Assessment of Brain Tumor Displacements after Skullbased Registration: A CT/MRI Fusion Study

Research Article

Austin J Radiat Oncol & Cancer. 2015; 1(3): 1011.

Assessment of Brain Tumor Displacements after Skullbased Registration: A CT/MRI Fusion Study

Xu Q1*, Hanna G1, Zhai Y1, Asbell A1, Fan J2, La Couture T1, Chen Y1 and Kubicek G1

1Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, NJ, USA

2Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA, USA

*Corresponding author: Xu Q, Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Suite C, 715 Fellowship Rd, Mt Laurel, NJ, USA

Received: May 21, 2015; Accepted: September 11, 2015; Published: September 30, 2015


Purpose: To assess brain tumor displacements between skull based and soft-tissue based matching during CT-MRI fusion for a total of 35 brain lesions.

Methods: Twenty-five patients who underwent CT and MRI scans in the same day were retrospectively recruited into the study. Semi-automatic skull based fusion was first performed and reviewed. A secondary fine-tuning of the fusion was performed, if mismatch was observed in the tumor or neighboring soft-tissues. Two physicists fine-tuned the secondary fusion until the best match could be agreed upon. The resulting rotations and translations after fine-tuning indicated local displacements between the two fusions. We further created a PTV to evaluate the coverage of the GTV after soft-tissue based fusion.

Results: In 29 of the 35 lesions, minor to no mismatch was found between the soft-tissue and skull based fusions. The translational and rotational shifts were 0.05±0.63 mm (LR), 0.01±0.79 mm (AP), 0.37±1.01 mm (SI); -0.15±0.67° (pitch), -0.19±0.34° (yaw), and -0.12±0.49° (roll). In the remaining 6 lesions, noticeable displacements were observed between the two fusions. For the outlier lesion, the GTV was nearly missed by the PTV, and for the rest of the 5 lesions, the mean coverage of the GTV was 98.9%.

Conclusion: In a small portion of lesions, our study showed noticeable brain tumor displacement with typical patient setup in CT and MRI scans between skull based and soft-tissue based fusion. Careful review of skull based fusion is suggested and adding a margin to the GTV is recommended, if fusion deviations are found.

Keywords: Fusion; Brain; Skull; Soft-Tissue; Displacement


Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) deliver high doses to benign and malignant intracranial tumors in a single (SRS) to multiple (SRT) fractions. For patients with high risk of surgical complications, it provides non-invasive alternative treatment to ensure local tumor control while sparing nearby critical structures. Reproducibility and accurate contouring of the tumor is of great importance since treatments consist of high doses delivered in a single or limited number of fractions. In the initial design of SRS and SRT, the skull was directly fixed to a frame to achieve high treatment precision [1]. Despite the invasive nature, the technique is still widely adopted in Gamma Knife treatments. In recent years, localization techniques have evolved into using non-invasive image guidance with similar treatment accuracy, including kV and MV cone beam computed tomography (CBCT) [2-5], optical systems [6- 8], orthogonal x-ray imaging systems [9-11] and in-room CT [12]. With many of these systems a CT scan is used for treatment planning but an MRI scan is required to contour and define the target. Thus, overall accuracy depends on the ability to properly register or fuse the MRI scan with the planning CT scan.

Among all types of image guidance, the skull has been universally adopted as the image matching anatomy during SRS and SRT due to its rigidity and great visibility in all imaging modalities. This reliance on skull matching is common in multi-modality image fusions before treatment and in image guidance and target localization during treatment. This convention, however, relies on the assumptions that brain tumors keep the same relative position to the skull, and that the accuracy of matching simple skull geometries can pick up matching differences in complicated soft-tissue geometries and then can ultimately represent accurate tumor matching. Guckenberger et al., recently evaluated the reliability of the skull in SRS treatments of brain metastases and reported three-dimensional (3D) displacements of brain tumors between the skull and nearby soft-tissue [13]. In the study, 18 patients with intravenous (IV) contrast injection hadan inroom CT scan followed by a CBCT scan before SRS their treatment. The in-room CT and CBCT were fused to the planning CT based on skull and soft-tissue matching. The tumor displacement in each axis between these two fusions had high correlation (r=0.88). However, the tumor centroids between the two fusions revealed amen 3D mismatch of1.7±0.7 mm (maximum 2.8 mm). Rotational shifts were not considered in the study. The mismatch indicates slight tumorskull relative displacements between CT fusions.

In the field of neurological studies actual brain displacements have been the focus of research over the past decades since brain tissue motions a biomechanical indication for most brain traumas. Various theoretical and experimental models have been developed to estimate brain motion relative to the skull when the brain was under mild impact and in static conditions (in different positions). In a typical design, the heads of volunteers moved on a short track and stopped by hitting a soft suspension. Meanwhile, MRI scans with high temporal resolution were acquired. The relative brain displacements (2-5 mm), as well as rotational shifts, were reported [14]. In another study, the volunteers had MRI scans of their brain while in different positions, and relative brain-skull motion was reported [15].

The setting in trauma studies is clearly different from a patient setting in SRS and SRT. However, the possibility of such motion is still insightful. We modified the work from Guckenberger et al. in our study by using MRI to take advantage of its superior soft-tissue contrast. After a skull-based fusion of the CT and MRI images, the alignment of nearby soft-tissue and tumors was reviewed. If any mismatch was observed, fine-tuning of the skull based fusion was performed until the best soft-tissue or tumor match was achieved. The two fusions were reviewed by two medical physicists (Zhai Y and Xu Q) and a radiation oncologist (Kubicek G), and the translational and rotational shifts between the two fusions were quantified. We further evaluated the coverage of the Gross Tumor Volume (GTV) by adding a small margin into an expanded Planning Target Volume (PTV) to account for such displacements of skull image guidance during treatment

Many institutions use rigid skull registration for MRI and CT fusion, the subsequent SRS and SRT plans derived from this is potentially incorrect if skull fusion does not faithfully correspond to the soft tissue. The purpose of this project is to explore potential differences between skull based and soft-tissue based fusion between CT and MRI.

Methods and Materials

Patient and imaging settings

Twenty-five previously treated patients with either primary or metastatic brain tumors, as part of institutional review boardapproved studies of retrospectively analysis of SBRT with Cyber Knife were enrolled in this study. The patient recruitment criterion was the visibility of the tumor or nearby soft-tissues in the planning CT. The characteristics of the patients and tumors are summarized in (Table 1). A total of 35 brain lesions were analyzed. All patients had a planning CT scan (Light Speed, GE Healthcare, WI, USA) with 1.25 mm slice thickness. The patient heads were immobilized by a thermoplastic mask with a plastic holder placed under the heads. The mask was rigidly attached to the CT couch to ensure setup reproducibility. The same setup was used through simulation and treatment. The patients were sent for an MRI scan (Magnetom, Siemens Healthcare, PA, USA) after their planning CT scan in the same day. IV contrast was administered to every patient andT1- weighted MRI scans were acquired with a Magnetization-Prepared Rapid Gradient-Echo (MP-RAGE) imaging technique with 1 mm slice thickness.