Department of Radiation Oncology, University of California, San Francisco, CA 94143-1708, USA.
Med Phys. 2011 Oct;38(10):5338-44. doi: 10.1118/1.3633946.
Currently, there are no successful long-term treatments or preventive strategies for radiation-induced cognitive impairments, and only a few possibilities have been suggested. One such approach involves reducing the dose to neural stem cell compartments (within and outside of the hippocampus) during whole-brain radiation treatments for brain metastases. This study investigates the fundamental physics issues associated with the sparing of neural stem cells during photon radiotherapy for brain metastases.
Several factors influence the stem cell dose: intracranial scattering, collimator leakage, beam energy, and total number of beams. The relative importance of these factors is investigated through a set of radiation therapy plans, which are all variations of an initial 6 MV intensity-modulated radiation therapy (IMRT) plan designed to simultaneously deliver a whole-brain dose of 30 Gy and maximally reduce stem cell compartment dose. Additionally, an in-house leaf segmentation algorithm was developed that utilizes jaw motion to minimize the collimator leakage.
The plans are all normalized such that 50% of the PTV receives 30 Gy. For the initial 6 MV IMRT plan, 50% of the stem cells receive a dose greater than 6.3 Gy. Calculations indicate that 3.6 Gy of this dose originates from intracranial scattering. The jaw-tracking segmentation algorithm, used in conjunction with direct machine parameter optimization, reduces the 50% stem cell dose to 4.3 and 3.7 Gy for 6 and 10 MV treatment beams, respectively.
Intracranial scattering alone is responsible for a large dose contribution to the stem cell compartment. It is, therefore, important to minimize other contributing factors, particularly the collimator leakage, to maximally reduce dose to these critical structures. The use of collimator jaw tracking in conjunction with modern collimators can minimize this leakage.
目前，对于放射性诱导的认知障碍，还没有成功的长期治疗或预防策略，只有少数几种可能性被提出。一种方法是在脑转移的全脑放射治疗中，减少神经干细胞隔室（海马内外）的剂量。本研究探讨了光子放射治疗脑转移时保留神经干细胞的基本物理问题。
有几个因素影响干细胞剂量：颅内散射、准直器漏泄、射束能量和射束总数。通过一组放射治疗计划研究了这些因素的相对重要性，这些计划都是初始 6 MV 调强放射治疗（IMRT）计划的变化，旨在同时给予全脑 30 Gy 剂量，并最大限度地降低干细胞隔室剂量。此外，还开发了一种内部叶片分割算法，该算法利用颌运动将准直器漏泄最小化。
这些计划都是归一化的，使 50%的 PTV 接受 30 Gy。对于初始的 6 MV IMRT 计划，50%的干细胞接受的剂量大于 6.3 Gy。计算表明，该剂量的 3.6 Gy 来自颅内散射。与直接机器参数优化结合使用的颌跟踪分割算法，将 6 MV 和 10 MV 治疗射束的 50%干细胞剂量分别降低到 4.3 和 3.7 Gy。
仅颅内散射就会导致干细胞隔室的大量剂量贡献。因此，重要的是要尽量减少其他因素的影响，特别是准直器漏泄，以最大限度地降低这些关键结构的剂量。准直器颌跟踪与现代准直器的结合可以最小化这种漏泄。
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