Centre for Proton Therapy, Paul Scherrer Institute, 5232 Villigen, Switzerland.
Phys Med Biol. 2018 Jan 11;63(2):025022. doi: 10.1088/1361-6560/aaa2ad.
The lateral fall-off is crucial for sparing organs at risk in proton therapy. It is therefore of high importance to minimize the penumbra for pencil beam scanning (PBS). Three optimisation approaches are investigated: edge-collimated uniformly weighted spots (collimation), pencil beam optimisation of uncollimated pencil beams (edge-enhancement) and the optimisation of edge collimated pencil beams (collimated edge-enhancement). To deliver energies below 70 MeV, these strategies are evaluated in combination with the following pre-absorber methods: field specific fixed thickness pre-absorption (fixed), range specific, fixed thickness pre-absorption (automatic) and range specific, variable thickness pre-absorption (variable). All techniques are evaluated by Monte Carlo simulated square fields in a water tank. For a typical air gap of 10 cm, without pre-absorber collimation reduces the penumbra only for water equivalent ranges between 4-11 cm by up to 2.2 mm. The sharpest lateral fall-off is achieved through collimated edge-enhancement, which lowers the penumbra down to 2.8 mm. When using a pre-absorber, the sharpest fall-offs are obtained when combining collimated edge-enhancement with a variable pre-absorber. For edge-enhancement and large air gaps, it is crucial to minimize the amount of material in the beam. For small air gaps however, the superior phase space of higher energetic beams can be employed when more material is used. In conclusion, collimated edge-enhancement combined with the variable pre-absorber is the recommended setting to minimize the lateral penumbra for PBS. Without collimator, it would be favourable to use a variable pre-absorber for large air gaps and an automatic pre-absorber for small air gaps.
外侧剂量跌落对于质子治疗中保护危险器官至关重要。因此，尽量减小铅笔束扫描（PBS）的半影非常重要。研究了三种优化方法：边缘准直均匀加权光斑（准直）、非准直铅笔束的铅笔束优化（边缘增强）和边缘准直铅笔束的优化（准直边缘增强）。为了提供低于 70 MeV 的能量，这些策略与以下预吸收器方法结合进行评估：特定射野固定厚度预吸收（固定）、射程特定固定厚度预吸收（自动）和射程特定可变厚度预吸收（可变）。所有技术都在水箱中的蒙特卡罗模拟方野中进行评估。对于典型的 10 cm 气隙，如果没有预吸收器准直，仅在水当量范围为 4-11 cm 时，半影最多可减小 2.2 mm。通过准直边缘增强可实现最陡峭的横向下降，将半影降低至 2.8 mm。使用预吸收器时，当将准直边缘增强与可变预吸收器结合使用时，可获得最陡峭的下降。对于边缘增强和大的气隙，重要的是要将光束中的材料量最小化。然而，对于小的气隙，当使用更多的材料时，可以利用更高能量束的优越相空间。总之，为了最小化 PBS 的横向半影，推荐使用准直边缘增强并结合可变预吸收器。如果没有准直器，对于大的气隙，使用可变预吸收器是有利的，而对于小的气隙，则使用自动预吸收器。
