Timothy C. Hain,
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The "gamma knife", invented by Lars Leksell in 1971, is a rapidly evolving radiation treatment. It is well suited, and often the treatment of choice, for small tumors such as arise in the inner ear -- e.g. Acoustic Neuroma and Glomus. There actually is no "knife" in Gamma Knife -- Gamma "beam" would be a better name. The main alternatives to gamma knife are doing nothing (also called "watchful waiting"), and "true" surgery using a knife. Gamma knife is normally not used for pituitary adenoma's, malignant brain tumors, or dural arteriovenous malformations. Gamma knife is also avoided in persons with acoustic neuroma's due to neurofibromatosis, because of inability to restore hearing with implantable devices and questionable radiosensitivity.
This procedure instead works by converging beams of radiation. A "frame" is created by placing pins into the skull. This provides a reference position for the radiation source.Radiation beams, about 12-13 Gy, are administered to the target tumor. The tumor is localized by CT and MRI images.
Tumor and normal cells at the focus die in between 1-6 months, vascular and white matter brain damage occur about 6 months and later.
The name "gamma knife" is a poor one as there is no cutting involved at all.
The procedure has changed greatly over the last 3 decades. According to Wackym and associates (2004), it has now become appreciated that the high radiation doses used for malignant tumors in other contexts result in unacceptably high rates of late facial nerve and trigeminal nerve disturbances
Generally gamma knife is used for benign, slow-growing tumors, and one must compare the gamma-knife to doing nothing at all ("watchful waiting"), or more aggressive procedures such as true surgery. There has been some dissension about whether it is preferable to use radiation or surgery (or do nothing), and the literature sometimes seems a bit slanted.
For Acoustic Neuroma's, according to the radio "surgeon" literature, tumor "control" rate is 98.6%, meaning that the tumor stops growing quickly. It doesn't go away, however, and may continue to slowly enlarge. Facial nerve function is "preserved" in 100% -- meaning that it still works - -the author of this page has observed patients who develop facial twitching post gamma knife. Trigeminal nerve function is preserved in 95%. According to the radiosurgery literature, hearing is unchanged in 70%. (Lunsford, Niranjan et al. 2005; Paek, Chung et al. 2005). Other literature suggests that hearing tends to deteriorate no matter what treatment is used, and that it is unrealistic to expect that one will not eventually go deaf in the affected ear. The author of this page agrees with this idea.
In early studies where large radiation doses were used, delayed facial weakness, and facial numbness occured in roughly 1 third of patients after gamma knife. From the University of Pittsburgh series, an average dose at the tumor margin of 13 Gy units controls the tumor in 97% but with minimal (1%) facial nerve and (2%) trigeminal nerve disturbance. Doses of 16 Gy resulted in much higher rates of facial nerve disturbance (31%) and trigeminal disturbance (27%).
As of 1996, approximately 25 percent of all acoustic neuromas were treated with Gamma knife. In most instances the reason was that patients refused surgery. Gamma knife does not generally make acoustic tumors go away -- the figure above is actually that of a patient who had gamma knife surgery several years prior. Patients are typically followed with periodic MRI scans for the remainder of their lives. Tumor is "controlled" about 98% of the time with gamma knife using high doses, and is the only treatment needed in roughly 90% of patients using the newer lower dose protocol (Timmer, 2011). Hearing tends to gradually decline over several years in the treated ear. (Kaplan et al, 2003; Wackym et al, 2004). Vestibular function tends to worsen substantially in the first 6 months and remain stable thereafter (Wackym et al, 2004).
If surgery is eventually required after gamma knife, surgical complications in this situation such as severe facial nerve weakness are nearly 100%. Surgery is much more difficult (Limb et al, 2005). This occurs because the facial nerve often becomes "fused" to the tumor. Like surgery, hearing loss is common after gamma knife. In older studies, hydrocephalus has been reported to occur in between 3 and 12.8 percent (Noren et al, Pollock et al). Studies in which radiation dose is lower do not seem to have this complication (Wackym et al, 2004). Disequilibrium is reported in 8-31%, a figure which is analogous to surgical management. This is likely due to worsening of vestibular disturbance which is expected for both treatment modalities. Malignant transformation of acoustics after radiosurgery occurs extremely rarely (Wackym et al, 2004).
Occasionally tumors enlarge after treatment (Ho and Kveton, 2002), and some enlargement should be expected due to swelling in the first 6 months (Wackym et al, 2004).
The author of this review, while not a surgeon, 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 much better 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). Typical doses are 11.0 Gy (Timmer et al, 2011).
An interesting consequence of the low-dose protocols is that patients are now seen who do not have complete loss of hearing or vestibular function after the radiation. In some cases this can be annoying as it may result in nerve irritibility symptoms such as hyperventilation induced nystagmus. Here the nystagmus beats towards the lesion (unlike vibration induced nystagmus which beats away from the lesion).
An very strong example of HVT induced nystagmus is shown here in this movie. The procedure is to have the person deep rapidly and deeply for 30 breaths, and then observe under video Frenzel goggles in the dark. Many other movies can be found on the site DVD:
For Glomus tumors, tumor "control" is 95% over 10 years. Glomus tumors are difficult to remove operatively, watchful waiting is less likely, and thus the gamma knife may be more important than in other tumors (Pollock 2004; Gerosa, Visca et al. 2006)
All radiation is damaging, and the Gamma Knife is no exception. The beams of the gamma knife traverse normal tissue, and consquently damage normal structures. All radiation treatments also tend to cause late complications -- years later, patients develop facial twitching, facial numbness, or other symptoms that are more likely to be due to radiation than tumor growth.
The author has also seen several patients who developed a meniere's like syndrome in the opposite ear. This is most likely due to release of inner ear antigen into the circulation and formation of an immune response.
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