Nuclear Metrology, Ecole Polytechnique, Universite Libre de Bryxellers, Belgium.
Radiotherapy Department, Institute Jules Bordet, Belgium.
Phys Med. 2018 Jul;51:1-6. doi: 10.1016/j.ejmp.2018.05.023. Epub 2018 Jun 15.
In IOERT breast treatments, a shielding disk is frequently used to protect the underlying healthy structures. The disk is usually composed of two materials, a low-Z material intended to be oriented towards the beam and a high-Z material. As tissues are repositioned around the shield before treatment, the disk is no longer visible and its correct alignment with respect to the beam is guaranteed. This paper studies the dosimetric characteristics of four possible clinical positioning scenarios of the shielding disk. A new alignment method for the shielding disk in the beam is introduced. Finally, it suggests a new design for the shielding disk.
As the first step, the IOERT machine "Mobetron 1000" was modeled by using Monte Carlo simulation, tuning the MC model until an excellent match with the measured PDDs and profiles was achieved. Four possible shielding disk positioning scenarios were considered, determining the dosimetric impact. Furthermore, in our center, to prevent beam misalignment, we have developed a shielding disk equipped with guiding rods. Having ascertained a correct alignment between the disk and the beam, we can propose a new internal design of the shielding disk that can improve the dose distribution with a better coverage of the treated area.
All MC simulations were performed with a 12 MeV beam, the maximum energy of Mobetron 1000 and a 5.5 cm diameter flat tip applicator, this applicator being the most clinically used. The simulations were compared with measurements performed in a water phantom and showed good results within 2.2% of root mean square difference (RMSD). The misplacement positions of the shielding disk have dosimetric impacts in the treatment volume and a small translation could have a significant influence on healthy tissues. The D-scenario is the worst which could happens when the shielding disk is flipped upside down, giving up to 144% dose instead of 90% at the surface of the Pb/Al shielding disk. A new shielding design used, together with our alignment tool, is able to give a more homogeneous dose in the target area.
The accuracy of shielding disk position can still be problematic in IOERT dosimetry. Any method that can ascertain the good alignment between the shielding disk and the beam is beneficial for the dose distribution and is a prerequisite for an optimized shield internal design that could improve the coverage of the treated area and the protection of healthy tissues.
在 IOERT 乳房治疗中，经常使用屏蔽盘来保护下方的健康组织。该圆盘通常由两种材料组成，一种是低 Z 材料，旨在面向射束，另一种是高 Z 材料。由于在治疗前将组织重新定位在屏蔽盘周围，因此圆盘不再可见，并且其相对于射束的对准得到了保证。本文研究了屏蔽盘的四种可能临床定位场景的剂量学特征。引入了一种新的屏蔽盘在射束中的对准方法。最后，提出了一种新的屏蔽盘设计。
作为第一步，使用蒙特卡罗模拟对 IOERT 机器“Mobetron 1000”进行建模，调整 MC 模型，直到与测量的 PDD 和剖面达到极好的匹配。考虑了四种可能的屏蔽盘定位场景，确定了剂量学影响。此外，在我们的中心，为了防止光束失准，我们开发了一种带有导向杆的屏蔽盘。确保了磁盘和光束之间的正确对准之后，我们可以提出一种新的屏蔽盘内部设计，通过更好地覆盖治疗区域来改善剂量分布。
所有 MC 模拟均使用 12 MeV 束、Mobetron 1000 的最大能量和 5.5 cm 直径的平底尖端施源器进行，该施源器是最常用的临床施源器。模拟结果与在水模体中进行的测量结果进行了比较，在均方根差（RMSD）的 2.2%以内显示出良好的结果。屏蔽盘的错位位置会对治疗体积中的剂量产生影响，即使很小的平移也会对健康组织产生重大影响。D 场景是最坏的情况，当屏蔽盘上下颠倒时可能会发生这种情况，导致 Pb/Al 屏蔽盘表面的剂量高达 144%，而不是 90%。一起使用的新屏蔽设计和我们的对准工具能够在靶区提供更均匀的剂量。
在 IOERT 剂量学中，屏蔽盘位置的准确性仍然可能存在问题。任何能够确定屏蔽盘和射束之间良好对准的方法都有利于剂量分布，并且是优化屏蔽内部设计的前提，这可以改善治疗区域的覆盖范围和对健康组织的保护。
