2718 - Targeting Accuracy of Frameless Linear Accelerator Thalamotomy

R. A. Popple¹, E. M. Thomas², E. H. Middlebrooks³, H. Walker¹, N. J. Bentley¹, M. T. Holland¹, J. B. Fiveash¹, B. L. Guthrie¹, and M. Bredel¹; ¹University of Alabama at Birmingham, Birmingham, AL, ²Department of Radiation Oncology, The James Cancer Center, Ohio State University Wexner Medical Center, Columbus, OH, ³Mayo Clinic, Jacksonville, FL

Purpose/Objective(s): Radiosurgical thalamotomy is one method to treat medically refractory tremor for patients that are not candidates for deep-brain stimulation. It ablates aberrant cerebello-thalamo-cortical circuitry by targeting the dentato-rubro-thalamic tract (DRTT) within the ventral intermediate nucleus (VIM) of the thalamus. We report on the accuracy of radiosurgical thalamotomy using a linear accelerator equipped with a multileaf collimator (MLC), a thermoplastic mask for immobilization, and surface-imaging for intra-fraction motion monitoring. Materials/

Methods: 40 patients underwent SRS thalamotomy on a linear accelerator on an IRB-approved clinical trial (ClinicalTrials.gov Identifier: NCT03305588) using our previously described virtual cone technique to deliver 135 Gy to isocenter. Patients were immobilized using a non-invasive thermoplastic Immobilization system and a treatment planning CT was obtained with 0.8 mm slice spacing. MR images were registered to the treatment planning CT and the VIM identified using stereotactic coordinates based on the anterior and posterior commissure locations. Patient position was monitored real-time during treatment using optical surface imaging. At 3, 6, and 12 months after treatment, post Gadolinium-contrast T1 images were acquired on a 3T MRI scanner. For each patient, the first image set for which a lesion was clearly visible was selected and registered to the treatment planning CT using mutual information rigid registration. The lesion was manually segmented by two investigators, a radiation oncologist and a medical physicist. For each lesion, the centroid of the segmented volume was determined. For each patient, the mid-point of the two centroids was compared with the isocenter position.
Results: Post-treatment imaging data was available for 39/40 patients. The mean distance between the centroids identified by the two observers was 0.3 mm (range 0.1-0.6 mm, interquartile range 0.1 mm). The mean distance between the centroid mid-points and the isocenter was 0.7 mm (range 0.2-1.6 mm, interquartile range 0.4 mm). The mean difference (standard deviation) in the x, y, and z directions of the treatment planning CT coordinate system was -0.1 (0.3), -0.2 (0.4), and 0.3 (0.4) mm, respectively.
Conclusion: Our findings demonstrate sub-millimeter accuracy of frameless MLC-based linear accelerator SRS using optical surface imaging for intra-fraction position monitoring. These results are comparable to accuracies reported using other techniques for radiosurgical thalamotomy and to open surgical lesioning/stimulation procedures.