Jasper Katie, Liu Baochang, Olson Robert, Matthews Quinn
BC Cancer-Vancouver, Vancouver, British Columbia, Canada.
Division of Radiation Oncology, Department of Surgery, University of British Columbia, Vancouver, British Columbia, Canada.
Adv Radiat Oncol. 2021 Jul 3;6(6):100750. doi: 10.1016/j.adro.2021.100750. eCollection 2021 Nov-Dec.
Standard planning target volume (PTV) margins for lung stereotactic ablative radiation therapy (SABR) are 5 mm. High-dose-rate volumetric modulated arc therapy delivered using flattening filter-free (FFF) beams with modern immobilization systems may allow for PTV margin reduction. This study assesses whether PTV margins can be reduced from 5 to 3 mm.
Target intrafractional motions derived from pretreatment and posttreatment cone beam computed tomography (CBCT) scans for 33 patients receiving lung SABR treated with 10XFFF energy and 5-mm PTV margins from 2016 to 2019 were used to calculate the required PTV margin. Deformable registration of the planning CT scan and internal gross tumor volume (IGTV) contour to posttreatment CBCT scans for 36 consecutive patients with 4 fraction schedules was completed to capture volume changes and intrafractional movement. Plans were replanned with 3-mm margins and recalculated on each deformed CT scan to assess deformed IGTV (d-IGTV) coverage and organ-at-risk doses.
Margin analysis showed PTV margins may be reduced to 3 mm. The mean d-IGTV coverage (percentage of the d-IGTV receiving ≥100% of the prescription dose [V100%] and the minimum dose covering 99.9% of the d-IGTV volume [D99.9%]) over 4 fractions for each patient was >95% with both margins. With 5-mm PTV margins, all 144 fractions had a d-IGTV V100% of >95% and a D99.9% >95%. With 3-mm PTV margins, the d-IGTV V100% was >95% in 99.3% of fractions (143 of 144) and the D99.9% was >95% in 98.6% of fractions (142 of 144). With 3-mm PTV margins, significant reductions in body V50%, body V80%, the volume of the lung receiving ≥20 Gy, and the mean lung dose and chest wall dose to 0.035 cm and 30 cm were observed (all < .001). Using theoretical models, the normal tissue complication probability for radiation pneumonitis decreased by a mean of 0.8% (range, 0.1%-2.7%), and the mean 2-year tumor control probability was 96.1% and 95.2% with 5-mm and 3-mm PTV margins, respectively.
With modern treatment and immobilization techniques in lung SABR, 3-mm PTV margins maintain acceptable IGTV coverage, modestly reduce toxicity to organs at risk, and maintain a calculated 2-year local control rate of >95%.
肺部立体定向消融放疗（SABR）的标准计划靶区（PTV）边界为5毫米。使用无 flattening 滤波器（FFF）束流并结合现代固定系统进行的高剂量率容积调强弧形放疗，可能允许减少PTV边界。本研究评估PTV边界是否可以从5毫米减少到3毫米。
利用2016年至2019年期间接受肺部SABR治疗、采用10XFFF能量和5毫米PTV边界的33例患者治疗前和治疗后的锥形束计算机断层扫描（CBCT）扫描得出的靶区内分次运动，来计算所需的PTV边界。对连续36例接受4分次治疗方案的患者，完成计划CT扫描与内部大体肿瘤体积（IGTV）轮廓到治疗后CBCT扫描的可变形配准，以捕捉体积变化和分次内运动。采用3毫米边界重新规划计划，并在每次变形的CT扫描上重新计算，以评估变形后的IGTV（d-IGTV）覆盖情况和危及器官的剂量。
边界分析表明PTV边界可减少至3毫米。两种边界情况下，每位患者4次分次治疗中d-IGTV的平均覆盖情况（接受≥100%处方剂量的d-IGTV百分比[V100%]以及覆盖99.9%的d-IGTV体积的最小剂量[D99.9%]）均>95%。采用5毫米PTV边界时，144次分次治疗中所有d-IGTV的V-100%均>95%且D99.9%>95%。采用3毫米PTV边界时，99.3%的分次治疗（144次中的143次）中d-IGTV的V100%>95%，98.6%的分次治疗（144次中的142次）中D99.9%>95%。采用3毫米PTV边界时，观察到身体V50%、身体V80%、接受≥20 Gy的肺体积以及至0.035厘米和30厘米处的平均肺剂量和胸壁剂量均显著降低（均<.001）。使用理论模型，放射性肺炎的正常组织并发症概率平均降低0.8%（范围为0.1%-2.7%），5毫米和3毫米PTV边界时的平均2年肿瘤控制概率分别为96.1%和95.2%。
在肺部SABR中采用现代治疗和固定技术时，3毫米的PTV边界可维持可接受的IGTV覆盖，适度降低对危及器官的毒性，并维持计算得出的>95%的2年局部控制率。
