Radiotherapy Clinics of Georgia, Decatur, Georgia, USA.
Radiation oncology department, Texas Oncology, Houston, Texas, USA.
J Appl Clin Med Phys. 2024 Oct;25(10):e14470. doi: 10.1002/acm2.14470. Epub 2024 Jul 23.
The aim of this study is to find optimal gantry, collimator, and couch angles for performing single isocenter, multiple target stereotactic radiosurgery (SIMT-SRS). Nineteen angle sets were tested across seven linear accelerators for radiation-isocenter coincidence and off-isocenter coincidence. The off-isocenter Winston-Lutz test was performed to evaluate the accuracy of isocenter alignment for each angle set, and optimal angle sets as well as maximum off-isocenter distance to target for each angle set was determined. The influence of simulated patient weight on off-iso Winston-Lutz test accuracy was also inspected.
The SNC MultiMet-WL phantom and MultiMet-WL QA Software v2.1 were used for the direct measurement and analysis of the off-iso Winston-Lutz test (also referred to as Winston-Lutz-Gao test). A two-step method was developed to ensure precise initial placement of the target. Nineteen beams were delivered at 6X energy and 2 × 2 cm field size to each of six targets on the MultiMet Cube with couch kicks at five cardinal angles (90°, 45°, 0°, 315°, and 270°). To reduce imaging uncertainty, only EPID was used in target alignment and test image acquisition. A total of 200 Ibs (90.7 kg) of weight was also used to mimic patient weight. All tests were performed on both the free table and the weighted table.
For two new TrueBeam machines, the maximum offset was within the 1 mm tolerance when the off-iso distance is less than 7 cm. Two older VitalBeam machines exhibited unfavorable gantry, couch, and collimator (GCC) angle sets: Linac No. 3 at (0,90,0), (0,270,0) and Linac No. 4 at (0,45,45) and (0,90,0). The C-Series Linacs failed in the majority of GCC angle sets, with Linac No. 5 exhibiting a maximum offset of 1.53 mm. Four of seven machines show a clear trend that offset increases with off-isocenter distance. Additionally, the IGRT table was less susceptible to the addition of simulated patient weight than the ExactCouch.
Among the seven linear accelerators addressed, newer model machines such as the Varian TrueBeam were more precise than older models, especially in comparison to the C-Series Linacs. The newer machines are more suitable for delivering SIMT-SRS procedures in all GCC angle sets, and the results indicate that newer TrueBeams are capable of performing SIMT-SRS procedures at all angle sets for targets of off-iso distances up to 7 cm. The trend that offset between the target center and radiation field center increases with off-iso distance, however, does not always hold true across machines. This may be comprised by the EPID's severe off-axis horn effect. Lastly, the IGRT couch was less susceptible to patient weight compared to ExactCouch in the off-isocenter Winston-Lutz test.
本研究旨在寻找执行单等中心、多靶立体定向放射外科 (SIMT-SRS) 的最佳旋转机架、准直器和治疗床角度。在 7 台直线加速器上测试了 19 组角度，以实现射束中心与等中心的重合和离等中心的重合。进行离等中心 Winston-Lutz 测试以评估每组角度的等中心对准精度，并确定最佳角度组以及每组角度的最大离靶目标距离。还检查了模拟患者体重对离等中心 Winston-Lutz 测试准确性的影响。
使用 SNC MultiMet-WL 体模和 MultiMet-WL QA 软件 v2.1 进行离等中心 Winston-Lutz 测试（也称为 Winston-Lutz-Gao 测试）的直接测量和分析。开发了两步法以确保目标的精确定位初始放置。在 MultiMet Cube 上的六个目标上，以 6X 能量和 2×2cm 射野大小，以五个方位角（90°、45°、0°、315°和 270°）进行 19 束照射。为了降低成像不确定性，仅在目标对准和测试图像采集时使用 EPID。还使用了总共 200 磅（90.7 公斤）的重量来模拟患者体重。所有测试均在自由床和加重床上进行。
对于两台新的 TrueBeam 机器，当离等中心距离小于 7cm 时，最大偏移量在 1mm 公差内。两台较旧的 VitalBeam 机器显示出不利的旋转机架、治疗床和准直器（GCC）角度组：Linac No.3 为 (0,90,0)、(0,270,0) 和 Linac No.4 为 (0,45,45) 和 (0,90,0)。C 系列直线加速器在大多数 GCC 角度组中失败，Linac No.5 显示最大偏移量为 1.53mm。七台机器中有四台显示出明显的趋势，即随着离等中心距离的增加，偏移量也会增加。此外，IGRT 治疗床比 ExactCouch 更不容易受到模拟患者体重的影响。
在所涉及的七台直线加速器中，新型机器（如瓦里安 TrueBeam）比旧型机器更精确，尤其是与 C 系列直线加速器相比。新型机器更适合在所有 GCC 角度组中进行 SIMT-SRS 手术，结果表明新型 TrueBeams 能够在所有角度组中进行 SIMT-SRS 手术，对于离等中心距离高达 7cm 的目标。然而，目标中心和射束中心之间的偏移量随离等中心距离增加的趋势并不总是适用于所有机器。这可能是由于 EPID 的严重的轴外 horn 效应造成的。最后，与 ExactCouch 相比，IGRT 治疗床在离等中心 Winston-Lutz 测试中对患者体重的影响较小。
