Frensch Carla, Bäcker Claus Maximilian, Jentzen Walter, Lüvelsmeyer Ann-Kristin, Teimoorisichani Mohammadreza, Wulff Jörg, Timmermann Beate, Bäumer Christian
West German Proton Therapy Centre Essen, Essen, Germany.
West German Cancer Center (WTZ), University Hospital Essen, Essen, Germany.
Med Phys. 2025 Feb;52(2):1293-1304. doi: 10.1002/mp.17530. Epub 2024 Nov 28.
Treatment planning in radiation therapy (RT) is performed on image sets acquired with commercial x-ray computed tomography (CT) scanners. Considering an increased frequency of verification scans for adaptive RT and the advent of alternatives to x-ray CTs, there is a need to review the requirements for image sets used in RT planning.
This study aims to derive the required image quality (IQ) for the computation of the dose distribution in proton therapy (PT) regarding spatial resolution and the combination of spatial resolution and noise. The knowledge gained is used to explore the potential for dose reduction in tomography-guided PT.
Mathematical considerations indicate that the required spatial resolution for dose computation is on the scale of the set-up margins fed into the robust optimization. This hypothesis was tested by processing retrospectively 12 clinical PT cases, which reflect a variety of tumor localizations. Image sets were low-pass filtered and were made noisy in a generic manner. Dose distributions on the modified CT scans were computed with a Monte-Carlo dose engine. The similarity of these dose distributions with clinical ones was quantified with the gamma-index (1 mm/1%). The potential reduction of the x-ray exposure compared to the planning CT scan was estimated.
Dose distributions within the irradiated volume were robust against low-pass filtering of the CTs with kernels up to a full-width-at-half-maximum of 4 mm, that is, the gamma pass rate (1 mm/1%) was 98%. The limit of the filter width was 6 mm for brain tumors and 8 mm for targets in the abdomen. These pass rates remained approximately unchanged if a limited amount of noise was added to the CT image sets. The estimated potential reductions of the x-ray exposure were at least a factor of 20.
The requirements on IQ in terms of spatial resolution in combination with noise for computing the dose in PT are clearly lower than the IQ of current clinical planning. The results apply, for example, to ultra-low dose x-ray CTs, proton CTs with coarse spatial detection, and attenuation images from the joint reconstruction of time-of-flight PET scans.
放射治疗（RT）中的治疗计划是基于使用商用X射线计算机断层扫描（CT）扫描仪获取的图像集来进行的。鉴于自适应RT的验证扫描频率增加以及X射线CT替代技术的出现，有必要重新审视RT计划中所用图像集的要求。
本研究旨在得出质子治疗（PT）中剂量分布计算所需的图像质量（IQ），涉及空间分辨率以及空间分辨率与噪声的组合。所获得的知识用于探索断层扫描引导的PT中剂量降低的潜力。
数学考量表明，剂量计算所需的空间分辨率处于输入稳健优化的设置边界尺度上。通过回顾性处理12例临床PT病例对这一假设进行了检验，这些病例反映了多种肿瘤定位情况。对图像集进行低通滤波并以一般方式添加噪声。使用蒙特卡罗剂量引擎计算经修改的CT扫描上的剂量分布。用伽马指数（1毫米/1%）对这些剂量分布与临床剂量分布的相似性进行量化。估计了与计划CT扫描相比X射线曝光的潜在减少量。
对于半高宽达4毫米的内核的CT低通滤波，照射体积内的剂量分布具有稳健性即伽马通过率（1毫米/1%）为98%。对于脑肿瘤，滤波宽度的极限为6毫米，对于腹部靶区为8毫米。如果向CT图像集添加有限量的噪声，这些通过率大致保持不变。估计的X射线曝光潜在减少量至少为20倍。
在PT中计算剂量时，在空间分辨率与噪声方面对IQ的要求明显低于当前临床计划的IQ。例如，这些结果适用于超低剂量X射线CT、具有粗略空间检测的质子CT以及飞行时间PET扫描联合重建的衰减图像。
