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PURPOSE

Proton treatment plan perturbation by common dental fixtures such as amalgams (Am) and porcelain-fused-to-metal (PFM) crowns has, to date, been uncharacterized. Previous studies have been conducted to determine the physical effect of these materials within the beam path for single spots, but their effects on complex treatment plans and clinical anatomy have not yet been quantified. The present manuscript aims to study the effect of Am and PFM fixtures on proton treatment planning in a clinical setting.

METHODS

An anthropomorphic phantom with removable tongue, maxilla, and mandible modules was simulated on a clinical computed tomography (CT) scanner. Spare maxilla modules were modified to include either a 1.5 mm depth central groove occlusal amalgam (Am) or a porcelain-fused-to-metal (PFM) crown, implanted on the first right molar. Modified tongue modules were 3D printed to accommodate several axial or sagittal oriented pieces of EBT-3 film. Clinically representative spot-scanning proton plans were generated in Eclipse v.15.6 using the proton convolution superposition (PCS) algorithm v.15.6.06 using a multi-field optimization (MFO) technique with the goal of delivering a uniform 54 Gy dose to a clinical target volume (CTV) typical of a base-of-tongue (BoT) treatment. A typical geometric beam arrangement of two anterior oblique (AO) beams and a posterior beam was employed. Plans optimized without any material overrides were delivered to the phantom A) without implants; B) with Am fixture; or C) with PFM crown. Plans were also reoptimized and delivered with inclusion of material overrides to equate relative stopping power of the fixture with that of a previously measured result.

RESULTS

Plans exhibit slightly greater dose weight towards AO beams. The optimizer accounted for inclusion of fixture overrides by increasing beam weights to the beam closest to the implant. Film measurements exhibited cold spots directly within the beam path through the fixture in plans with and without overridden materials. Cold spots were somewhat mitigated in plans including overridden materials in the structure set but were not entirely eliminated. Cold spots associated with Am and PFM fixtures were quantified at 17% and 14% for plans without overrides, respectively, and 11% and 9% with using Monte Carlo simulation. Compared with film measurements and Monte Carlo simulation, the treatment planning system underestimates the dose shadowing effect in plans including material overrides.

CONCLUSIONS

Dental fixtures create a dose shadowing effect directly in line with the beam path through the material. This cold spot is partially mitigated by overriding the material to measured relative stopping powers. Due to uncertainties in modeling perturbation through the fixture, the magnitude of the cold spot is underestimated using the institutional TPS when compared to measurement and MC simulation.

PURPOSE

METHODS

RESULTS

CONCLUSIONS

目的

质子治疗计划因常见的牙科修复体（如汞合金（Am）和金属烤瓷冠（PFM））而发生的改变，迄今为止尚未得到描述。以前的研究已经确定了这些材料在单个光斑射束路径中的物理效应，但它们对复杂治疗计划和临床解剖结构的影响尚未量化。本手稿旨在研究 Am 和 PFM 修复体在临床环境中对质子治疗计划的影响。

方法

在临床 CT 扫描仪上模拟了具有可移动舌、上颌和下颌模块的人体模型。多余的上颌模块经过修改，在上颌第一磨牙处包含一个 1.5 毫米深的中央凹槽汞合金（Am）或金属烤瓷冠（PFM），并植入其中。经过修改的舌模块使用 EBT-3 胶片的几个轴向或矢状方向的片材 3D 打印，以适应需要。使用多野优化（MFO）技术，在 Eclipse v.15.6 中使用质子卷积叠加（PCS）算法 v.15.6.06 生成具有代表性的临床点扫描质子计划，目标是将典型的舌底（BoT）治疗靶区（CTV）的 54Gy 剂量均匀输送。采用典型的两个前斜（AO）光束和一个后束的几何光束排列。计划优化没有任何材料覆盖，A）没有植入物；B）有 Am 修复体；或 C）有 PFM 冠。还重新优化并使用材料覆盖物进行了计划传输，以将夹具的相对阻止能力与之前测量的结果相匹配。

结果

计划显示 AO 光束的剂量权重略高。优化器通过增加最接近植入物的光束权重来考虑夹具覆盖物的包含，从而考虑到夹具覆盖物的包含。在包括和不包括覆盖材料的计划中，胶片测量结果均显示在穿过夹具的射束路径中存在冷点。在包括结构集内覆盖材料的计划中，冷点得到了一定程度的缓解，但并未完全消除。Am 和 PFM 夹具的冷点在没有覆盖物的计划中分别量化为 17%和 14%，使用蒙特卡罗模拟分别量化为 11%和 9%。与胶片测量和蒙特卡罗模拟相比，治疗计划系统在包括材料覆盖物的计划中低估了剂量阴影效应。