compared in terms of distribution of the mean shift (systematic error) and their standard deviations (random error).Van Herk′s setup margin defined as MPTV=2.5Σ＋0.7δ was calculated in patients with or without mask immobilization. Results Altogether,56 images and 109 images were acquired in breast board only group and head mask group,respectively. Shifts of the breast board only group and the head mask group in x,y,z were 0.212±0.174 cm vs.0.272±0.242 cm (P=0.070);0.364±0.246 cm vs.0.242±0.171 cm (P=0.001);0.423±0.302 cm vs.0.364±0.269 cm (P=0.204).Proportion of the shift less than 5 mm in the breast board only group and the head mask group were 91.07% vs. 85.32%(P=0.294);67.86% vs.89.91%(P=0.001);67.86% vs.74.31%(P=0.381).The CTV to PTV margin in x,y,z were 0.645 cm,0.981 cm,1.317 cm in breast board only group and 0.873 cm,0.709 cm,0.961 cm in head mask group,respectively. Setup error in the x direction was significantly correlated with BMI (P=0.001). Conclusions For the alignment and immobilization of patients who received whole breast and supraclavicular nodal region irradiation,using a breast board in combination of a thermoplastic head mask may significantly help to reduce the shift variance in superior-inferior direction compared to using breast board only. The anterior-posterior error is relatively large,other immobilization device or patient alignment methods are needed to be further explored to improve the accuracy.