Nuclear and Radiological Engineering Program, The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
Med Phys. 2013 Aug;40(8):081708. doi: 10.1118/1.4812427.
Emission guided radiation therapy (EGRT) is a new modality that uses PET emissions in real-time for direct tumor tracking during radiation delivery. Radiation beamlets are delivered along positron emission tomography (PET) lines of response (LORs) by a fast rotating ring therapy unit consisting of a linear accelerator (Linac) and PET detectors. The feasibility of tumor tracking and a primitive modulation method to compensate for attenuation have been demonstrated using a 4D digital phantom in our prior work. However, the essential capability of achieving dose modulation as in conventional intensity modulated radiation therapy (IMRT) treatments remains absent. In this work, the authors develop a planning scheme for EGRT to accomplish sophisticated intensity modulation based on an IMRT plan while preserving tumor tracking.
The planning scheme utilizes a precomputed LOR response probability distribution to achieve desired IMRT planning modulation with effects of inhomogeneous attenuation and nonuniform background activity distribution accounted for. Evaluation studies are performed on a 4D digital patient with a simulated lung tumor and a clinical patient who has a moving breast cancer metastasis in the lung. The Linac dose delivery is simulated using a voxel-based Monte Carlo algorithm. The IMRT plan is optimized for a planning target volume (PTV) that encompasses the tumor motion using the MOSEK package and a Pinnacle3™ workstation (Philips Healthcare, Fitchburg, WI) for digital and clinical patients, respectively. To obtain the emission data for both patients, the Geant4 application for tomographic emission (GATE) package and a commercial PET scanner are used. As a comparison, 3D and helical IMRT treatments covering the same PTV based on the same IMRT plan are simulated.
3D and helical IMRT treatments show similar dose distribution. In the digital patient case, compared with the 3D IMRT treatment, EGRT achieves a 15.1% relative increase in dose to 95% of the gross tumor volume (GTV) and a 31.8% increase to 50% of the GTV. In the patient case, EGRT yields a 15.2% relative increase in dose to 95% of the GTV and a 20.7% increase to 50% of the GTV. The organs at risk (OARs) doses are kept similar or lower for EGRT in both cases. Tumor tracking is observed in the presence of planning modulation in all EGRT treatments.
As compared to conventional IMRT treatments, the proposed EGRT planning scheme allows an escalated target dose while keeping dose to the OARs within the same planning limits. With the capabilities of incorporating planning modulation and accurate tumor tracking, EGRT has the potential to greatly improve targeting in radiation therapy and enable a practical and effective implementation of 4D radiation therapy for planning and delivery.
发射制导放射治疗（EGRT）是一种新的模式，它在放射治疗过程中使用 PET 发射实时直接跟踪肿瘤。射线束通过由线性加速器（Linac）和 PET 探测器组成的快速旋转环治疗单元沿着正电子发射断层扫描（PET）线的响应（LOR）输送。在我们之前的工作中，已经使用 4D 数字体模证明了肿瘤跟踪和原始调制方法补偿衰减的可行性。然而，实现与传统强度调制放射治疗（IMRT）治疗相同的剂量调制的基本能力仍然缺失。在这项工作中，作者开发了一种 EGRT 计划方案，以在保留肿瘤跟踪的同时，基于 IMRT 计划实现复杂的强度调制。
该计划方案利用预先计算的 LOR 响应概率分布，在考虑不均匀衰减和非均匀背景活动分布的情况下，实现所需的 IMRT 计划调制。在一个模拟肺肿瘤的 4D 数字患者和一个肺部有移动乳腺癌转移的临床患者上进行评估研究。使用基于体素的蒙特卡罗算法模拟 Linac 剂量输送。使用 MOSEK 软件包和 Pinnacle3™工作站（Philips Healthcare，Fitchburg，WI）分别为包含肿瘤运动的计划靶区（PTV）优化 IMRT 计划，用于数字和临床患者。为了获得这两个患者的发射数据，使用 Geant4 应用程序进行断层发射（GATE）包和商业 PET 扫描仪。作为比较，模拟了基于相同 IMRT 计划的覆盖相同 PTV 的 3D 和螺旋 IMRT 治疗。
3D 和螺旋 IMRT 治疗显示出相似的剂量分布。在数字患者病例中，与 3D IMRT 治疗相比，EGRT 使 95%的大体肿瘤体积（GTV）的剂量增加了 15.1%，使 50%的 GTV 的剂量增加了 31.8%。在患者病例中，EGRT 使 95%的 GTV 的剂量增加了 15.2%，使 50%的 GTV 的剂量增加了 20.7%。在这两种情况下，EGRT 使危及器官（OARs）的剂量保持相似或更低。在所有 EGRT 治疗中，都观察到了存在计划调制时的肿瘤跟踪。
与传统的 IMRT 治疗相比，所提出的 EGRT 计划方案允许在保持相同计划限制下，提高靶区剂量。通过结合计划调制和精确的肿瘤跟踪能力，EGRT 有可能大大提高放射治疗的靶向性，并为 4D 放射治疗的计划和输送实现实用有效的方法。
