Chan Eujin, Goodall Simon K, Finnegan Robert, Moorfoot Paul, Jameson Michael, Dunn Leon
GenesisCare Victoria, Melbourne, Victoria, Australia.
GenesisCare Western Australia, Wembley, Western Australia, Australia.
J Appl Clin Med Phys. 2025 Jan;26(1):e14539. doi: 10.1002/acm2.14539. Epub 2024 Oct 3.
The aim of this study is to determine the impact of rectal air volume changes on treatment plan quality, and subsequently inform daily cone-beam computed tomography (CBCT) evaluation constraints, in terms of acceptable rectal air volume during treatment.
Twelve rectal cancer patients who exhibited rectal air within the PTV on their planning CT were selected. A study was conducted to evaluate the deterioration in plan quality due to expanding air volume. For each case, the air cavity volume was isotropically expanded in three dimensions using predefined margins of 3, 5, 7, and 10 mm, while deforming bladder and rectum contours. A constraint was applied to the bony anatomy to restrict the deformation. Treatment plans were then generated for all twelve patients by recalculating the reference plan with the expanded air cavity volume.
As the air cavity expanded, the maximum relative change in D98% coverage, compared to the reference plan, decreased by 10.8% ± 3.5%, while the D2% increased by 3.5% ± 0.9%. The positioning of the air cavity notably influenced the D98% variability with the 3 mm expansion. D98% coverage falls below 95% when the air cavity volume exceeds 17 cm. On average, D2% coverage increased by 0.5% with each expansion. At the largest expansion, extensive coverage of 102% and 105% isodoses was observed compared to the reference plan.
Air cavity volumes above 17 cm can potentially degrade the high-dose PTV coverage while increasing the regions covered by the 102% and 105% isodoses. Clinical CBCT guidelines were deduced, recommending a maximum threshold of 3.2 cm in diameter in any direction.
本研究旨在确定直肠空气体积变化对治疗计划质量的影响，并随后根据治疗期间可接受的直肠空气体积，为每日锥形束计算机断层扫描（CBCT）评估限制提供参考。
选择12例在计划CT上PTV内显示有直肠空气的直肠癌患者。开展一项研究以评估因空气体积扩大导致的计划质量恶化情况。对于每个病例，使用3、5、7和10毫米的预定义边界在三个维度上各向同性地扩大气腔体积，同时使膀胱和直肠轮廓变形。对骨骼解剖结构施加一个约束以限制变形。然后通过用扩大后的气腔体积重新计算参考计划，为所有12例患者生成治疗计划。
随着气腔扩大，与参考计划相比，D98%覆盖率的最大相对变化降低了10.8%±3.5%，而D2%增加了3.5%±0.9%。气腔的位置对3毫米扩大时的D98%变异性有显著影响。当气腔体积超过17立方厘米时，D98%覆盖率降至95%以下。平均而言，每次扩大时D2%覆盖率增加0.5%。在最大扩大时，与参考计划相比，观察到102%和105%等剂量线的覆盖范围扩大。
气腔体积超过17立方厘米可能会降低高剂量PTV覆盖率，同时增加102%和105%等剂量线覆盖的区域。推导了临床CBCT指南，建议在任何方向上直径的最大阈值为3.2厘米。
