Influence of MRI-based bone outline definition errors on external radiotherapy dose calculation accuracy in heterogeneous pseudo-CT images of prostate cancer patients

Background. This work evaluates influences of susceptibility-induced bone outline shift and perturbations, and bone segmentation errors on external radiotherapy dose calculation accuracy in magnetic resonance imaging (MRI)-based pseudo-computed tomography (CT) images of the male pelvis.Material and methods. T1/T2*-weighted fast gradient echo, T1-weighted spin echo and T2-weighted fast spin echo images were used in bone detection investigation. Bone edge location and bone diameter in MRI were evaluated by comparing those in the images with actual physical measurements of fresh deer bones positioned in a gelatine phantom. Dose calculation accuracy in pseudo-CT images was investigated for 15 prostate cancer patients. Bone outlines in T1/T2*-weighted images were contoured and additional segmentation errors were simulated by expanding and contracting the bone contours with 1 mm spacing. Heterogeneous pseudo-CT images were constructed by adopting a technique transforming the MRI intensity values into Hounsfield units with separate conversion models within and outside of bone segment.Results. Bone edges and diameter in the phantom were illustrated correctly within a 1 mm-pixel size in MRI. Each 1 mm-sized systematic error in bone segment resulted in roughly 0.4% change to the prostate dose level in the pseudo-CT images. The prostate average (range) dose levels in pseudo-CT images with additional systematic bone segmentation errors of −2 mm, 0 mm and 2 mm were 0.5% (−0.5–1.4%), −0.2% (−1.0–0.7%), and −0.9% (−1.8–0.0%) compared to those in CT images, respectively, in volumetric modulated arc therapy treatment plans calculated by Monte Carlo algorithm.Conclusions. Susceptibility-induced bone outline shift and perturbations do not result in substantial uncertainty for MRI-based dose calculation. Dose consistency of 2% can be achieved reliably for the prostate if heterogeneous pseudo-CT images are constructed with ≤± 2 mm systematic error in bone segment.

研究背景：本研究旨在评估基于磁共振成像（Magnetic Resonance Imaging, MRI）构建的男性骨盆伪计算机断层扫描（pseudo-computed tomography, pseudo-CT）图像中，磁化率诱发的骨骼轮廓移位与扰动、以及骨骼分割误差对体外放射治疗剂量计算精度的影响。

材料与方法：本研究采用T1/T2*加权快速梯度回波、T1加权自旋回波及T2加权快速自旋回波图像开展骨骼检测相关研究。通过将明胶体模中植入的新鲜鹿骨的实际物理测量值，与MRI图像中的骨骼边缘位置及骨骼直径进行比对，以此评估MRI图像中的骨骼相关参数。针对15例前列腺癌患者，探究了伪CT图像中的剂量计算精度。对T1/T2*加权图像中的骨骼轮廓进行勾画，并通过以1mm为间隔扩张与收缩骨骼轮廓来模拟额外的分割误差。采用将MRI强度值转换为亨氏单位（Hounsfield Unit, HU）的技术构建异质性伪CT图像，且在骨骼区域内外分别使用独立的强度转换模型。

结果：在像素尺寸为1mm的MRI图像中，体模内的骨骼边缘与直径可被准确识别。骨骼分割中每1mm的系统误差，会使伪CT图像中的前列腺剂量水平产生约0.4%的变化。基于蒙特卡罗算法（Monte Carlo algorithm）计算的容积调强弧形治疗计划中，当额外施加-2mm、0mm及2mm的系统性骨骼分割误差时，伪CT图像对应的前列腺平均（范围）剂量水平与CT图像相比分别为0.5%（-0.5%~1.4%）、-0.2%（-1.0%~0.7%）及-0.9%（-1.8%~0.0%）。

结论：磁化率诱发的骨骼轮廓移位与扰动不会对基于MRI的剂量计算造成显著不确定性。若骨骼分割的系统误差控制在±2mm以内，通过构建异质性伪CT图像，可使前列腺剂量的一致性可靠达到2%以内。