Physics department, University of Trento, via Sommarive 14, Povo, TN, Italy.
Trento Institute for Fundamental Physics and Applications (TIFPA), Italian National Institute of Nuclear Physics (INFN), via Sommarive, 14, Povo, TN, Italy.
Phys Med Biol. 2024 Jun 26;69(13). doi: 10.1088/1361-6560/ad56f5.
To present and characterize a novel method for x-ray computed tomography (xCT) calibration in proton treatment planning, based on proton CT (pCT) measurements on biological phantoms.A pCT apparatus was used to perform direct measurements of 3D stopping power relative to water (SPR) maps on stabilized, biological phantoms. Two single-energy xCT calibration curves-i.e. tissue substitutes and stoichiometric-were compared to pCT data. Moreover, a new calibration method based on these data was proposed, and verified against intra- and inter-species variability, dependence on stabilization, beam-hardening conditions, and analysis procedures.Biological phantoms were verified to be stable in time, with a dependence on temperature conditions, especially in the fat region: (-2.5 0.5) HU °C. The pCT measurements were compared with standard xCT calibrations, revealing an average SPR discrepancy within ±1.60% for both fat and muscle regions. In the bone region the xCT calibrations overestimated the pCT-measured SPR of the phantom, with a maximum discrepancy of about +3%. As a result, a new cross-calibration curve was directly extracted from the pCT data. Overall, the SPR uncertainty margin associated with this curve was below 3%; fluctuations in the uncertainty values were observed across the HU range. Cross-calibration curves obtained with phantoms made of different animal species and anatomical parts were reproducible with SPR discrepancies within 3%. Moreover, the stabilization procedure did not affect the resulting curve within a 2.2% SPR deviation. Finally, the cross-calibration curve was affected by the beam-hardening conditions on xCTs, especially in the bone region, while dependencies below 2% resulted from the image registration procedure.Our results showed that pCT measurements on biological phantoms may provide an accurate method for the verification of current xCT calibrations and may represent a tool for the implementation of a new calibration method for proton treatment planning.
为了在质子治疗计划中提出并描述一种基于质子 CT（pCT）在稳定生物体模上的直接测量来实现 X 射线计算机断层扫描（XCT）校准的新方法，我们使用 pCT 设备对 3D 相对于水的阻止本领（SPR）图谱进行了直接测量。我们比较了两种单能量 XCT 校准曲线——组织替代物和化学计量法——与 pCT 数据。此外，还提出了一种基于这些数据的新校准方法，并验证了其对种内和种间变异性、对稳定性的依赖性、束硬化条件和分析程序的依赖性。生物体模被证明在时间上是稳定的，其对温度条件具有依赖性，尤其是在脂肪区域：（-2.5 0.5）HU°C。pCT 测量与标准 XCT 校准进行了比较，结果显示脂肪和肌肉区域的 SPR 平均差异在±1.60%以内。在骨区域，XCT 校准高估了体模中 pCT 测量的 SPR，最大差异约为+3%。因此，我们直接从 pCT 数据中提取了新的交叉校准曲线。总体而言，该曲线相关的 SPR 不确定度在 3%以下；在 HU 范围内观察到不确定性值的波动。使用不同动物物种和解剖部位制成的体模获得的交叉校准曲线具有在 3%SPR 差异以内的可重复性。此外，稳定化过程在 2.2%SPR 偏差内不会影响最终曲线。最后，交叉校准曲线受到 XCT 上的束硬化条件的影响，特别是在骨区域，而来自图像配准过程的依赖度低于 2%。我们的结果表明，生物体模上的 pCT 测量可能为验证当前 XCT 校准提供一种准确的方法，并可能成为质子治疗计划中新校准方法的实现工具。
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