A Study of Dosimetric Feasibility of a New Approach of Radiosurgery for Multiple Sclerosis-Related Trigeminal Neuralgia

Multiple sclerosis-related trigeminal neuralgia (MS-TN) presents challenges due to multifocal demyelination and diffuse neural involvement. Conventional single-isocenter gamma knife stereotactic radiosurgery (SRS) treats approximately 6 mm of the trigeminal nerve, which is often inadequate for MS-TN. This study evaluated the feasibility of a double-isocenter technique to expand nerve coverage to 10 mm while maintaining brainstem dose limits.

A dosimetric analysis was conducted on 10 patients previously treated for trigeminal neuralgia (TN) using conventional SRS. The double-isocenter configuration involved manually positioning two isocenters of a 4 mm gamma knife collimator, with an average separation of 4.8 mm, to create a consistent dose distribution covering a 10 mm segment of the trigeminal nerve. The treatment was prescribed to the 50% isodose line. To compare the single- and double-isocenter plans, dosimetric parameters including geometric nerve coverage, maximum brainstem dose (Dmax), and brainstem volume receiving ≥10 Gy (V(10Gy)) were analyzed.

Replacing the single-isocenter with a double-isocenter configuration increased the prescription dose coverage along the trigeminal nerve from approximately 6.0 mm to 10.0 mm. Although the width of the 50% prescription isodose line perpendicular to the nerve axis slightly decreased from 5.9 ± 0.3 mm to 5.6 ± 0.5 mm, it remained sufficient to encompass the trigeminal nerve, which typically measures 2 mm to 3 mm in diameter. Similarly, the higher isodose lines, particularly the 80%, became slightly narrower, decreasing from 3.8 ± 0.2 mm to 3.1 ± 0.2 mm. This narrowing was effectively offset by an increased axial coverage along the nerve, from 4.0 ± 0.2 mm to 6.2 ± 0.8 mm. The mean brainstem Dmax was 23.4±2.9 Gy for single-isocenter and 22.5±3.3 Gy for double-isocenter plans (p=0.115), showing no statistically significant difference. The mean brainstem V(10Gy) increased from 0.054±0.011 cc (single-isocenter) to 0.079±0.024 cc (double-isocenter) (p<0.003).

The double-isocenter technique effectively expanded nerve coverage without compromising brainstem safety, as the brainstem Dmax remained within acceptable limits. Although the increase in brainstem V(10Gy) was statistically significant, its absolute value remained well below established tolerance guidelines, suggesting that the clinical impact is likely negligible. This approach demonstrates feasibility and supports future clinical trials to evaluate efficacy and long-term outcomes in MS-TN patients.