Elsevier

World Neurosurgery

Volume 113, May 2018, Pages e399-e407
World Neurosurgery

Original Article
Brainstem Dose Constraints in Nonisometric Radiosurgical Treatment Planning of Trigeminal Neuralgia: A Single-Institution Experience

https://doi.org/10.1016/j.wneu.2018.02.042Get rights and content

Highlights

  • With nonisocentric radiosurgery strategies, isodose plans accommodating a decreased maximum brainstem dose do not lead to poorer outcomes relative to historical controls.

  • A decrease in brainstem dosing constraints can maintain a comparably high level of pain control without increasing the risk of facial numbness.

  • A longer trigeminal nerve may confer an advantage in achieving durable pain control, and may augment outcomes from conservative brainstem dosing.

Background

CyberKnife stereotactic radiosurgery (SRS) for trigeminal neuralgia (TGN) administers nonisometric, conformational high-dose radiation to the trigeminal nerve with risk of subsequent hypoesthesia.

Methods

We performed a retrospective, single-institution review of 66 patients with TGN treated with CyberKnife SRS to compare outcomes from 2 distinct treatment periods: standard dosing (n = 38) and reduced dosing (n = 28). Standard and reduced dosing permitted a maximum brainstem dose of 45 Gy and 25 Gy, respectively, each with a prescription dose of 60 Gy. Primary and secondary outcomes were Barrow Neurologic Institute pain and numbness scores. Maximum brainstem dose, prepontine nerve length, and treatment history were recorded for their predictive contributions by logistic regression.

Results

After matching, patients in the standard dosing and reduced dosing groups were followed for a median of 25 months and 19.5 months, respectively. Mean trigeminal nerve length was 8.55 mm in the standard dosing group and 9.46 mm in the reduced dosing group. Baseline rates of poorly controlled pain were 97% and 88%, respectively, which improved to 23.4% and 8.3%, respectively (P < 0.001 for both). The baseline rates of bothersome numbness were null in both groups, and increased to 25% in the standard group (P = 0.006) and to 21% in the reduced group (P = 0.07). Regression analyses suggested that reduced brainstem exposure (P = 0.01), as well as a longer trigeminal nerve (P = 0.01), were predictive of durable pain control.

Conclusions

These outcomes demonstrate that a lower maximum brainstem dose can provide excellent pain control without affecting facial numbness. Longer nerves may achieve better long-term outcomes and help optimize individual plans.

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.

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