RADIOSURGERY TREATMENT OF ACOUSTIC NEUROMA

Timothy C. Hain, MD • Page last modified: February 20, 2022

Related pages: acoustic neuroma treatment of acoustic neuroma.

From https://gammaknife.com/what-is-gamma-knife/

There are three distinct management options:

Here we are going to discuss the third option, use of radiation (gamma rays, protons, or X-rays) to treat acoustic neuromas, and compare this with other types of treatment. We will use the jargon of "acoustics" for a shorthand of "acoustic neuroma". As a general observation, over the last 30 years (through 2018), less and less surgery is being done for acoustics, and more "watchful waiting" and radiation type treatments.

Gamma Knife -- stereotactic radiotherapy using gamma rays.

gammaknife: https://upload.wikimedia.org/wikipedia/commons. This shows how all of the beams converge onto a single target, which is aligned in the machine using previously obtained Xray markers (stereotaxic positioning).

According to Carlson et al (2015), about 29% of acoustics are initially treated with radiation. When the risk of surgery is high because of other medical problems, or where the patient simply refuses surgery, the "gamma knife" or an X-ray or Proton beam procedure may be used. These are all methods of irradiating the tumor, hoping to kill it directly or damage the blood supply to the tumor sufficiently so that it starves.

The Gamma knife was invented by Lars Leksell in 1971. The head is placed in a stereotactic "Frame", and radioactive cobalt is used to expose the tumor to multiple intersecting beams. The GammaKnife is used at the author's institution, Northwestern. https://www.nm.org/conditions-and-care-areas/treatments/gamma-knife-radiosurgery)

Gamma knife is an "old" technology (invented in 1971). One might wonder how it compares with newer methodology, such as the "cyberknife" (invented in 1987, see below). One observation is that the gamma knife cobalt sources are relatively close to the head -- in theory, this might result in greater beam spread and collateral damage than procedures with longer beams (e.g. linac). As gamma sources emit radiation in all directions, there are an intrinsic trade-off between beam spread and beam intensity.

The gamma knife procedure as well as other types of radiation treatment avoids surgery with its attendant risks. In the past, this option was usually recommended only for higher risk surgical cases because of the possibilities of late radiation complications, and the need for ongoing MRI monitoring of the results of the procedure.

Billing for this procedure is high, as well as all other methods of stereotactic radiotherapy, so this is not something that can be generally used without health insurance.

As acoustic neuromas recently seem to have become more benign, one might argue that radiation is being overused, and that there should be more "medical management". This is called "watchful waiting", and we do think this is pretty reasonable.

Acoustic neuroma's are generally not "killed" by radiation. They are damaged, they are scarred, but it is just not the nature of radiation to completely obliterate tumors. (Jacob et al, 2015) For this reason, periodic MRI scans are prudent post gamma knife surgery. Generally speaking, this is once/year and then less often if there is no change.

 

Newer variants of stereotactic radiotherapy of acoustic neuroma (including X-ray and Proton)

This is really just another way to radiate the tumor.

Cyberknife (from https://en.wikipedia.org/wiki/Cyberknife#/media/File:CyberKnifeSchematic2.png)

Radiation other than gamma rays can also be used to treat acoustic neuroma. They include Xrays from a linear accelerator (LINAC), such as the Cyberknife. The Cyberknife is one brand of a robotic system that integrates together a robot used for positioning, a small linear accelerator (i.e. X-ray machine), and a diagnostic X-ray system. The CyberKnife is similar to gamma knife in overall features and use. The Cyberknife was invented by John Adler in 1987, roughly 16 years after the gamma knife was invented. The Cyberknife is available at many locations in Chicago (unlike the GammaKnife).

Proton therapy is another method of irradiation -- protons are much larger than electrons (X-rays), and in theory, might be less likely to cause collateral damage. This theory regarding better outcome has yet to be proven as of 2017. We are not so sure that we understand this concept that a proton might be less damaging than an electron or gamma ray either, and think that there are more important things to consider if one is radiation shopping (such as experience and results of the facility doctors), at least right now, than the type of radiation.

The Cyberknife and proton beam differ from the GammaKnife in that it uses X-rays or protons rather than gamma rays. In addition, the CyberKnife is "Frameless". The positioning of the Cyberknife is done by a robot, making it somewhat more flexible than the helmet/frame positioning system of the GammaKnife. How do these two methods differ in outcome ? We know of no head-head studies in Acoustic Neuroma as yet (pun intended).

