Acoustic neuroma radiation therapy

Radiation therapy
It is done in a variety of ways, but mainly by two methods: gamma knife radiosurgery or fractionated stereotactic radiotherapy. In the gamma knife approach, 201 beams of gamma radiation are focused on the tumor in a single session. The damage to the tumor at the convergence point may cause it to stop growing but usually does not cause it to shrink in the long term. It may cause short-term shrinkage due to necrosis in the tumor. The damage may be to the tumor cells and/or to the tumor vasculature.

It is not clear what percentage of tumors are controlled by this method for long periods. In earlier times when higher radiation doses were used, the failure rate was about 12% (which then required surgery). Most surgeons feel that these tumors are much more difficult to remove after radiation treatment. Radiation does not remove the tumor, and when irradiated tumors are surgically removed, it is often found that they have growing tumor cells in them.

Two risks of radiation treatment are carcinogenic progression of the acoustic neuroma (conversion from benign tomalignant) or induction of other tumors (such as glioblastoma) in the nearby irradiated brain tissue. The incidence of these events appears to be low, and it is often said to be one in one thousand or less. (However, the incidence is markedly higher in patients with neurofibromatosis type 2.) This calculation is done by dividing the number of obvious cases oftumorigenic progression or secondary tumor reported in the medical literature by the estimated number of gamma knife procedures done in the world to date. This is not a scientifically valid method of estimating the carcinogenic risk of medical radiation exposures, and involves a list of very questionable assumptions.

The proper and scientifically valid way to estimate such risks can be found at the web site of the Health Physics Society (http://www.hps.org/), where estimates of the risks of CT scans and other procedures can be found. These calculations have never been made for gamma knife radiosurgery.

Due to the possibility of regrowth and the possibility of tumorigenic progression or secondary tumors, it is essential that radiation treatments for acoustic neuromas be followed by yearly MRI for the rest of the patient's life. MRI at this time (2007) cost about 300€. Long-term secondary effects (for instance cognitive effects) on a scale of 10-20 years are not yet established for gamma knife surgery.

Fractionated stereotactic therapy involves a beam of ionizing radiation focused on the tumor from a moving gantry. The beam is wider and less accurate than that of the gamma knife. The total dose is also much higher than that used in gamma knife radiosurgery, but the fractionation of the dose (done on many different days) spares normal tissue. This method has not been done on as many patients as gamma knife procedures and there have not been as many years of follow-up study. This means that the tumor control by this method is not yet established, and the incidence of secondary effects of the radiation are not yet known.