Odense University Hospital, Department of Oncology, Odense, Denmark; University of Southern Denmark, Department of Clinical Research, Odense, Denmark.
Odense University Hospital, Department of Oncology, Odense, Denmark; University of Southern Denmark, Department of Clinical Research, Odense, Denmark.
Z Med Phys. 2023 Nov;33(4):567-577. doi: 10.1016/j.zemedi.2023.02.002. Epub 2023 Mar 27.
The precision of the dose delivery in radiation therapy with high-field MR-linacs is challenging due to the substantial variation in the beam attenuation of the patient positioning system (PPS) (the couch and coils) as a function of the gantry angle. This work aimed to compare the attenuation of two PPSs located at two different MR-linac sites through measurements and calculations in the treatment planning system (TPS).
Attenuation measurements were performed at every 1° gantry angle at the two sites with a cylindrical water phantom with a Farmer chamber inserted along the rotational axis of the phantom. The phantom was positioned with the chamber reference point (CRP) at the MR-linac isocentre. A compensation strategy was applied to minimise sinusoidal measurement errors due to, e.g. air cavity or setup. A series of tests were performed to assess the sensitivity to measurement uncertainties. The dose to a model of the cylindrical water phantom with the PPS added was calculated in the TPS (Monaco v5.4 as well as in a development version Dev of an upcoming release), for the same gantry angles as for the measurements. The TPS PPS model dependency of the dose calculation voxelisation resolution was also investigated.
A comparison of the measured attenuation of the two PPSs yielded differences of less than 0.5% for most gantry angles. The maximum deviation between the attenuation measurements for the two different PPSs exceeded ±1% at two specific gantry angles 115° and 245°, where the beam traverses the most complex PPS structures. The attenuation increases from 0% to 25% in 15° intervals around these angles. The measured and calculated attenuation, as calculated in v5.4, was generally within 1-2% with a systematic overestimation of the attenuation for gantry angles around 180°, as well as a maximum error of 4-5% for a few discrete angles in 10° gantry angle intervals around the complex PPS structures. The PPS modelling was improved compared to v5.4 in Dev, especially around 180°, and the results of those calculations were within ±1%, but with a similar 4% maximum deviation for the most complex PPS structures.
Generally, the two tested PPS structures exhibit very similar attenuation as a function of the gantry angle, including the angles with a steep change in attenuation. Both TPS versions, v5.4 and Dev delivered clinically acceptable accuracy of the calculated dose, as the differences in the measurements were overall better than ±2%. Additionally, Dev improved the accuracy of the dose calculation to ±1% for gantry angles around 180°.
由于患者定位系统（治疗床和线圈）的束流衰减随机架角度的变化很大，因此，在高磁场磁共振直线加速器中进行放射治疗时，精确控制剂量输送具有挑战性。本研究旨在通过治疗计划系统（TPS）中的测量和计算，比较两个位于不同磁共振直线加速器位置的患者定位系统的衰减。
在两个位置，使用圆柱形水模体和插入旋转轴的圆柱形水模体中的 Farmer 室，在每个机架角度进行衰减测量。水模体的中心位于磁共振直线加速器的等中心。应用补偿策略以最小化由于空气间隙或设置等原因引起的正弦测量误差。进行了一系列测试以评估对测量不确定性的敏感性。在 TPS（Monaco v5.4 以及即将发布的新版本的开发版本 Dev）中计算了添加患者定位系统的圆柱形水模体的模型的剂量，与测量的机架角度相同。还研究了 TPS 患者定位系统模型依赖性对剂量计算体素化分辨率的影响。
两个患者定位系统的测量衰减值之间的比较，大多数机架角度下的差异小于 0.5%。在两个特定的机架角度 115°和 245°处，两个不同的患者定位系统的衰减测量值之间的最大偏差超过±1%，此时光束穿过最复杂的患者定位系统结构。在这些角度周围的 15°间隔内，衰减从 0%增加到 25%。在 v5.4 中计算的测量和计算衰减值通常在 1%到 2%之间，对于机架角度在 180°左右，存在系统高估衰减的情况，对于复杂患者定位系统结构周围的 10°机架角度间隔中的几个离散角度，存在最大误差为 4%到 5%的情况。与 v5.4 相比，Dev 中的患者定位系统建模得到了改善，尤其是在 180°左右，并且这些计算的结果在±1%以内，但对于最复杂的患者定位系统结构，最大偏差仍为 4%。
一般来说，两种测试的患者定位系统结构在机架角度的束流衰减方面表现出非常相似的特性，包括衰减急剧变化的角度。TPS 版本 v5.4 和 Dev 都提供了可接受的计算剂量精度，因为总体上测量值之间的差异优于±2%。此外，Dev 提高了 180°左右机架角度的剂量计算精度，达到±1%。
