Another treatment option for an acoustic neuroma is radiation. Stereotactic radiation can be delivered as single-fraction stereotactic radiosurgery (SRS) or as multi-session fractionated stereotactic radiotherapy (FSR). Both techniques are performed in the outpatient setting, not requiring general anesthesia or a hospital stay. The purpose of these techniques is to arrest the growth of the tumor causing the tumor to die, which is called necrosis.
In single-dose treatments, many hundreds of small beams of radiation are aimed at the tumor. This results in a high dose of radiation to the tumor and very little to any surrounding brain structures. Many patients have been treated this way with high success rates. Facial weakness or numbness, in the hands of experienced radiation physicians, occurs in only a small percent of cases. Hearing can be preserved in some cases, with a slightly greater opportunity with FSR.
The multi-dose treatment, FSR, delivers smaller doses of radiation over a period of time, requiring the patient to return to the treatment location on a daily basis, from 3 to 30 times, generally over several weeks. Each visit lasts a few minutes and most patients are free to go about their daily business before and after each treatment session.
The treatment team should consist of a neurosurgeon, and/or a neurotologist (ear and skull base surgeon), a radiation oncologist and a physicist. Follow-up after SRS and FSR typically involves an MRI scan and an audiogram at six months, one year, then yearly for several years, then every second or third year indefinitely to make sure the tumor does not start to grow again.
Patients should understand that all types of radiation therapy for acoustic neuromas may result in “tumor control” in which the tumor cells die and necrosis occurs. Tumor control means that the tumor growth may slow or stop and, in some cases, the tumor may shrink in size. In almost no cases have acoustic neuroma tumors been completely eliminated by radiation treatments. In other words, radiation does not remove the tumor like microsurgery can. Furthermore, radiated patients require lifetime follow-up with MRI scans. Tumors under 2.5 – 3.0 cm, without significant involvement of the brainstem, are more favorable for radiation treatment. Side effects can occur when the brainstem is irradiated and in cases of large tumors, radiation is contraindicated. Patients should understand there have been rare reports of malignant degeneration (a benign tumor becoming malignant) after radiotherapy.
In some cases, the tumor does not die and continues to grow. In those instances, another treatment is necessary – either microsurgery or another dose of radiation. Retreatment must be done as always, in the hands of experienced physicians.
Several types of machines deliver focused radiation treatment suitable for treating acoustic neuromas, such as Gamma Knife® and linear accelerator (LINAC), such as CyberKnife®, Novalis® and Trilogy®. The underlying premise is to treat the tumor with a high dose of radiation while sparing the nerves and brain tissues. Much of the long-term data comes from the Gamma Knife literature since this was one of the earliest techniques used to radiate acoustic neuromas on a large scale.
The Gamma Knife uses 195-201 fixed Cobalt-60 radiation sources that are “collimated” to intersect at the site of the tumor and is a single-dose treatment. In this way, each individual beam of radiation has very little effect, but where they all intersect produces a maximum effect on the tumor. Very similar results can be obtained using a linear accelerator (LINAC) as the radiation source, such as with the Novalis or CyberKnife with multi-dose treatment.
Studies are beginning to appear for the other modalities. All of the techniques use computers to create three-dimensional models of the tumor and surrounding neural structures. Radiation physicists then create dosimetry maps showing the level of radiation to be received by the tumor and the normal tissues. Surgeons, radiation therapists and physicists then modify the dosimetry to maximize tumor doses and minimize radiation toxicity to surrounding normal tissues. The head is stabilized with a metal frame pinned to the head (Gamma Knife) or a fitted mask shield (CyberKnife, linear accelerator, fractionated XRT). Treatments generally last 30-60 minutes. Just like for surgery, the experience of the team in treating acoustic neuromas with all modalities (surgery and radiation) can affect outcomes.
There are a multitude of studies supporting short-term (<5 yrs.) and long-term (over 10 yrs.) tumor control with radiation. Unfortunately, as is the case with microsurgical studies, most have inconsistent follow-up to draw definitive conclusions.