3493 - A Non-Invasive Stereotactic Mimicking Radiosurgery with a Frameless Robotic Radiosurgery: A Comparison of Dose Prescription Methodologies for Treatment of Trigeminal Neuralgia

Purpose/Objective(s): In treatment planning for trigeminal neuralgia, a frameless robotic radiosurgery can be prescribed to a longer, volumetric segment of the trigeminal nerve, or using an isocentric technique that mimics the spherical distribution of Gamma-knife. The aim of this study was to compare these two distinct radiosurgical planning approaches.

Materials/Methods: The authors performed a retrospective review of consecutive patients treated between 2010 and 2022 with a frameless robotic radiosurgery for trigeminal neuralgia (TN). The early experience (cohort 1), modeled on the early Stanford experience, involved treating a roughly 6mm length of the retrogasserian segment of the trigeminal nerve truncated at least 2 mm from the dorsal root entry zone at the brainstem. Subsequent patients (cohort 2) were treated using an isocentric approach targeting a spherical volume overlying the trigeminal nerve either 2-3 mm anterior to the dorsal root entry zone or in a retrogasserian location depending on assessment of nerve and brainstem anatomy. Pain responses were assessed using the Barrow's Neurological Institute (BNI) scale. Dosimetric analysis was performed to investigate the relationship between pain response and various dose parameters to the trigeminal nerve, brainstem, and Meckel’s cave.

Results: One hundred forty-two patients were studied - 100 with Type I TN and 42 with Type II. The initial 55 patients (cohort 1) received a median dose of 60 Gy and a maximum dose 74.7 Gy prescribed to a roughly 6 mm volume contoured along the cisternal segment of the trigeminal nerve. The subsequent 87 patients (cohort 2) were treated to a maximum dose of 85 Gy prescribed with an isocentric approach resulting in a spherical dose distribution. Initial adequate pain relief (BNI I-IIIb) was seen in 88% of patients overall and was similar between cohort 1 and cohort 2 (89% vs 87%; p=0.9999). Favorable pain responses (BNI I-IIIa) were seen in 69% of cohort 1 patients and in 59% of cohort 2 patients (p=0.2309); however, at last follow-up numerical rates of ultimate pain control were marginally higher in cohort 2 at 58%, compared with 49% for cohort 1 (p=0.3883). Complete pain response (BNI I) was seen in 42% of cohort 1 patients and in 33% of cohort 2 patients (p=0.3574). Type I patients had more favorable outcomes both initially, (94% vs. 74%; p=0.0015) and at last follow-up (62% vs 36%; p=0.0055) compared with type II patients. Type I patients treated to = 85 Gy had more favorable BNI outcomes at last follow up (p=0.0252). Treatment related numbness occurred in 20/54 (37%) of cases in cohort 1 at a median time of 16 months and in 39/83 at risk patients (47%) in cohort 2 at a median time of 10.4 months (p=0.1864).

Conclusion: Volumetric SRS prescription to a lower dose/longer segment of nerve and isocentric prescription to a higher dose/tighter volume are both acceptable options for treatment of trigeminal neuralgia.