Department of Radiation Oncology, University of Oklahoma Health Sciences Center, 800 NE 10th street SCC L100, Oklahoma City, OK, 73104, USA.
Oklahoma Proton Center, 5901 W Memorial Rd, Oklahoma City, OK, 73142, USA.
Med Phys. 2021 Apr;48(4):1508-1519. doi: 10.1002/mp.14771. Epub 2021 Mar 4.
To quantitatively access penumbra sharpening and scattering by adaptive aperture (AA) under various beam conditions and clinical cases for a Mevion S250i compact pencil beam scanning proton therapy system.
First, in-air measurements were performed using a scintillation detector for single spot profile and lateral penumbra for five square field sizes (3 × 3 to 18 × 18 cm ), three energies (33.04, 147.36, and 227.16 MeV), and three snout positions (5, 15, and 33.6 cm) with Open and AA field. Second, treatment plans were generated in RayStation treatment planning system (TPS) for various combination of target size (3- and 10-cm cube), target depth (5, 10, and 15 cm) and air gap (5-20 cm) for both Open and AA field. These plans were delivered to EDR2 films in the solid water and penumbra reduction by AA was quantified. Third, the effect of the AA scattered protons on the surface dose was studied at 5 mm depth by EDR2 film and the RayStation TPS computation. Finally, dosimetric advantage of AA over Open field was studied for five brain and five prostate cases using the TPS simulation.
The spot size changed dramatically from 3.8 mm at proton beam energy of 227.15 MeV to 29.4 mm at energy 33.04 MeV. In-air measurements showed that AA substantially reduced the lateral penumbra by 30% to 60%. The EDR2 film measurements in solid water presented the maximum penumbra reduction of 10 to 14 mm depending on the target size. The maximum increase of 25% in field edge dose at 5 mm depth as compared to central axis was observed. The substantial penumbra reduction by AA produced less dose to critical structures for all the prostate and brain cases.
Adaptive aperture sharpens the penumbra by factor of two to three depending upon the beam condition. The absolute penumbra reduction with AA was more noticeable for shallower target, smaller target, and larger air gap. The AA-scattered protons contributed to increase in surface dose. Clinically, AA reduced the doses to critical structures.
为了在各种射束条件和临床情况下，对 Mevion S250i 紧凑型笔形束扫描质子治疗系统的自适应孔径（AA）进行半影锐化和散射的定量评估。
首先，使用闪烁探测器在空气中进行单点轮廓和横向半影的测量，用于五个方形射野尺寸（3×3 至 18×18cm）、三种能量（33.04、147.36 和 227.16MeV）和三种探头位置（5、15 和 33.6cm）的开放和 AA 射野。其次，在 RayStation 治疗计划系统（TPS）中生成了各种靶区大小（3 和 10cm 立方体）、靶区深度（5、10 和 15cm）和空气间隙（5-20cm）的治疗计划，用于开放和 AA 射野。这些计划被输送到固体水中的 EDR2 胶片上，并量化了 AA 对半影的减小。第三，通过 EDR2 胶片和 RayStation TPS 计算研究了 AA 散射质子对 5mm 深度表面剂量的影响。最后，通过 TPS 模拟研究了 AA 相对于开放射野在五个脑和五个前列腺病例中的剂量学优势。
质子束能量从 227.15MeV 的 29.4mm 到 33.04MeV 的 3.8mm 时，点尺寸变化很大。空气测量表明，AA 可使横向半影减少 30%至 60%。固体水 EDR2 胶片测量结果显示，靶区大小不同，半影最大减少 10 至 14mm。在 5mm 深度，与中心轴相比，场边缘剂量最大增加 25%。对于所有前列腺和脑病例，AA 显著减少了半影，从而减少了对关键结构的剂量。
自适应孔径可根据射束条件将半影锐化两到三倍。对于较浅的靶区、较小的靶区和较大的空气间隙，AA 的绝对半影减小更为明显。AA 散射质子会导致表面剂量增加。临床上，AA 可减少对关键结构的剂量。
