Stereotactic Radiation (STR), pioneered by Lars Lexsell at the Karolinska Institute in Stockholm, was first used to treat Vestibular Schwannomas (VS) in the early 1970’s. With each subsequent decade indications, and thus the number of patients treated, have expanded. Today, more patients diagnosed with a VS undergo active monitoring or STR versus microsurgery.
Despite the expanding use of STR, questions still persist, such as: Is this a hearing preservation treatment? What is the rate of tumor control? How does tumor size influence use of STR? What is the rate of malignant transformation after SRT? With accumulation of long-term data, a number of articles have now begun to address these questions. Hearing preservation, for example, was recently the subject of a systemic review by Coughlin, Willman & Gubbels
1. These authors analyzed 47 articles involving 2,195 patients. Two-thirds of these patients were treated with the Gamma Knife and one-third with a linear accelerator (LINAC/CyberKnife). The mean age of treatment was 56 years (45-69) and the mean tumor size was 15mm (7-21). The mean marginal dose of radiation was 12.5 Gy (11-18).
Hearing preservation was high initially, with nearly 75 percent of treated patients maintaining serviceable hearing (Class A/B) with a follow-up less than two years. Over time, hearing diminished. For example, between two to five years follow-up only 60 percent of patients retained a Class A or B hearing level, and this rate further diminished to 50 percent between 5 to 10 years follow-up. At > 10 years follow-up only 23 percent of patients had preserved hearing.
Interestingly, hearing preservation was not dependent on tumor size, patient age, radiation technique, or the number of fractions/prescription dose. Other studies have shown a direct correlation between hearing preservation and the mean cochlear dose, but this article did not have sufficient data to examine that relationship. Watanabe et al, for example, have shown that a mean cochlear dose more than 4.2 Gy is associated with poor hearing outcomes.
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Our own data of 31 consecutive patients treated over the last five years show similar hearing outcomes. My personal approach is to inform patients that STR may preserve hearing in 50 to 60 percent of patients in the short-term (two to five years), but that significant hearing loss is expected by 10 years follow-up.
The review article by Couglin, Willman and Gubbels did not review tumor control rates, but most studies show 85 to 90 percent control at 8 to10 years follow-up. There are no cases of tumor regrowth in our patient cohort with maximum follow-up of five years.
Most otolaryngologists employing STR acknowledge that the purpose of this treatment is to maintain, or possibly slightly diminish, the size of the tumor. Thus, most recommend documentation of tumor growth of at least two mm before recommending STR. With increased observation of VS, it has been noted that a subset of patients will in fact show no tumor growth over many years. The possible exception to this practice of documenting tumor growth is the development of a significant hearing loss during the observation period. In those patients SRT could then be employed in an attempt to stop that progression. It should be realized, however, that if the tumor is close to the fundus of the internal auditory canal, the likelihood of radiation exposure to the cochlea increases with hearing loss as a consequence. This issue can be mitigated to some degree with appropriate shielding of the cochlea as the radiation plan is formulated.
Finally, the risk of malignancy with STR is extremely low, but not zero. In general, studies have shown the incidence of malignancy to be one to two per ten thousand. This risk is increased when STR is used for neurofibromatosis Type 2 patients.
In addition to protecting the cochlea, it is also very important to limit radiation exposure to the brain stem. Shielding can be used to construct a plan that does minimize exposure. This becomes more problematic for larger VS that efface the brainstem. Thus, tumors greater than three cm in size are more a concern for brainstem radiation exposure.
In summary, STR can be expected to provide very good tumor control rates with minimal side effects. Long term hearing preservation, however, remains an issue.
