359 - Stereotactic Radiosurgery for Spinal Cord Arteriovenous Malformations: Dosimetric Correlates of Spinal Cord Toxicity

09:55am - 10:05am ET

Room 160

Presenter(s)

Emily Smith, MD - Stanford Health Care, Stanford, CA

E. J. Smith¹, K. S. Jin¹, J. P. Obeid², E. L. Pollom¹, S. M. Hiniker¹, G. A. Szalkowski¹, L. Wang¹, S. D. Chang³, J. R. Adler Jr³, I. C. Gibbs¹, R. L. Dodd³, and S. G. Soltys¹; ¹Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, ²Department of Radiation Oncology, University of Miami, Miami, FL, ³Department of Neurosurgery, Stanford University School of Medicine, Stanford, CA

Purpose/Objective(s):

Stereotactic radiosurgery (SRS) is a standard treatment for intracranial arteriovenous malformations (AVMs), but is underutilized for intramedullary spinal cord AVMs (scAVMs) due to concerns of spinal cord toxicity. We report dosimetric outcomes for patients with scAVMs treated with SRS. We hypothesize that despite greatly exceeding conventional spinal cord dosimetric constraints, toxicity is acceptable in this unique patient population where the cord is both the target and the organ at risk.

Materials/Methods:

In this IRB-approved study, we retrospectively analyzed patients from a single institution treated with a single course of SRS for scAVM from 2002-2024. Patients with more than one treatment course were excluded. Spinal cord dosimetry and toxicity (graded with CTCAE v5) outcomes were extracted and analyzed. As the targets were intrinsic to the cord, dosimetric values were analyzed as the spinal cord inclusive of the scAVM target. The single fraction equivalent dose (SFED) was calculated with the linear-quadratic model and an a/ß of 2. Conventional dosimetric constraints were considered as per HyTEC; e.g., a D0.03cc of <14 Gy and a D0.35 of <10 Gy if 1 fraction.

Results:

From a cohort of 61 consecutive patients with scAVMs, 45 patients met inclusion criteria; 16 patients were excluded for: treatment plans not available (n=6), more than 1 course of SRS (n=7), or no follow-up data (n=3). The median clinical follow up was 53.6 months (range 7.3-155.6 months). A median scAVM volume of 1.4 cm3 (range 0.1-15.0 cm3) received a median prescription SFED of 14.5 Gy2 (range 12.2 – 20.0 Gy2); the most common fractionation regimens were 16-20 Gy in 1 fraction (n=3), 20-22 Gy in 2 fractions (n=32), and 20-24 Gy in 3 fractions (n=10). Maximum conventional spinal cord constraints were exceeded in 98% (44/45) of patients: median spinal cord Dmax was 23.2 Gy (12.0-25.6 Gy) in 1 fraction, 25.7 Gy (22.9-29.5 Gy) in 2 fractions, and 27.2 Gy (23.1-30.7 Gy) in 3 fractions. For all patients, the SFED metrics were a median Dmax of 18.1 Gy2, D0.03cc of 17.2 Gy2, and D0.35cc of 15.7 Gy2. Three patients (7%) had toxicity: new G1 numbness, new G2 weakness, return of G3 weakness (pre-SRS paraplegia that improved then worsened again after SRS). These 3 patients had the highest spinal cord SFED D0.35cc of 18.4 Gy2, 19.5 Gy2, and 20.0 Gy2. Of note, in the 6 patients excluded due to non-dearchivable treatment plans, none had toxicity. Although only 27% (12/45) had complete obliteration of their scAVM, no patients had hemorrhage after SRS, despite 60% having a hemorrhage prior to SRS.

Conclusion:

Despite exceeding conventional spinal cord constraints in 98%, a 7% rate of clinical worsening (only 2% with G3+ and 0% with new G3+) was observed; these patients had the highest cord doses. Formal NTCP modeling of these data are ongoing to provide data-based spinal cord constraints to guide treatment of scAVMs.