2881 - LINAC-Based Fractionated Stereotactic Radiosurgery (fSRS) for Cervical Spine Arteriovenous Malformation (AVM): A Case-Based Treatment Planning Framework

Purpose/Objective(s):

To demonstrate the feasibility and favorable early outcomes of LINAC-based fractionated stereotactic radiosurgery (fSRS) in the management of high-risk, complex spinal arteriovenous malformations (AVMs), particularly in cases where invasive approaches such as microsurgical resection and trans-arterial embolization are technically prohibitive or clinically inadvisable.

Materials/Methods:

Treatment planning aimed to achieve highly conformal dose delivery to the AVM nidus while minimizing radiation to the adjacent spinal cord. Simulation was performed with 0.9 mm CT slices using an open-face thermoplastic mask to allow surface-guided radiotherapy (SGRT). MRI and MRA images were rigidly registered to the CT to define the AVM nidus (3.2 cc) and spinal cord. All volumes were contoured as high resolution structures per our SRS protocols. Planning risk volumes (PRVs) were generated to account for geometric uncertainties, and endovascular coils were contoured as fiducials.

Given the close proximity between the nidus and spinal cord, the plan prescribed 20 Gy in four fractions at the 80% isodose line, with a selective boost up to 24 Gy in a distal sub-volume comprising about half the nidus. This fSRS regimen balanced effective AVM obliteration with spinal cord protection. Constraints followed Timmerman’s 4-fraction SRS limits.

Planning was executed using 6X flattening filter-free beams and VMAT. Four 180° arcs with ± [5–10] ° collimator rotations and jaw tracking achieved optimal dose conformity and minimal interleaf leakage. The optimization process used manual normal tissue objectives and a 1.25 mm dose grid. Plan quality metrics showed a conformity index of 1.49, and gradient index 4.15. Dose-volume histograms confirmed D95% = 100% and V20 Gy = 95% for the nidus, while all spinal cord and PRV constraints were comfortably met, indicating a robust, conformal, and safe treatment plan.

Treatment was delivered on an Edge TrueBeam LINAC using SGRT, kV imaging, and high-resolution cone-beam CT for setup verification. SGRT ensured submillimeter motion control during each of four treatments delivered.

Results:

To date, the patient has had two follow-up visits and corresponding MRI/MRA studies at six and twelve months. He reported stable or resolved symptoms, along with improvement in some neurological deficits, correlating with a 63% reduction in nidus volume as demonstrated on follow-up imaging.

Conclusion:

This case underscores the potential of modern LINAC-based fSRS as a safe and effective therapeutic option for spinal AVMs, particularly when surgery or embolization is not feasible. The case also highlights the fSRS flexibility for personalized and case-specific modifications, accommodating each lesion's unique complexity and critical location within the spinal cord.