P. Ashley Wackym, MD, is the John C. Koss Professor and Chairman, Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin in Milwaukee.
There are three management options for patients with acoustic neuromas: microsurgical removal, stereotactic radiosurgery, and expectant management. Since there are relative advantages and disadvantages of each and every patient and his or her tumor requires special consideration, it is important that physicians caring for these individuals factor these issues into their counseling. In my practice, I have a strong experience in all three of these management options; the direct experience in performing both microsurgical removal and Gamma Knife® radiosurgery has helped me become much better at both of these techniques. My having this direct experience enables each patient and his or her family to participate in a balanced discussion of these options, which is essential for a true informed consent to proceed with one of the three management options.
Why Should a Neurotologist or an Otolaryngologist Perform Gamma Knife Radiosurgery?
The core group of participants in the radiosurgery team includes a radiation oncologist, a radiation physicist, and a surgeon. While traditionally the surgeon has been a neurosurgeon, because most of the applications are for the treatment of brain metastases and arteriovenous malformations, there is no reason that other surgical disciplines focused on the care of patients with diseases of the head and neck could not use this technology. I first became interested in Gamma Knife radiosurgery because of the treatment of patients with acoustic neuromas. After receiving numerous patient referrals from neurosurgeons, it became clear that they were in no position to manage the complications of treating small to medium-sized acoustic neuromas, including hearing loss, vestibular dysfunction, and facial and trigeminal nerve dysfunction.
In addition, based on my colleagues’ experience and training, and being able to generalize these observations after review of the world’s literature, it was clear that a neurotologist as Gamma Knife radiosurgeon was in a far better place to treat and manage the complications of treatment of acoustic neuromas than a neurosurgeon. As of January 2007, there are 42 neurotologists who have been trained to perform Gamma Knife radiosurgery, according to the International Radiosurgery Association. There is no question that our involvement will change the way patients with skull base diseases are treated.
The next generation of the Gamma Knife technology, termed the Perfexion™, will allow single-day treatment of benign or malignant lesions of the skull base, head and neck and structures down to the clavicles. Consequently, all otolaryngologists should be considering training and performance of Gamma Knife radiosurgery. In addition, there are other forms of stereotactic radiosurgery or radiotherapy that are available and there many neurotologists have been trained and use these systems.
What About Outcomes Data and Evidence of Clinical Efficacy?
It is important to understand all the issues and variables related to treatment, assessment of outcomes, and means of improving therapy. In the Medical College of Wisconsin Acoustic Neuroma and Skull Base Program, we established a protocol for all our patients undergoing Gamma Knife radiosurgery for primary or secondary treatment of their acoustic neuromas, which has been adopted by most neurotologists who now use this technology to treat patients with acoustic neuromas.
Following completion of the stereotactic radiosurgery, at six-month intervals, each patient undergoes a gadolinium-enhanced MRI as well as an audiometric test battery and caloric testing to assess peripheral vestibular function. Pretreatment they undergo a complete electronystagmography test battery to measure balance function, a complete audiometric assessment to measure hearing function, and facial nerve electromyography to measure facial nerve function. We have analyzed the early outcomes data and have summarized these data in an initial publication.1 Although there were a few patients who improved, most of the patients progressively lost hearing and balance function. The hearing and balance function outcomes over time were reported, as well as presenting these data for individual patients over time. This was unique in the stereotactic radiosurgery literature.
We have recently reassessed our outcomes with all of our patients and some patients have more than six years of follow-up after Gamma Knife radiosurgery. The outcomes, while similar to our preliminary findings, have emerged with an interesting pattern. We are now recognizing that the largest changes in vestibular function, pure tone averages, and speech discrimination occur within the first six months. This opens the possibility of intervention by the neurotologist when these acute changes occur. Clearly the need for multi-institutional clinical studies focused on determining the efficacy of specific interventions during this critical interval will need to be performed.
Is There Evidence of Reduction in Tumor Size over Time?
Another issue that needs to be addressed is tumor growth after radiosurgery. Based on our assessment of treatment outcomes, it is important to appreciate that there is an increased size of the tumor after radiosurgery. Typically this post-treatment edema will persist for six months; however, this may remain for up to one year. Consequently, pretreatment counseling should include these facts.
It is also the case that uniform methods to measure and report tumor size have not been used in papers published in the neurosurgery literature regarding Gamma Knife radiosurgery of acoustic neuromas. In fact, it has only been in the last few years that statistical comparison of tumor size before and after Gamma Knife radiosurgery treatment of acoustic neuromas has been completed and we published these outcomes in 2004 and 2005.1,2 There have been single cases discussed and occasionally reported that describe increased tumor size early after radiosurgery. The challenge is in making a decision about whether to operate these tumors and when. Pollock and colleagues3 emphasized the need to demonstrate sustained tumor growth by serial MRI before making the decision to operate, and also to review the case with the surgeon who performed the radiosurgery before a surgical decision is made.
