Fondazione Policlinico Universitario A. Gemelli, IRCCS, Largo Agostino Gemelli 8, 00168, Rome, Italy.
Radiol Med. 2020 Feb;125(2):157-164. doi: 10.1007/s11547-019-01090-0. Epub 2019 Oct 8.
MR-guided radiotherapy (MRgRT) relies on the daily assignment of a relative electron density (RED) map to allow the fraction specific dose calculation. One approach to assign the RED map consists of segmenting the daily magnetic resonance image into five different density levels and assigning a RED bulk value to each level to generate a synthetic CT (sCT). The aim of this study is to evaluate the dose calculation accuracy of this approach for applications in MRgRT.
A planning CT (pCT) was acquired for 26 patients with abdominal and pelvic lesions and segmented in five levels similar to an online approach: air, lung, fat, soft tissue and bone. For each patient, the median RED value was calculated for fat, soft tissue and bone. Two sCTs were generated assigning different bulk values to the segmented levels on pCT: The sCT uses the RED values recommended by ICRU46, and the sCT uses the median patient-specific RED values. The same treatment plan was calculated on two the sCTs and the pCT. The dose calculation accuracy was investigated in terms of gamma analysis and dose volume histogram parameters.
Good agreement was found between dose calculated on sCTs and pCT (gamma passing rate 1%/1 mm equal to 91.2% ± 6.9% for sCT and 93.7% ± 5.3% b or sCT). The mean difference in estimating V95 (PTV) was equal to 0.2% using sCT and 1.2% using sCT, respect to pCT values CONCLUSIONS: The bulk sCT guarantees a high level of dose calculation accuracy also in presence of magnetic field, making this approach suitable to MRgRT. This accuracy can be improved by using patient-specific RED values.
磁共振引导放射治疗（MRgRT）依赖于每天分配相对电子密度（RED）图，以允许进行分次剂量计算。一种分配 RED 图的方法包括将每天的磁共振图像分割成五个不同的密度水平，并为每个水平分配一个 RED 体值，以生成合成 CT（sCT）。本研究旨在评估该方法在 MRgRT 中的剂量计算准确性。
对 26 例腹部和盆腔病变患者进行了计划 CT（pCT）采集，并按类似于在线方法的五个水平进行分割：空气、肺、脂肪、软组织和骨。对于每个患者，计算了脂肪、软组织和骨的中位数 RED 值。为 pCT 上的分割水平分配了不同的体值，生成了两个 sCT：sCT 使用 ICRU46 推荐的 RED 值，sCT 使用患者特定的中位数 RED 值。在两个 sCT 和 pCT 上计算了相同的治疗计划。根据伽马分析和剂量体积直方图参数研究了剂量计算准确性。
在 sCT 和 pCT 上计算的剂量之间发现了良好的一致性（γ通过率 1%/1mm 等于 91.2%±6.9%的 sCT 和 93.7%±5.3%b 或 sCT）。使用 sCT 估计 V95（PTV）的平均差异等于 0.2%，而使用 sCT 则为 1.2%，相对于 pCT 值。
体 sCT 即使在磁场存在下也能保证高剂量计算准确性，使其适用于 MRgRT。通过使用患者特定的 RED 值，可以提高这种准确性。