In theory, radiation such as the CyberKnife might be better collimated (i.e. like a laser) compared to GammaKnife, as the longer the path that the beam travels, the less beam spread. According to Schoonbeek et al (2010), all available systems have problems targeting small tumors (defined as < 0.5 cc volume), so in other words, the problem is bigger for smaller targets. This is really just common sense. It appears that what has happened is that the design criteria for both CyberKnife and GammaKnife have resulted in devices with similar performance. It would have been nice, in our opinion, to design a clearly superior device.

If you are told that this method or that method is clearly superior -- be cautious concerning whoever is telling you this, and use common sense. We suggest trying to find someone to give you advice about destructive treatments who doesn't benefit one way or the other from whatever direction of treatment you choose. In other words, get an opinion, ideally from someone in a different health care system and who has no "iron in the fire". If someone tells you that gamma-knife or cyberknife can "preserve your hearing", as it magically can kill tumor without killing normal tissue, we think you should ask them how does radiation know the good cells from the bad ? Perhaps best to find another doctor if they say too many things contrary to evidence based medicine.

Not all that surprisingly, whether you cut something out right next to the brainstem and other working nerves or you blast it with radiation, there is a high likelihood of collateral damage. With surgery, it would seem conceivable that one might take the tumor out without damaging other things, but practically speaking, this seems to be a rare outcome. With blasts of radiation, it is obviously magical thinking to suppose that one can just kill the tumor and leave everything else spared, because radiation goes through lots of normal tissue. When the tumor dies, that normal tissue is going to be damaged too.

 

Hearing prior to gamma knife
Hearing prior to Gamma Knife
1 month post
Hearing shortly after Gamma Knife -- big mid-frequency notch
6 months post
Hearing 6 months post Gamma Knife. Smaller difference between ears.

 

Failed radiosurgery.

Mahboubi et al (2017) reported that "tumor control' for cyberknife was 95.3%. We also find this statistic to be a vague one, but do agree that in the author's experience, it would be rare for cyber/gamma knife to fail to stop the tumor from enlarging further. Rare is not the same as impossible, and below is an example of the opposite situation.

Pre radiosurgery MRI

Pre-radio surgery

Post radiosurgery MRI

In some patients, while there is good evidence that the radiation "did something", such as a transient loss of hearing as above, the tumor keeps growing. This is the reason for surveillance after the radiosurgery. . In the patient above, the portion of her tumor close to the brainstem, outside of the internal auditory canal, is larger 2 years after the radiation. The rest of the tumor is constrained by the bony margins of the IAC, and has no place to go.

Doing nothing (watchful waiting) is not a reasonable option here, and the two remaining are more radiosurgery or open surgery. Generally speaking, the reasoning would be that the radiosurgery didn't work the first time, and one should go to open surgery. Howeve, if not possible (perhaps due to general medical condition), then repeat radiosurgery would be the best option.

Short term worsening of symptoms after gamma knife:

In the short term, gamma knife may cause damage to hearing (presumably) due to damage to the 8th nerve from swelling. This may recover, but usually will be followed by long term hearing loss as the the tumor and normal 8th nerve is damaged by radiation. There is also a fairly high prevalence of dizziness -- about 20%, in the short term, after gamma knife (Horiba et al, 2016). According to Tuleasca et al (2016), "Acute effects after radiosurgery for VS are not rare. They concern predominantly de novo vertigo and gait disturbance and the exacerbation of preexistent hearing loss. In de novo vestibular symptoms, a vestibular dose of more than 8 Gy is thought to play a role."

Don't expect to be cured -- acoustic neuroma tumors do not "vanish" with gamma knife.

Long term hearing preservation is very rare in persons with stereotactic radiotherapy (6.7% according to Lin et al, 2005). In other words, although the "goal" is to preserve hearing, practically this is unrealistic.  The main reason to seek out stereotactic radiotherapy rather than gamma knife, is the theoretical one of beam spread. The chance of recurrent tumor using current dose regimens is roughly 5-10%. Tumor growth is rare in patients who remain stable 6-7 years post therapy.

Contrary to the idea that long term hearing preservation for radiotherapy is just not very likely, recently Mahboubi et al (2017) reported "hearing preservation was 79.1% after Cyberknife treatment. Cakir et al (2018) reported 69% preservation at 16 months. This may be an apples/oranges comparison, as Lin discussed long-term hearing preservation, while the aforementioned authors reported on "hearing preservation". Radiotherapy damage is almost always delayed (see bottom of this page about vestibular damage). We think Lin is more likely to be correct here and that it is imprudent to expect hearing preservation after a destructive treatment of any kind for acoustic neurinoma.

