Department of Radiation Oncology, Academic Medical Center/University of Amsterdam, Amsterdam, The Netherlands.
Department of Radiation Oncology, Academic Medical Center/University of Amsterdam, Amsterdam, The Netherlands.
Int J Radiat Oncol Biol Phys. 2017 Nov 1;99(3):710-718. doi: 10.1016/j.ijrobp.2017.06.016. Epub 2017 Jun 28.
To quantify magnetic resonance imaging (MRI) distortions on a plastic intracavitary/interstitial applicator with plastic needles at a field strength of 3 T and to determine the dosimetric impact, using patient data.
For 11 cervical cancer patients, our clinical MRI protocol was extended with 3 scans. From the first scan, a multi-echo acquisition, a map of the magnetic field (B) was calculated and used to quantify the field inhomogeneity. The expected displacements of the applicator were quantified for the clinical sequence using the measured field inhomogeneity and the clinical sequence's bandwidth. The second and third scan were our routine clinical sequence (duration: <5 minutes each), acquired consecutively using opposing readout directions. The displacement of the applicator between these scans is approximately twice the displacement due to B inhomogeneity. The impact of the displacement on the dose was determined by reconstructing the applicator on both scans. The applicator was then shifted and rotated the same distance as the observed displacement to create a worst-case scenario (ie, twice the actual displacement due to B inhomogeneity). Next, the dose to 98%/90% (D/D) of the clinical target volume at high risk, as well as the dose to the most irradiated 2 cm for bladder and rectum, were calculated for the original plan as well as the shifted plan.
For a volume of interest containing the intrauterine device and the ovoids the 95th percentile of the absolute displacement ranged between 0.2 and 0.75 mm, over all patients. For all patients, the difference in D/D in the opposing readout scans with the original plan was at most 4.7%/4.3%. For the dose to the most irradiated 2 cm of bladder/rectum, the difference was at most 6.0%/6.3%.
The dosimetric impact of distortions on this plastic applicator with plastic needles is limited. Applicator reconstruction for brachytherapy planning purposes is feasible at 3 T MRI.
在 3T 场强下，量化带有塑料针的腔内/间质应用器的磁共振成像（MRI）变形，并使用患者数据确定剂量学影响。
对 11 例宫颈癌患者，在我们的临床 MRI 方案中额外增加了 3 次扫描。第一次扫描时，采用多回波采集，计算磁场（B）图，并用于量化场不均匀性。使用测量的场不均匀性和临床序列的带宽，对临床序列中的应用器预期位移进行量化。第二次和第三次扫描是我们的常规临床序列（每次扫描持续时间<5 分钟），采用相反的读出方向连续采集。两次扫描之间应用器的位移大约是由 B 不均匀性引起的位移的两倍。通过在两次扫描上重建应用器，确定位移对剂量的影响。然后将应用器移动和旋转与观察到的位移相同的距离，以创建最坏情况场景（即，由于 B 不均匀性导致的实际位移的两倍）。接下来，为高风险的临床靶体积 98%/90%（D/D）以及膀胱和直肠受照 2cm 内最高剂量，计算原始计划和移位计划的剂量。
对于包含宫内节育器和子宫旁容器的感兴趣体积，所有患者的绝对位移 95%的范围在 0.2 至 0.75mm 之间。对于所有患者，原始计划与相反读出扫描的 D/D 差异最大为 4.7%/4.3%。对于膀胱/直肠受照 2cm 内最高剂量，差异最大为 6.0%/6.3%。
在带有塑料针的这种塑料应用器上，变形的剂量学影响是有限的。在 3T MRI 下，用于近距离治疗计划的应用器重建是可行的。
