Department of Medical Physics, Fakultät für Physik, Ludwig-Maximilians-Universität München (LMU Munich), D-85748 Garching bei München, Germany.
Department of Radiation Oncology, University Hospital, LMU Munich, D-81377 Munich, Germany.
Phys Med Biol. 2021 Mar 2;66(6):064001. doi: 10.1088/1361-6560/abe3d2.
Particle therapy treatment planning requires accurate volumetric maps of the relative stopping power, which can directly be acquired using proton computed tomography (pCT). With fluence-modulated pCT (FMpCT) imaging fluence is concentrated in a region-of-interest (ROI), which can be the vicinity of the treatment beam path, and imaging dose is reduced elsewhere. In this work we present a novel optimization algorithm for FMpCT which, for the first time, calculates modulated imaging fluences for joint imaging dose and image variance objectives. Thereby, image quality is maintained in the ROI to ensure accurate calculations of the treatment dose, and imaging dose is minimized outside the ROI with stronger minimization penalties given to imaging organs-at-risk. The optimization requires an initial scan at uniform fluence or a previous x-ray CT scan. We simulated and optimized FMpCT images for three pediatric patients with tumors in the head region. We verified that the target image variance inside the ROI was achieved and demonstrated imaging dose reductions outside of the ROI of 74% on average, reducing the imaging dose from 1.2 to 0.3 mGy. Such dose savings are expected to be relevant compared to the therapeutic dose outside of the treatment field. Treatment doses were re-calculated on the FMpCT images and compared to treatment doses re-recalculated on uniform fluence pCT scans using a 1% criterion. Passing rates were above 98.3% for all patients. Passing rates comparing FMpCT treatment doses to the ground truth treatment dose were above 88.5% for all patients. Evaluation of the proton range with a 1 mm criterion resulted in passing rates above 97.5% (FMpCT/pCT) and 95.3% (FMpCT/ground truth). Jointly optimized fluence-modulated pCT images can be used for proton dose calculation maintaining the full dosimetric accuracy of pCT but reducing the required imaging dose considerably by three quarters. This may allow for daily imaging during particle therapy ensuring a safe and accurate delivery of the therapeutic dose and avoiding excess dose from imaging.
粒子治疗计划需要准确的相对阻止本领体素图，这可以直接通过质子计算机断层扫描（pCT）获得。使用调强质子 CT（FMpCT）成像，射线束在感兴趣区域（ROI）中聚焦，该 ROI 可以是治疗束路径附近，而在其他区域减少成像剂量。在这项工作中，我们提出了一种用于 FMpCT 的新的优化算法，该算法首次为联合成像剂量和图像方差目标计算调制成像射线束强度。通过这种方式，在 ROI 中保持图像质量以确保治疗剂量的准确计算，并且在 ROI 之外通过对成像风险器官更强的最小化惩罚来最小化成像剂量。该优化需要在均匀射线束强度下进行初始扫描或进行先前的 X 射线 CT 扫描。我们对三个患有头部肿瘤的儿科患者进行了 FMpCT 图像的模拟和优化。我们验证了在 ROI 内达到了目标图像方差，并证明了 ROI 外的成像剂量平均降低了 74%，将成像剂量从 1.2 降低到 0.3 mGy。与治疗场之外的治疗剂量相比，这种剂量节省预计是相关的。我们在 FMpCT 图像上重新计算了治疗剂量，并使用 1%的标准将其与在均匀射线束强度 pCT 扫描上重新计算的治疗剂量进行了比较。所有患者的通过率均高于 98.3%。所有患者的 FMpCT 治疗剂量与实际治疗剂量的通过率均高于 88.5%。使用 1mm 标准评估质子射程，通过率高于 97.5%（FMpCT/pCT）和 95.3%（FMpCT/实际）。联合优化的调强质子 CT 图像可用于质子剂量计算，在保持 pCT 完全剂量学准确性的同时，将所需的成像剂量减少四分之三。这可能允许在粒子治疗期间进行日常成像，从而确保治疗剂量的安全准确传递，并避免成像带来的过量剂量。
