As Mark said, it is difficult to prove a negative, especially when there aren't enough cases to study. However, that won't stop me from trying.
If you are considering radiosurgery, though, it is worth taking a look at the serious literature on the subject. You can't go wrong to start off with U. of Pittsburgh and Dr. Kondziolka, who changed the perception of radiosurgery for many. The main web site is here:
http://www.acousticneuroma.neurosurgery.pitt.edu/index.htmlThere is a wealth of good information there under various categories. It is worth noting that much of it dates to 1998, and includes a study of those treated between 1984 and 1992, giving them some 10-15 year results at the time. Some of those patients are near the 25 year mark now, if they haven't passed away from old age in the mean time. Many were treated under the old protocol, with a radiation dose twice what the standard today is. The reference list at that site includes many useful articles.
At the bottom of this page (
http://www.acousticneuroma.neurosurgery.pitt.edu/or.html), Kondziolka says:
"Malignant schwannomas are rare, but have been reported de novo, after prior resection (34), and after irradiation. We answer that this is always a risk after irradiation, but that the risk should be very low. We have not seen this yet in any of our 5,400 patients during our first 15 years experience with radiosurgery, but quote the patients a risk between 1:1000 and 1: 20,000." The next article,
http://www.ncbi.nlm.nih.gov/pubmed/17704364, 2007, was quoted by HEI in the last year as a way of warning people off of radiation, despite its actual content. Here are some quotes:
"There are currently 19 reported cases of tumors linked with stereotactic radiotherapy/radiosurgery, to which we add (one more)..." Thats not 19 ANs; that is the number for all benign tumor radiosurgery treatments, worldwide, ever.
"The exact carcinogenic risk after radiotherapy is unknown but likely extremely low. However, the risk is not zero and requires discussion with the patient, with specific consideration in young patients and those with a cancer predisposition." Fair enough, as long as "likely extremely low" is part of the discussion.
A recent overview of radiosurgery, dated Oct 2008:
http://emedicine.medscape.com/article/857604-overview"Eight case reports of malignant transformation of acoustic neuroma can be found: 2 followed external beam radiotherapy, 2 followed stereotactic radiotherapy, and only 4 were spontaneous. Although lacking statistical power, this comparison clearly highlights the greater risk of malignant transformation of acoustic neuromas following irradiation." It clearly highlights something, anyway.
How many people have had radiation for an AN? This editorial, from 2006, says 21,000, starting in 1969:
http://thejns.org/doi/full/10.3171/jns.2006.105.5.655This editorial is from the Journal of Neurosurgery, with a full reference section if you want to dig in. The response at the end concludes:
"The issue of malignant tumorigenesis has recently been the subject of much discussion. It probably exists but at a frequency tremendously lower than the risk of a serious complication from open surgery. Let’s use our hard-won microsurgical skills on larger tumors and accept the facts about radiosurgery discussed in our paper and many others."It is worth noting that a malignant AN is rare enough that each case warrants its own published article, and as far as I know, no one has documented a case of someone dying from one, or of one metastasizing (spreading). In the end, it is up to you to decide if the risk is acceptable. If I ever get one, I will just have it zapped with CK.
Steve
