Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, USA.
Applications of Detectors and Accelerators to Medicine, Meyrin, Switzerland.
Acta Oncol. 2020 Oct;59(10):1186-1192. doi: 10.1080/0284186X.2020.1769859. Epub 2020 Jun 5.
Pelvic target dose from intensity-modulated proton therapy (IMPT) is sensitive to patient bowel motion. Robustly optimized plans in regard to bowel filling may improve the dose coverage in the treatment course. Our purpose is to investigate the effect of air volume in large and small bowel and rectum on target dose from IMPT plans.
Data from 17 cancer patients (11 prostate, 3 gynecologic, 2 colon, and 1 embryonal rhabdomyosarcoma) with planning CT (pCT) and weekly or biweekly scanned quality assurance CTs (QACTs; 82 QACT scans total) were studied. Air in bowels and rectum traversed by proton pencil beams was contoured. The robust treatment plan was made by using 3 CT sets: the pCT set and 2 virtual CT sets that were copies of pCT but in which the fillings of bowels and rectum were overridden to be either air or muscle. Each plan had 2-5 beams with a mean of 3 beams. Targets in the pCT were mapped to the QACTs by deformable image registration, and the dose in QACTs was calculated. Dose coverage (D99 and D95) and correlations between dose coverage and changes in air volume were analyzed. The significance of the correlation was analyzed by test.
Mean changes of D99 in QACTs were within 3% of those in the pCT for all prostate and colon cases but >3% in 2 of the 3 gynecologic cases and in the embryonal rhabdomyosarcoma case. Of these three cases with mean change of D99 > 3%, air volume may be the main cause in 2. For the prostate cases, correlation coefficients were <0.7 between change in air volume and change in D99 and D95, because other anatomy changes also contributed to dose deviation. Correlation coefficients in the non-prostate cases were >0.9 between D99 change and rectum and between D95 change and small bowel, indicating a greater effect of the air volume on target dose.
The air volume may still have an important effect on target dose coverage in treatment plans using 3 CT sets, particularly when the air is traversed by multiple beams.
盆腔靶区剂量受患者肠道运动影响较大。对于肠道充盈进行稳健优化的计划可能会改善治疗过程中的靶区剂量覆盖。本研究旨在探讨大、小肠和直肠内的空气体积对调强质子治疗（IMPT）计划靶区剂量的影响。
本研究纳入了 17 例癌症患者（11 例前列腺癌、3 例妇科癌、2 例结肠癌和 1 例胚胎性横纹肌肉瘤）的计划 CT（pCT）和每周或每两周扫描的质量保证 CT（QACT；共 82 次 QACT 扫描）数据。勾画质子笔束穿过的肠道和直肠内的空气。通过使用 3 套 CT 来制定稳健的治疗计划：pCT 套和 2 个虚拟 CT 套，它们是 pCT 的副本，但肠道和直肠的填充内容被覆盖为空气或肌肉。每个计划有 2-5 个射束，平均 3 个射束。通过变形图像配准将 pCT 中的靶区映射到 QACTs 上，并计算 QACTs 中的剂量。分析靶区剂量覆盖（D99 和 D95）和剂量覆盖与空气体积变化之间的相关性。采用 t 检验分析相关性的显著性。
对于所有前列腺癌和结肠癌病例，QACT 中的 D99 平均变化在 pCT 中的变化的 3%以内，但在 3 例妇科病例中的 2 例和胚胎性横纹肌肉瘤病例中超过 3%。在这 3 例 D99 平均变化大于 3%的病例中，2 例的主要原因可能是空气体积。对于前列腺癌病例，D99 和 D95 的变化与空气体积的变化之间的相关系数<0.7，因为其他解剖结构的变化也导致了剂量偏差。非前列腺癌病例中，D99 变化与直肠、D95 变化与小肠之间的相关系数>0.9，表明空气体积对靶区剂量的影响更大。
在使用 3 套 CT 的治疗计划中，空气体积可能仍然对靶区剂量覆盖有重要影响，尤其是当空气被多个射束穿过时。
