(1) Background: Synthetic CT images of the pelvis were generated from daily CBCT images to monitor changes in water equivalent path length (WEPL) and determine the dosimetric impact of anatomy changes along the proton beam’s path; (2) Methods: Ten pediatric patients with pelvic tumors treated using proton therapy with daily CBCT were included. The original planning CT was deformed to the same-day CBCT to generate synthetic CT images for WEPL comparison and dosimetric evaluation; (3) Results: WEPL changes of 20 proton fields at the distal edge of the CTV ranged from 0.1 to 12 mm with a median of 2.5 mm, and 75th percentile of 5.1 mm for (the original CT—rescanned CT) and ranged from 0.3 to 10.1 mm with a median of 2.45 mm and 75th percentile of 4.8 mm for (the original CT—synthetic CT). The dosimetric impact was due to proton range pullback or overshoot, which led to reduced coverage in CTV Dmin averaging 12.1% and 11.3% in the rescanned and synthetic CT verification plans, respectively; (4) Conclusions: The study demonstrated that synthetic CT generated by deforming the original planning CT to daily CBCT can be used to quantify proton range changes and predict adverse dosimetric scenarios without the need for excessive rescanned CT scans during large interfractional variations in adaptive proton therapy of pediatric pelvic tumors.
(1) 背景：本研究通过日常锥形束CT图像生成骨盆合成CT影像，用于监测水等效路径长度变化，并评估质子束路径上解剖结构改变对剂量的影响；(2) 方法：纳入10例接受质子治疗且每日行锥形束CT扫描的儿童盆腔肿瘤患者。将原始计划CT配准至当日锥形束CT生成合成CT图像，用于水等效路径长度比较与剂量学评估；(3) 结果：在CTV远端边缘处，20个质子野的水等效路径长度变化显示：原始CT与重扫CT对比的中位数为2.5毫米（范围0.1-12毫米），第75百分位数为5.1毫米；原始CT与合成CT对比的中位数为2.45毫米（范围0.3-10.1毫米），第75百分位数为4.8毫米。剂量学影响主要源于质子射程回退或过冲，导致重扫CT与合成CT验证计划中CTV最小剂量覆盖率平均分别降低12.1%和11.3%；(4) 结论：本研究证实，通过将原始计划CT配准至日常锥形束CT生成的合成CT，可在儿童盆腔肿瘤自适应质子治疗出现较大分次间变化时，量化质子射程变化并预测不良剂量学场景，从而避免过度依赖重复CT扫描。
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