Hybrid isocenter technique for Gamma-Knife Perfexion treatment of trigeminal neuralgia

Gamma-Knife (GK), Perfexion (PFX) has 8 sectors that can be blocked compared with the older model 4C, which has 201 independent source blocking options using plugs. This limits the ability of PFX to shape a single isocenter plan, such as trigeminal neuralgia. In this retrospective review and prospective re-planning comparison study of trigeminal neuralgia radiosurgery plans, a hybrid isocenter technique (HIT) is compared with single isocenter GK radiosurgery plans on 2 models (4C and PFX) dosimetric parameters. With ethics approval, patients treated on 4C (n = 100) and PFX (n = 50) between 2005 and 2014 for trigeminal neuralgia were reviewed. All clinical plans were produced using 1 isocenter with a 4-mm collimator and a prescription dose of 80 Gy at 100% while limiting brainstem dose to < 15 Gy (D1mm3). Dose shaping was performed using plugs in 4C and sector blocking in PFX to optimize the plan, if required. Re-planning using HIT was retrospectively attempted for PFX plans with brainstem dose > 15 Gy. In the implementation of HIT, we used 2 shots at the same isocenter, but with different sector block configuration and weights, to further reduce brainstem dose while optimizing dose coverage of the nerve. Planning parameters such as dose to the treated nerve, brainstem, and surrounding tissue were compared between the clinical plans and the new plans with HIT. Source blocking was applied in 66/100 4C plans with mean blocked sources of 40.2/201 (ranging 1 to 99) and in 36/50 PFX plans with mean blocked sectors of 2.2/8 (1 to 4). There is no significant differences in mean dose, integral dose, irradiated volume of the treated nerve between 4C, and PFX plans. The 4C plans had smaller irradiated volume of tissue (90.6 mm3) than PFX plans (103.5 mm3) (p < 0.001). The irradiated volume of tissue increases by 17% when half of the sources are blocked for both 4C and PFX plans. In 9/50 PFX plans, brainstem D1mm3 was 15.8 Gy on average (15.2 to 16.5 Gy) and D10mm3 was 12.2 Gy (11.4 to 13.2 Gy). Re-plans of these 9 cases using HIT resulted in a significant reduction in D1mm3 to 13.5 Gy (12.2 to 15.1) and D10mm3 to 10.5 Gy (8.1 to 12.2) (p < 0.001). There was < 1% difference between the original PFX and re-planned HIT plans in dose to the nerve, and treatment time (p = 0.186). It is noted that the increase in the irradiated volume of tissue is proportional to the use of source blocking. Larger irradiated volume may be a reason that higher pain control rate and complication rate at the same time. Hybrid isocenter technique results in greater sparing of the brainstem without compromise in the dose to the nerve or treatment time on GK PFX planning for the treatment of trigeminal neuralgia.