Pritzker School of Medicine, The University of Chicago, Chicago, IL, USA.
Department of Radiation and Cellular Oncology, The University of Chicago, Chicago, IL, USA.
J Appl Clin Med Phys. 2020 Dec;21(12):146-154. doi: 10.1002/acm2.13080. Epub 2020 Oct 30.
To evaluate the dosimetric impact of daily positioning variations measured with cone-beam computed tomography (CBCT) on whole-breast radiotherapy patients treated in the prone position.
Daily CBCT was prospectively acquired for 30 consecutive patients positioned prone. Treatment for early-stage (≤II) breast cancer was prescribed with standard dose (50 Gy/25 fractions) or hypofractionation (42.56 Gy/16 fractions) for 13 and 17 patients, respectively. Systematic and random errors were calculated from the translational CBCT shifts and used to determine population-based setup margins. Mean translations (±one standard deviation) for each patient were used to simulate the dosimetric impact on targets (PTV_eval and lumpectomy cavity), heart, and lung. Paired Student's t tests at α = 0.01 were used to compare dose metrics after correction for multiple testing (P < 0.002). Significant correlation coefficients were used to identify associations (P < 0.01).
Of 597 total fractions, 20 ± 13% required patient rotation. Mean translations were 0.29 ± 0.27 cm, 0.41 ± 0.34 cm, and 0.48 ± 0.33 cm in the anterior-posterior, superior-inferior, and lateral directions leading to calculated setup margins of 0.63, 0.88, and 1.10 cm, respectively. Average three-dimensional (3D) shifts correlated with the maximum distance of breast tissue from the sternum (r = 0.62) but not with body-mass index. Simulated shifts showed significant, but minor, changes in dose metrics for PTV_eval, lung, and heart. For left-sided treatments (n = 18), mean heart dose increased from 109 ± 75 cGy to 148 ± 115 cGy. Shifts from the original plan caused PTV_eval hotspots (V105%) to increase by 5.2% ± 3.8%, which correlated with the total MU of wedged fields (r = 0.59). No significant change in V95% to the cavity was found.
Large translational variations that occur when positioning prone breast patients had small but significant dosimetric effects on 3DCRT plans. Daily CBCT may still be necessary to correct for rotational variations that occur in 20% of treatments. To maintain planned dose metrics, unintended beam shifts toward the heart and the contribution of wedged fields should be minimized.
评估使用锥形束计算机断层扫描（CBCT）测量的日常定位变化对接受俯卧位全乳房放疗的患者的剂量学影响。
前瞻性地对 30 例连续俯卧位患者进行每日 CBCT 采集。为 13 例早期（≤II 期）乳腺癌患者和 17 例接受标准剂量（50Gy/25 次）或低分割（42.56Gy/16 次）治疗的患者分别处方治疗。从平移 CBCT 移位中计算系统误差和随机误差，并用于确定基于人群的设置边界。每位患者的平均平移（±一个标准差）用于模拟对靶区（PTV_eval 和乳房切除术腔）、心脏和肺的剂量学影响。使用配对学生 t 检验（α=0.01），对多次检验（P<0.002）进行校正后比较剂量指标。使用显著相关系数（P<0.01）来识别相关性。
在 597 个总分次中，有 20%±13%需要患者旋转。平均平移为 0.29±0.27cm、0.41±0.34cm 和 0.48±0.33cm，在前-后、上-下和侧-方向上分别导致 0.63、0.88 和 1.10cm 的设置边界。平均三维（3D）移位与乳房组织距胸骨的最大距离相关（r=0.62），但与体重指数无关。模拟移位导致 PTV_eval、肺和心脏的剂量指标发生显著但较小的变化。对于左侧治疗（n=18），平均心脏剂量从 109±75cGy 增加到 148±115cGy。原始计划中的移位导致 PTV_eval 热点（V105%）增加了 5.2%±3.8%，与楔形野的总 MU 相关（r=0.59）。未发现对腔的 V95%有显著变化。
当俯卧位定位患者时，发生的大的平移变化对 3DCRT 计划有微小但显著的剂量学影响。为了纠正 20%治疗中发生的旋转变化，可能仍需要每日进行 CBCT。为了维持计划剂量指标，应尽量减少射束向心脏的意外移位和楔形野的贡献。