The other related controversy is whether the facial nerve dissection and subsequent preservation are more difficult during microsurgical resection after radiosurgery. On one end of the spectrum, descriptions of no increased difficulty have been reported, and on the other end of the spectrum, markedly increased difficulty in separating the tumor from the facial nerve and poorer facial nerve function outcome has been reported. The report of Watanabe et al.4 included a histopathologic analysis of the resected facial nerve. They found microvasculitis of the facial nerve, axonal degeneration/loss of axons, and proliferation of Schwann cells. In light of the mechanism of delayed effects following radiosurgery, these findings are not surprising and based on my experience, the facial nerve is compromised and consequentially the neurotologist must be certain that the treatment plan avoids high radiation doses adjacent to the facial nerve. For this reason, if the neurotologist/neurosurgeon team and the patient have made a decision to resect a tumor previously treated with radiosurgery, it is important to review the treatment plan to determine the amount of radiation delivered to the facial nerve in order to appropriately counsel the patient preoperatively.
What Trends Should We Be Worried About for Our Patients?
A concerning early trend in stereotactic radiosurgery is the concept of debulking the tumor and subsequently radiating the tumor for the purpose of hearing preservation and facial nerve preservation. The single biggest variable during this process is how much tumor is resected prior to radiation. In the absence of applying intraoperative MRI to visualize the remaining tumor volume, it is difficult to be certain when or if the preoperative goal for debulking has been achieved. This approach is not being used in high-volume acoustic neuroma programs nationally, and the traditional neurotologist/neurosurgeon team is not involved with innovative surgical approach. I have seen and cared for several patients who had a neurosurgeon complete a debulking procedure that, based on MRI scans that I have reviewed, represents merely a biopsy rather than a debulking procedure. Aside from the unnecessary expensive of completing both the craniotomy and Gamma Knife radiosurgery, the ethical and moral questions presented by this practice are troubling.
In light of the current outcomes in microsurgery or stereotactic radiosurgery, there is no justification for this type of management algorithm. In contrast, Iwai and colleagues5 applied this concept in a more appropriate way. They reported a series of 14 patients managed over a six-year interval with acoustic neuromas too large (range 3.0 to 5.8 cm) to treat primarily with radiosurgery. Subtotal resection was achieved in 13 and partial resection due to hypervascularity was performed in one patient. After recovery, radiosurgery was performed to treat the remaining tumor. This latter management algorithm would be appropriate in selected patients.
What Should We Anticipate for the Future?
Gamma Knife radiosurgery continues to evolve rapidly and advances are being made in improving accuracy, effective radiation dose, and other variables necessary to maximize patient outcome. Gamma Knife radiosurgery, just as any other treatment method, has advantages and disadvantages that must be discussed with a patient who has an acoustic neuroma. An informed decision to pursue observation, microsurgery, or stereotactic radiosurgery-or a combination of these methods-must be made, and it remains the responsibility of the surgeon to provide a balanced view as to the relative advantages and disadvantages of each method. With the introduction of new technologies that will be able to address disease extending to the clavicles, our entire otolaryngology community should prepare to meet the opportunity to integrate stereotactic radiosurgery into their practices.
References
- Wackym PA, Runge-Samuelson CL, Poetker DM, et al. Gamma knife radiosurgery for acoustic neuromas performed by a neurotologist: early experiences and outcomes. Otol Neurol 2004;25:752-61.
- Poetker DM, Jursinic PA, Runge-Samuelson CL, Wackym PA. Distortion of magnetic resonance images used in Gamma Knife radiosurgery treatment planning: implications for acoustic neuroma outcomes. Otol Neurotol 2005;26:1220-8.
- Pollock BE, Lunsford LD, Kondziolka D, et al. Vestibular schwannoma management. Part II. Failed radiosurgery and the role of delayed microsurgery. J Neurosurg 1998;89:949-55.
- Watanabe T, Saito N, Hirato J, et al. Facial neuropathy due to axonal degeneration and microvasculitis following Gamma Knife surgery for vestibular schwannoma: a histological analysis. J Neurosurg 2003;99:916-20.
- Iwai Y, Yamanaka K, Ishiguro T. Surgery combined with radiosurgery of large acoustic neuromas. Surg Neurol 2003;59:283-9.
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