Medical Dosimetry Program, University of Wisconsin, La Crosse, WI, 54601, USA; Wexner Medical Center, The Ohio State University, Columbus, OH, 43210, USA.
Medical Dosimetry Program, University of Wisconsin, La Crosse, WI, 54601, USA; Wexner Medical Center, The Ohio State University, Columbus, OH, 43210, USA.
Med Dosim. 2023;48(4):256-260. doi: 10.1016/j.meddos.2023.06.001. Epub 2023 Jul 16.
Volumetric modulated arc therapy (VMAT) is a frequently employed and guideline-recommended radiotherapy (RT) modality for extremity soft tissue sarcomas (eSTS). Prior studies have demonstrated that significant tumor volume changes during treatment result in loss of target volume coverage with highly conformal techniques such as VMAT, but few solutions exist to these issues aside from adaptive replanning. Here, we describe a related but novel phenomenon in which relatively minor changes in surface volume contour (whether due to daily setup uncertainty, edema of peritumoral tissue, or progression or pseudo-progression of tumor volume itself) can result in unexpected subcutaneous hotspots. This phenomenon is of significant clinical concern given the known association between skin dose and major wound complications during preoperative RT for eSTS. By evaluating daily cone-beam CT (CBCT) images from thirteen eSTS patients treated with VMAT RT, we identify daily surface contour changes (range: 2 mm-15 mm, median: 8 mm) which are frequently below conventional adaptive replanning thresholds. When applied under experimental conditions, these external contour changes did not have major impacts on target volume coverage (range: 30.2%-91.2%, mean: 72.5%) but did result in unexpected hotspots of 125.8% on average (range: 110.0%-142.2%) in the subcutaneous tissues. To mitigate this issue, we develop a methodology for VMAT treatment planning using flash PTV and virtual bolus (VB) to produce robust treatment plans that are more resistant to target volume changes, surface contour changes, and setup uncertainties than conventional planning methods. With this methodology, robust plans were equivalent to standard plans at baseline, but, after incorporation of surface volume changes, both maintained target volume coverage (p < 0.001) and prevented development of subcutaneous hotspots (p < 0.001) better than standard plans. As such, this treatment planning methodology may facilitate development of robust VMAT treatment plans that minimize development of subcutaneous hotspots and preserve target volume coverage in the context of routine volumetric changes during preoperative RT.
容积旋转调强放疗（VMAT）是一种常用于治疗肢体软组织肉瘤（eSTS）的放疗技术，也是指南推荐的治疗方法。先前的研究表明，治疗过程中肿瘤体积的显著变化会导致靶区覆盖丢失，而对于 VMAT 等高度适形技术而言，除了自适应重计划外，目前几乎没有其他解决方案。在这里，我们描述了一种相关但新颖的现象，即表面体积轮廓的相对较小变化（无论是由于日常摆位不确定性、肿瘤周围组织水肿还是肿瘤体积本身的进展或假性进展）可能导致意想不到的皮下热点。鉴于术前 RT 治疗 eSTS 中皮肤剂量与主要伤口并发症之间的已知关联，这种现象具有重要的临床意义。通过评估 13 例接受 VMAT RT 治疗的 eSTS 患者的每日锥形束 CT（CBCT）图像，我们发现每日表面轮廓变化（范围：2mm-15mm，中位数：8mm）经常低于常规自适应重计划阈值。在实验条件下应用这些外部轮廓变化时，对靶区覆盖范围没有重大影响（范围：30.2%-91.2%，平均值：72.5%），但会导致皮下组织中意外出现平均 125.8%的热点（范围：110.0%-142.2%）。为了解决这个问题，我们开发了一种使用快速 PTV 和虚拟敷贴（VB）的 VMAT 治疗计划方法，该方法可以生成更能抵抗靶区变化、表面轮廓变化和摆位不确定性的稳健治疗计划，优于常规计划方法。使用这种方法，稳健计划在基线时与标准计划等效，但在纳入表面体积变化后，两者均能更好地维持靶区覆盖（p<0.001）并防止皮下热点的形成（p<0.001），优于标准计划。因此，这种治疗计划方法可能有助于制定稳健的 VMAT 治疗计划，在术前 RT 过程中常规体积变化的情况下，最大限度地减少皮下热点的形成并保持靶区覆盖。
