SU-E-T-446：剂量计算网格大小变异性对脊柱立体定向体部放射治疗中脊髓剂量耐受性设定的影响。

SU-E-T-446: Effect of Dose Calculation Grid Size Variability on the Specification of Spinal Cord Dose Tolerance for Spinal Stereotactic Body Radiation Therapy.

作者信息

Gros S, Descovich M, Barani I, Ma L

机构信息

University of California San Francisco, San Francisco, CA.

出版信息

Med Phys. 2012 Jun;39(6Part17):3807. doi: 10.1118/1.4735535.

DOI:10.1118/1.4735535

PMID:28517484

Abstract

PURPOSE

To investigate the effect of the dose grid resolution variability on the spinal cord dose for spinal SBRT treatments.

METHOD

10 CyberKnife plans were selected for the proximity of the PTV to the spinal cord. All dose distributions were calculated with Monte Carlo using high spatial resolution and minimal relative uncertainty. The plans were renormalized to a 16 Gy prescription dose and to ensure a target coverage > 95% in order to compare the calculated dose distributions. Each dose matrix was resampled 12 times, covering a grid resolution range of 0.95 mm to 13.52 mm. The spinal cord DVHs were generated for each resampled dose grid. The variations of the maximum point dose (DmaxCord) and dose-coverage to partial volumes (D[V]) up to 5 cc were investigated against the grid resolution.

RESULTS

The mean variation of DmaxCord with grid resolution is characterized by an inverse power law, with a sharp initial decrease leading to potentially large underestimates of DmaxCord (24%, 40% and 55% at resolutions of 2 mm, 4 mm and 8 mm). The variability of mean D[V] values decreases from smaller to larger grid resolutions, however large disparities are observed between patient plans. We introduced the variability threshold volume (Vth) as a constraint to express the dose coverage independently from the grid resolution. For resolutions up to 8 mm, the mean Vth value is (0.96±0.10) cc with a corresponding dose coverage of (26± 12) % relative to the initial DmaxCord value.

CONCLUSIONS

Dose distributions calculated with grid resolutions larger than 2 mm could Result in significant underestimates of DmaxCord. Furthermore, the sensitivity of the dose coverage to grid resolution variability is patient dependent. Consequently, a specified cord dose tolerance should be quoted at a particular grid resolution uniformly adopted between institutions; 2 mm or less is an appropriate value.

摘要

目的

研究剂量网格分辨率变异性对脊柱立体定向放射治疗（SBRT）中脊髓剂量的影响。

方法

选择10个射波刀治疗计划，其靶区（PTV）靠近脊髓。所有剂量分布均使用蒙特卡罗方法以高空间分辨率和最小相对不确定性进行计算。将计划重新归一化为16 Gy的处方剂量，并确保靶区覆盖率>95%，以便比较计算出的剂量分布。每个剂量矩阵重新采样12次，覆盖0.95 mm至13.52 mm的网格分辨率范围。为每个重新采样的剂量网格生成脊髓剂量体积直方图（DVH）。研究了最大点剂量（DmaxCord）和高达5 cc的部分体积剂量覆盖率（D[V]）随网格分辨率的变化。

结果

DmaxCord随网格分辨率的平均变化符合反幂律，最初急剧下降，可能导致DmaxCord被大幅低估（在2 mm、4 mm和8 mm分辨率下分别为24%、40%和55%）。平均D[V]值的变异性从较小网格分辨率到较大网格分辨率逐渐减小，但不同患者计划之间存在较大差异。我们引入变异性阈值体积（Vth）作为一种约束，以独立于网格分辨率来表示剂量覆盖率。对于高达8 mm的分辨率，平均Vth值为（0.96±0.10）cc，相对于初始DmaxCord值，相应的剂量覆盖率为（26±12）%。