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End-To-End Targeting Accuracy of the Gamma Knife for Trigeminal Neuralgia

I Brezovich

I Brezovich*, X Wu , J Duan , S Benhabib , M Huang , S Shen , R Cardan , R Popple , Univ Alabama Birmingham, Birmingham, AL


WE-G-BRD-8 Wednesday 4:30PM - 6:00PM Room: Ballroom D

Purpose: Current QA procedures verify accuracy of individual equipment parameters, but may not include CT and MRI localizers. This study uses an end-to-end approach to measure the overall targeting errors in individual patients previously treated for trigeminal neuralgia.

Methods: The trigeminal nerve is simulated by a 3 mm long, 3.175 mm (1/8 inch) diameter MRI contrast-filled cavity embedded within a PMMA plastic capsule. The capsule is positioned within the head frame such that the cavity position matches the Gamma Knife coordinates of 10 previously treated patients. Gafchromic EBT2 film is placed at the center of the cavity in coronal and sagittal orientations. The films are marked with a pin prick to identify the cavity center. Treatments are planned for delivery with 4 mm collimators using MRI and CT scans acquired with the clinical localizer boxes and acquisition protocols. Coordinates of shots are chosen so that the cavity is centered within the 50% isodose volume. Following irradiation, the films are scanned and analyzed. Targeting errors are defined as the distance between the pin prick and the centroid of the 50% isodose line.

Results: Averaged over 10 patient simulations, targeting errors along the x, y and z coordinates (patient left-to-right, posterior-anterior, head-to-foot) were, respectively, -0.060 +/- 0.363, -0.350 +/- 0.253, and 0.364 +/- 0.191 mm when MRI was used for treatment planning. Planning according to CT exhibited generally smaller errors, namely 0.109 +/- 0.167, -0.191 +/- 0.144, and 0.211 +/- 0.94 mm. The largest errors in MRI and CT planned treatments were, respectively, y = -0.761 and x = 0.428 mm.

Conclusion: Unless patient motion or stronger MRI image distortion in actual treatments caused additional errors, all patients received the prescribed dose, i.e., the targeted section of the trigĀ±eminal nerve was contained within the 50% isodose surface in all cases.

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