Accuracy Of Stereotactic Radiosurgery Research Articles

Optimized MR Simulation Protocol for Framed Stereotactic Radiosurgery at 3T

Magnetic resonance imaging is often the primary imaging modality for framed stereotactic radiosurgery (SRS). However, eddy currents, chemical shift, gradient nonlinearities, and the magnetic susceptibility distribution of the patient can all contribute to geometric distortions that may affect the accuracy of SRS if not properly controlled. The goal of this work was to establish an optimized MR simulation protocol for framed SRS at 3T, and assess the severity of any residual geometric distortions. Imaging was performed on a 3T scanner using an SRS phantom (CIRS, model 603A). A titanium Leksell head frame was secured to the phantom. The fiducial box was then affixed to the head frame. The combined, phantom, head frame, and fiducial box assembly were then immobilized in treatment position using a holder of our design. The combination of two flexible phase-array coils and the integrated spine RF coil was used for signal reception. Magnetic field mapping was performed using a dual echo GRE sequence. Different shimming modes (tune up, standard, standard + local shim, standard + brain shim, advanced, advanced + local shim, advanced + brain shim) were applied prior to acquisition of each field map. In addition, 3D T1 FLASH images were acquired with different gradient slew rate modes (whisper, normal, fast) and readout bandwidths. Vendor-provided 3D gradient nonlinearity distortion correction was applied to all images prior to them being sent to a commercial deformable registration algorithm system for analysis. Large magnetic field variations were observed with tune up and standard shim modes. The advanced + brain shim mode resulted in smallest magnetic field variations over the brain. Shimming over the brain did not affect the geometric accuracy of the fiducial markers. The low slew rate whisper gradient mode reduced eddy current-induced distortion by 1.5mm compared to normal gradient mode. Reducing the readout bandwidth from 980 Hz/pixel to 440 Hz/pixel reduced eddy current-induced distortion further by 2mm and increased SNR, while minimizing the severity of chemical shift. Eddy currents induced by the gradients in the titanium frame can result in geometric distortions of fiducial that exceed 1mm in regions of the fiducial box within 2cm of the SRS frame. We established an optimized MR simulation protocol for framed SRS at 3T. Our recommendations include: 1) use of low slew rate gradient modes to reduce eddy current induced distortions, 2) reduce readout bandwidths to 440 Hz/pixel as a tradeoff between eddy current induced distortions and water-fat shift, 3) scan with the prescription volume centered at isocenter, 4) perform advanced high-order shimming with shim volume localized to brain, 5) confirm A-P phase encode direction to avoid flow artifacts, 6) apply 3D gradient nonlinearity distortion correction prior to using images for treatment planning, 7) avoid registering images in treatment planning system using fiducials within 2cm of the SRS frame.

(P57) The Accuracy of Stereotactic Radiosurgery With Cone Beam Computed Tomography Image Guidance for Trigeminal Neuralgia

(P57) The Accuracy of Stereotactic Radiosurgery With Cone Beam Computed Tomography Image Guidance for Trigeminal Neuralgia

Delivery Accuracy of Stereotactic Radiosurgery (SRS) With Tomotherapy as Compared to Linear Accelerator and Robotic-based Radiosurgery

Delivery Accuracy of Stereotactic Radiosurgery (SRS) With Tomotherapy as Compared to Linear Accelerator and Robotic-based Radiosurgery

Quality assurance in stereotactic space. A system test for verifying the accuracy of aim in radiosurgery.

A detailed quality assurance (QA) program is essential for high precision single dose irradiations. The accuracy of stereotactic radiosurgery is limited by the errors of each step in the chain for optimal treatment beginning with the diagnostic imaging and target localization leading to the dose planning and ending up with the treatment of the patient. Two main goals were followed on the way to finding a concept for a suitable and sufficient quality assurance routine. First, the chain of items in terms of a complete patient simulation should be followed and second, the stereotactic MR image data should be verified against a reference in our case stereotactic radiographic projection images. Target point verifications were performed using the so-called, unknown target method based on MRI, CT, and stereotactic projection images. A marked radiochromic film, embedded between inserts of the phantom is fixed parallel to either the xy or the xz plane of the stereotactic coordinate system. After imaging and planning, the phantom is adjusted and irradiated. At the end, the film, dyed by the radiation field around the premarked cross, is evaluated. The measured distance between the unit center point (shadow) and the localization of the marked film leads to the deviation to be minimized. This is referred to as the displacement vector. The results, evaluating 170 system tests within 5 years. show that the mean displacement vector of the complete system is 0.48 mm +/-0.23 mm (mean+/- sd). Factors having a significant influence on the overall accuracy are associated with MRI parameters. Test results based on axial images (xy plane; 0.42 mm +/- 0.24 mm) are significantly superior to coronal images (xz plane; x = 0.60 mm +/- 0.02 mm). Further on, the 3D-mpr sequence (0.40 mm +/- 0.19 mm) is significantly superior to the T1 weighted SE sequences (0.66 mm +/- 0.24 mm). Given the high mechanical accuracy of the Leksell gamma knife, the most sensitive technical factor having an influence on the overall precision of radiosurgery is the MRI study. However, using the appropriate imaging sequences and parameters the dislocation error inferred by MRI can be kept very low and restricted to the rare patient inherent distortion factors. With these precautions in mind, MRI is recommended as the imaging method of choice in radiosurgery.

The Use of the Laitinen Stereoadapter for Three-Dimensional Conformal Stereotactic Radiotherapy

3-D conformal therapy is used to treat a variety of tumors by delivering a high dose to the target volume while minimizing dose to surrounding tissue. Conventional irradiation of intracranial lesions involves treating substantial portions of healthy brain. Stereotactic radiosurgery (SRS) delivers a single fraction to a small lesion targeted by an invasive head frame. The Laitinen stereoadapter is a noninvasive localization/immobilization device that provides the precision of stereotactic techniques (within 2 mm) with the radiobiologic advantage of fractionation. This report describes the Northwestern experience of 3-D conformal Stereotactic radiotherapy (SRT) for intracranial lesions using the Laitinen Stereoadapter for reproducible, noninvasive immobilization. 17 patients with 18 isocenters were treated with SRT via noncoplanar, static, and/or rotational beams. Patient age ranged from 3 to 77. Total doses ranged from 1000 to 5580 cGy at 180 to 400 cGy per fraction. Histologies included primitive neuroectodermal tumor (PNET), recurrent medulloblastoma, mixed germ cell tumor, recurrent juvenile pilocytic astrocytoma, optic glioma, oligodendroglioma, cran-iopharyngioma, meningioma, metastatic carcinoma, and one orbital rhabdomyosarcoma. Patient positioning was checked daily, and verification port films were taken every other day. Port films showed excellent reproducibility of isocenter placement with the Laitinen stereoadapter. Follow-up ranged from 1 to 24 months. Post-treatment radiographic responses were similar to those seen with conventional radiation therapy. The Laitinen stereoadapter is very well tolerated with minimal acute effects. Its use does not require excessive preplanning (i.e., customized oral appliances) and can accommodate any cranial size. Patients can initiate treatment within 24 hours, if necessary. The relocatable frame exploits the accuracy of SRS with the biological advantage of fractionation. We currently use the Laitinen for primary adult and pediatric brain tumors as well as metastatic lesions too large for SRS. Further studies are currently underway to further define its utility.