Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Garching, Germany.
Division of Radiation Biophysics, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA.
Med Phys. 2022 Jul;49(7):4693-4704. doi: 10.1002/mp.15644. Epub 2022 May 15.
In proton therapy, dose distributions are currently often conformed to organs at risk (OARs) using the less sharp dose fall-off at the lateral beam edge to reduce the effects of uncertainties in the in vivo proton range. However, range uncertainty reductions may make greater use of the sharper dose fall-off at the distal beam edge feasible, potentially improving OAR sparing. We quantified the benefits of such novel beam arrangements.
For each of 10 brain or skull base cases, five treatment plans robust to 2 mm setup and 0%-4% range uncertainty were created for the traditional clinical beam arrangement and a novel beam arrangement making greater use of the distal beam edge to conform the dose distribution to the brainstem. Metrics including the brainstem normal tissue complication probability (NTCP) with the endpoint of necrosis were determined for all plans and all setup and range uncertainty scenarios.
For the traditional beam arrangement, reducing the range uncertainty from the current level of approximately 4% to a potentially achievable level of 1% reduced the brainstem NTCP by up to 0.9 percentage points in the nominal and up to 1.5 percentage points in the worst-case scenario. Switching to the novel beam arrangement at 1% range uncertainty improved these values by a factor of 2, that is, to 1.8 percentage points and 3.2 percentage points, respectively. The novel beam arrangement achieved a lower brainstem NTCP in all cases starting at a range uncertainty of 2%.
The benefits of novel beam arrangements may be of the same magnitude or even exceed the direct benefits of range uncertainty reductions. Indirect effects may therefore contribute markedly to the benefits of reducing proton range uncertainties.
在质子治疗中，目前通常使用束边缘较低的剂量陡度来调整危及器官（OAR）剂量分布，以降低体内质子射程不确定性的影响。然而，降低射程不确定性可能会使更充分地利用远端束边缘的陡度成为可能，从而可能提高 OAR 保护。我们量化了这种新束流布局的优势。
对于 10 个脑或颅底病例中的每一个，为传统临床束流布局和一种新的束流布局分别创建了 5 个对 2mm 摆位误差和 0%-4%射程不确定性稳健的治疗计划，新束流布局更充分地利用远端束边缘来调整剂量分布以符合脑干形状。对所有计划和所有摆位误差和射程不确定性场景，确定了包括以坏死为终点的脑干正常组织并发症概率（NTCP）在内的指标。
对于传统束流布局，将射程不确定性从当前的约 4%降低到潜在的可达到的 1%，可使脑干 NTCP 在名义情况下降低多达 0.9 个百分点，在最坏情况下降低多达 1.5 个百分点。在 1%的射程不确定性下切换到新的束流布局将这些值提高了 2 倍，即分别提高到 1.8 个百分点和 3.2 个百分点。从射程不确定性为 2%开始，新束流布局在所有情况下都实现了更低的脑干 NTCP。
新束流布局的优势可能与射程不确定性降低的直接优势相当，甚至更大。因此，间接影响可能对降低质子射程不确定性的优势有显著贡献。