More recently, Gallogly et al (2018) reported 'RESULTS: Forty patients met inclusion criteria. Twenty-two underwent CyberKnife stereotactic radiotherapy. Eighteen remain in watchful waiting. Crude tumor control was 86.4% at mean radiographic follow-up of 52.3 months. Kaplan-Meier progression-free survival was 76.9% at 5 years. Kaplan-Meier survival from radiographic growth was 61.5% at 5 years. Kaplan-Meier hearing preservation was 17.5% at 5 years. All patients undergoing watchful waiting presenting with serviceable hearing maintained serviceable hearing. Serviceable hearing among CyberKnife stereotactic radiotherapy patients was 42.9% prior to treatment and 14.2% through mean follow-up of 53.7 months. One patient experienced trigeminal nerve toxicity 45 months after SRT. 95.5% of CyberKnife stereotactic radiotherapy patients were complication-free." In other words, preservation of "servicable hearing" is better if you do nothing (watchful waiting), than if you use radiation. There is no free lunch.

A case example of a patient who did not have hearing preservation is shown here.

General problems with radiotherapy of any kind (i.e. gamma, X-ray, proton)

Issues in radiotherapy are recurrence (5-10%), hearing loss (eventually 93%), risk of radiating large (>2 cm) tumors due to swelling of the tumor in the first year, risk for malignancy (Tanbouzi et al, 2011; Markou et al, 2011), hydrocephalus (rare), ruptured IAC aneurysm (rare), and accelerated vertebrobasilar atherosclerosis (e.g. Jackler, 2007).

Any kind of radiation treatment can cause collateral damage. The facial nerve is very close to the 8th nerve, and facial nerve neuropathy, trigeminal neuropathy, as well as hearing loss are all potential consequences. The auditory artery can also be damaged by radiation causing narrowing or rupture. Malignant transformation is a risk of all types of radiation therapy -- this is rare and we have not encountered any comparison data between the two radiation methods (gamma, X-ray).

Muzevic et al (2014), in a Cochrane review, stated that "There is no high quality evidence in the literature from RCTs to determine whether stereotactic radiotherapy is better than microsurgical resection or observation alone for patients with a vestibular schwannoma." So in other words, one must rely on common sense. The trend now is towards doing nothing (i.e. observation alone).

The author of this review does not favor high-dose gamma knife because of the possibility of radiation complications at 2 years and beyond. However, low dose gamma knife is looking quite reasonable now as of 2017 and there are certainly many times when it is the best option. Lower doses of radiation (e.g. 13 Gy) are presently advised because of the much lower risk of facial weakness and numbness (Wackym et al, 2004).

 

Lack of response to gamma knife 2 years later
Pre Gamma Knife acoustic
Post gamma knife

Occasionally acoustic neuromas continue to grow after radiotherapy. This is more common recently as radiation has been "cranked down" to avoid side effects. Illustrated above is an acoustic neuroma, imaged 2 years (bottom) after gamma knife. The tumor is now much larger -- so the radiosurgery failed. For situations like thhs, open surgery is the main option. Interestingly, this patient had good evidence for radiation effect (HVT nystagmus, see below), but the tumor grew. On the other hand, their hearing was good post radiotherapy -- this feature might predict poorer response to radiation.

Hyperventilation induced nystagmus from radiotherapy for acoustic neuroma.

Video of hyperventilation induced nystagmus in patient with left sided acoustic neuroma

An interesting consequence of the lower-dose radiation protocols is that patients are now seen who do not have complete loss of hearing or vestibular function after radiation. In some cases this can be very annoying as it may result in nerve irritability symptoms such as hyperventilation induced nystagmus. This is probably a consequence of using a treatment methodology that works more slowly than surgery. It seems likely that this complication is more common in persons who have small tumors.

Hyperventilation induced nystagmus for this situation beats towards the lesion (unlike vibration induced nystagmus which beats away from the lesion). It is often very powerful. In persons with this sign, one can either wait for it to go away (this may take several years), try a medication that reduces nerve irritability, or reconsider surgical treatment. We don't think the medications work well if at all, so time is the more common approach.

Increased dizziness due to radiation damage after radiotherapy.

Another issue is that the damage from radiation for acoustic neuroma is not immediate, but grows over time. It takes about 2 years for radiation damage to complete. An example of this is shown here. Usually everything is over by 2 years however.

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