Practical considerations of linear accelerator-based frameless extracranial radiosurgery for treatment of occipital neuralgia for nonsurgical candidates

Occipital neuralgia generally responds to medical or invasive procedures. Repeated invasive procedures generate increasing complications and are often contraindicated. Stereotactic radiosurgery (SRS) has not been reported as a treatment option largely due to the extracranial nature of the target as opposed to the similar, more established trigeminal neuralgia. A dedicated phantom study was conducted to determine the optimum imaging studies, fusion matrices, and treatment planning parameters to target the C2 dorsal root ganglion which forms the occipital nerve. The conditions created from the phantom were applied to a patient with medically and surgically refractory occipital neuralgia. A dose of 80 Gy in one fraction was prescribed to the C2 occipital dorsal root ganglion. The phantom study resulted in a treatment achieved with an average translational magnitude of correction of 1.35 mm with an acceptable tolerance of 0.5 mm and an average rotational magnitude of correction of 0.4 degrees with an acceptable tolerance of 1.0 degrees. For the patient, the spinal cord was 12.0 mm at its closest distance to the isocenter and received a maximum dose of 3.36 Gy, a dose to 0.35cc of 1.84 Gy, and a dose to 1.2cc of 0.79 Gy. The brain maximum dose was 2.20 Gy. Treatment time was 59min for 18, 323MUs. Imaging was performed prior to each arc delivery resulting in 21 imaging sessions. The average deviation magnitude requiring a positional or rotational correction was 0.96 +/- 0.25 mm, 0.8 +/- 0.41 degrees, whereas the average deviation magnitude deemed within tolerance was 0.41 +/- 0.12 mm, 0.57 +/- 0.28 degrees. Dedicated quality assurance of the treatment planning and delivery is necessary for safe and accurate SRS to the cervical spine dorsal root ganglion. With additional prospective study, linear accelerator-based frameless radiosurgery can provide an accurate, noninvasive alternative for treating occipital neuralgia where an invasive procedure is contraindicated.