Chang Jennifer S, Perez-Andujar Angelica, Barani Igor J, Ma Lijun, Larson David A
Department of Radiation Oncology, University of California, San Francisco, United States.
Department of Radiation Oncology, University of California, San Francisco, United States.
Radiother Oncol. 2016 Aug;120(2):248-52. doi: 10.1016/j.radonc.2016.05.030. Epub 2016 Jul 9.
PURPOSE/OBJECTIVES: Whole-brain radiation for brain metastases can result in cognitive side effects. Hippocampal-sparing techniques have been developed to decrease morbidity, but they carry the risk of underdosing lesions near the hippocampus due to the unavoidable dose gradient from the hippocampal surface to the prescription isodose surface. This study examines the impact of variable levels of hippocampal sparing on the underdosing of potential brain metastases.
MATERIALS/METHODS: Helical intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans were developed for hippocampal-sparing whole-brain treatment. For all plans, 30Gy was prescribed in 10 fractions to result in mean hippocampal doses of 6-12Gy. From a series of expanded shells, we determined the distance from the hippocampus at which the parenchyma would receive less than specified doses. Then, using published data, a mathematical model was constructed to predict the incident probability of potential brain metastases receiving different doses for different levels of hippocampal sparing.
Whole-brain radiation plans were able to spare the hippocampi to mean doses of 7-12Gy under our planning constraints; more stringent constraints compromised brain coverage. The dose gradients were ∼4% per mm, regardless of the hippocampal constraint, and they decreased sharply by a factor of almost 4 at approximately 15mm from the hippocampi. A mathematical model was constructed and combined the plan information with published data on the distribution of brain metastases, to determine the percentage of potential brain metastases receiving specified doses, as a function of technique and level of hippocampal sparing.
Our results describe the characteristics of an array of hippocampal-sparing whole-brain radiation dose distributions. These can be used as a decision-making guideline for weighing the benefit of decreased dose to the hippocampi against the cost of decreased dose to potential brain metastases when deciding on a hippocampal-sparing whole-brain irradiation treatment approach.
目的/目标：全脑放疗治疗脑转移瘤可能会导致认知方面的副作用。已开发出海马保护技术以降低发病率，但由于从海马表面到处方等剂量面不可避免的剂量梯度，这些技术存在海马附近病变剂量不足的风险。本研究考察了不同程度的海马保护对潜在脑转移瘤剂量不足的影响。
材料/方法：制定了用于海马保护全脑治疗的螺旋调强放射治疗（IMRT）和容积调强弧形治疗（VMAT）计划。对于所有计划，10次分割给予30Gy的处方剂量，以使海马平均剂量为6 - 12Gy。从一系列扩展的壳层中，我们确定了实质组织接受低于特定剂量时距海马的距离。然后，利用已发表的数据构建了一个数学模型，以预测不同程度海马保护下潜在脑转移瘤接受不同剂量的入射概率。
在我们的计划限制下，全脑放疗计划能够将海马保护至平均剂量7 - 12Gy；更严格的限制会损害脑区覆盖。无论海马限制如何，剂量梯度约为每毫米4%，并且在距海马约15mm处急剧下降近4倍。构建了一个数学模型，并将计划信息与已发表的脑转移瘤分布数据相结合，以确定作为技术和海马保护程度函数的接受特定剂量的潜在脑转移瘤的百分比。
我们的结果描述了一系列海马保护全脑放射剂量分布的特征。这些可作为决策指南，在决定海马保护全脑照射治疗方法时，权衡降低海马剂量的益处与降低潜在脑转移瘤剂量的代价。
