Original ArticleBrainstem Dose Constraints in Nonisometric Radiosurgical Treatment Planning of Trigeminal Neuralgia: A Single-Institution Experience
Introduction
Trigeminal neuralgia (TGN) is characterized by paroxysmal facial pain that has a profound impact on quality of life. Patients with TGN that is refractory to medication may benefit from less-invasive stereotactic radiosurgery (SRS).1, 2, 3 Because prescription dosing for single-fraction treatment is comparatively high relative to other SRS procedures, concerns for radiation overexposure to critical structures have led to more conservative treatment plans. These dose constraints may have a particular significance in nonisocentric treatment plans, where a less-steep dose dropoff leads to a greater irradiated volume of the brainstem.4 Typical brainstem dosing limits have ranged from 22.5 to 45 Gy to minimize post-SRS complications, particularly trigeminal nerve dysfunction.5, 6, 7, 8, 9, 10, 11 Pain relief and recurrence rates also have ranged widely in various nonisometric series, from 67% to 87% and from 15% to 33%, respectively.5, 11, 12 Thus, the extent to which brainstem dosing strategy governs outcomes warrants further exploration. In this study, we investigated the role of maximum brainstem dosing in achieving pain control, and we hypothesize that improved outcomes are more likely in patients with longer trigeminal nerves with their associated, larger treatment planning space.
Section snippets
Patient Selection and CyberKnife SRS Planning
Patients treated for TGN between April 2005 and June 2011 (standard dosing group; n = 38) and between July 2011 and September 2015 (reduced dosing group; n = 28) with at least 1 year of follow-up were included (Table 1). All patients were treated by CyberKnife SRS at Stanford University Hospital. The standard and reduced dosing plans targeted the trigeminal nerve at 2 mm and 2.5 mm from the brainstem, respectively, with a maximum brainstem dose of 45 Gy and 25 Gy, respectively (Figure 1).
Treatment Population and Parameters
After cohort weighting, the standard dosing plan included 38 patients (27% females), with a mean age of 69 years, and a median follow-up period of 25 months. Previous intervention had been sought by 50% of patients, including 33% with microvascular decompression, 13% with previous SRS, and 9% with injections. The mean trigeminal nerve length was 8.55 mm. After cohort matching, the reduced dosing group comprised 24 patients (18% females), with a mean age of 71 years and a median follow-up period
Discussion
Here we present the first cohort comparison of 2 treatment plans that investigated reduced brainstem exposure using nonisocentric radiosurgical planning. Adjusting the planned maximal brainstem exposure from 45 Gy to 25 Gy did not produce statistically significant changes in symptom frequency or timelines between treatment groups. Nevertheless, this work uniquely suggests that reduced brainstem dosing and patient anatomy may have long-term implications for pain relief.
Conclusions
Based on our present findings, there is evidence to support the continued optimization of TGN SRS treatment in dosimetry planning and patient selection. Although a mean difference of 20 Gy in maximum brainstem exposure yields comparable rates of pain control and facial dysesthesias, longer follow-up should be performed to affirm an associated predisposition for durable pain relief. In addition, larger sample sizes are needed to assess anatomic thresholds that are sensitive and specific for
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Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.