Page 30 - Delaware Medical Journal - January/February 2020

Figure 3. BNI scale
SRS are Gamma Knife, dedicated linear accelerator devices, and CyberKnife.
In cases where patients are at high
risk for surgery, SRS is becoming
more commonly used as an alternative method of treating TN by ablating the dorsal root of the trigeminal nerve with a high dose of radiation. The advantages of SRS versus surgery include: 1) it is non-invasive; 2) treatment is usually completed within 30 minutes; 3) there
is minimal acute toxicity. SRS avoids potential complications of anesthesia as well as the risks of bleeding or infection. A disadvantage of SRS as opposed to surgery is that SRS takes longer to provide pain relief, usually within six weeks but sometimes up to three months.
The Gamma Knife is a large, helmet- like device that holds 201 Cobalt-60 sources and is used to administer high- dose radiation.10 Pins are implanted
into the patient’s skull in order to immobilize the head during treatment. Because each beam is high-energy and converges on a target, Gamma Knife does not damage surrounding tissue and combines to deliver a powerful dose
to the target. As a result, conventional Gamma Knife cannot be fractionated, or be divided into multiple lower doses.
A LINAC, or linear accelerator, is a machine that emits high-energy X-rays that can be focused to deliver a high dose of radiation. CyberKnife is a robotic device with an arm containing a miniaturized LINAC with a real- time, stereoscopic X-ray tracking system that can account for patient motion, thus making it highly precise to 0.5 mm for skull targets. Due to this, patients do not need to be immobilized with pins screwed into the skull.
In addition, because the LINAC is mounted on a non-isocentric robotic arm that can move to all sides of the patient, radiation doses can be designed to deliver low doses at all angles of
the target. This leads to a high dose directed at the target, while delivering very low doses to the surrounding healthy tissue. Because CyberKnife
is LINAC-based, it can also deliver a single high dose to a target or can be split into multiple smaller daily fractions.
For instance, instead of treating someone with 65 Gy in a single fraction, using the CyberKnife, one could treat with three fractions of 33 Gy each to give the normal tissues time to heal between fractions.
RESULTS WITH SRS
The most extensive data on the use
of SRS for TN has been accumulated at the University of Pittsburgh using
a Gamma Knife device. A study conducted by Pollock et al, between April 1997 and December 1999, with trigeminal neuralgia treated to
a single point on the dorsal nerve

dose of 70 Gy and 41 received a high dose of 90 Gy. The researchers found that at an average follow-up of 14.4 months (range two-36 months), 41%
of low-dose and 61% of high-dose patients remained pain free. However, bothersome numbness occurred in 32% of high-dose patients, as opposed to
in 3% of low-dose patients. Pollock
et al concluded that while high-dose SRS results in a higher incidence of pain reduction, it also leads to a higher incidence of numbness and as such, they recommended an intermediate dosage.11 They suggested the inferior results compared to surgery may
be due to selection bias. Generally, patients with atypical symptoms or no neurovascular compromise on MRI imaging were more likely to undergo SRS rather than surgery.
The results of single-fraction CyberKnife treatment were first published by Dr. Adler from Stanford University. He reported on a technique treating the entire course of the trigeminal nerve in the cavernous sinus rather than a single point and reported superior pain relief to Gamma Knife.12 Another study, conducted
at Georgetown University Hospital between 2002 and 2013 by Karam
et al,13 looked at the radiosurgical results of 25 patients with medically intractable TN. The authors treated patients using the Stanford technique and reported that 42.9% of patients experienced some pain relief within
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Del Med J | January/February 2020 | Vol. 92 | No. 1