Some of the questions patients with an acoustic neuroma not uncommonly ask are about the issue of salvage surgery after failed radiotherapy: "What happens if the tumour grows despite radiation therapy?" "Is the tumour harder to remove?" "What will happen to my facial nerve/function?" The pertinence of these questions is reflected in the fact that it is a subject of interest written about in the medical literature and discussed at annual scientific conferences.
As outlined in the Acoustic Neuroma Clinical Care Pathway on the new ANAC website, many factors influence the individualized management of a patient with an acoustic neuroma. That being said, many small-to-medium-sized tumours for which treatment is recommended can be appropriate for either surgery or radiation therapy.
Many in this situation would argue that there is what is called “clinical equipoise” between surgery and radiation therapy, meaning one option has not been adequately shown to be better or inferior to the other. Beyond physician measured outcomes of tumour recurrence, hearing, facial function, etc., several studies have shown that the quality of life in acoustic neuroma patients who undergo either surgery or radiation therapy is very similar. In many cases, the decision to pursue radiation therapy over surgery can come down to patient preference.
The principle goal of radiation therapy is referred to as “tumour control” which means either an arrest of tumour growth or actual tumour shrinkage. When calculating treatment failure after radiation therapy, it is important to discriminate from transient increases in tumour size which can be part of the radiation response and is termed “pseudoprogression.” This occurs ~20% of the time and generally between 6-24 months following radiotherapy. In most of these cases, the tumour enlargement is self-limited and either stabilizes or the tumour subsequently shrinks. Although the mechanism by which this occurs is not completely understood, it rarely represents a situation of uncontrolled growth requiring salvage treatment.
In high volume centers of excellence with published long-term results, the proportion of patients with acoustic neuromas who truly fail radiation therapy necessitating “salvage surgery” is typically well under 10% and probably closer to 5%. One should account for several factors when interpreting this data, the most important being the size (specifically the volume) of tumour being irradiated. Other variables include the rate of tumour growth prior to radiation (if growing at all), and the type and dose of radiation being delivered. Nevertheless, the majority of patients with acoustic neuromas that were deemed appropriate for radiotherapy will achieve tumour control. But in the unlikely event that radiotherapy fails, what then?
To date there are a few published studies of salvage surgery for previously irradiated acoustic neuromas. The timing of tumour recurrence after radiotherapy was very variable and occurred at any time, including after a period of long-term (>5 to 10 year) stability. Observations noted at the time of surgery for post-radiation tumours include increased scarring at the interface between the tumour and the facial nerve, increased irritability of the facial nerve during dissection and a firmer tumour consistency. The extent of resection is more likely to be incomplete compared to series of sporadic (i.e. not previously irradiated) cases, in part due to the practice of some surgeons to purposely not aim for a complete resection. The results of post-operative facial nerve function are di cult to interpret because of variability in individual surgeon expertise however, a general relationship between attempts to achieve a gross total resection and more severe post-operative facial nerve dysfunction can be appreciated. Interestingly, a consistent finding is that the rate of recurrence in cases with incomplete resection is very low. For example, in one series of 73 patients with previously irradiated acoustic neuromas, there were no recurrences in those patients who underwent incomplete resection after a mean of 5.7 years of follow-up.
My personal experience is that previously irradiated tumours are heterogenous in consistency, with some areas of tumour looking very typical (i.e. virgin) without significant scarring and can be removed much like non-irradiated tumours. In other areas, the tumour is more rm or fused to the surrounding tissues, including sometimes to the facial nerve or brainstem. As a result, completely removing a previously irradiated tumour can be more challenging.
Many surgeons (including myself) will rely very heavily on intra-operative monitoring of the facial nerve. This refers to placing electrodes at the time of surgery to measure the reaction of the facial muscles to stimulation over the course of the operation. Several techniques for stimulating and monitoring the facial nerve are possible and some allow verification of function of the nerve and not just irritability or anatomic continuity of the nerve, neither of which are particularly accurate in predicting post-operative facial nerve function. Facial nerve monitoring is especially useful to me in previously irradiated acoustic neuromas, where the usual visual or tactile “cues” the surgeon uses to safely dissect around the facial nerve are disrupted. If the facial nerve monitoring reaches a specific threshold beyond which post-operative facial weakness is likely, then I am likely to stop removing the tumour and leave a small residual if necessary. As mentioned above, we have relatively good data that shows that even if a small volume of tumour is left behind, it is likely not to grow, and by proceeding based on the intra-operative monitoring data, the patients’ facial nerve function can be preserved.
In summary, radiotherapy is a very reasonable option for many small-to medium-sized acoustic neuromas, and most patients who need to undergo treatment will have good results. Tumour growth after radiotherapy is generally uncommon. Surgical resection of previously irradiated acoustic neuromas is more challenging as described above. Besides surgeon experience and expertise, intra-operative monitoring of the facial nerve is an important adjunct to ensure maximal safe resection. Leaving some tumour along the facial nerve probably occurs more frequently compared to non-irradiated cases; however, most studies concur that the residual tumour is unlikely to recur. When patients ask me about salvage surgery in the event radiotherapy doesn’t work, and despite the challenges described above, I respond that most people will still do well in that scenario and I would not worry about that when deciding which upfront treatment to pursue.
Salvatore Di Maio is a skull base neurosurgeon and assistant professor at the Jewish General Hospital in Montreal, Quebec. He attended medical school at McGill University and completed his neurosurgical training at the University of British Columbia. He also received in-residency training in endoscopic skull base surgery under Paolo Cappabianca in Naples, Italy, and a clinical fellowship in skull base and cerebrovascular surgery with Dr. Laligam Sekhar at the University of Washington in Seattle, USA.
Dr. Di Maio specializes in open and minimally invasive procedures for skull base tumours, meningiomas, acoustic neuromas, pituitary tumours, and neuro-oncology.