Acoustic neuroma: What the evidence says about evaluation and treatment

Watchful waiting has been recommended for the elderly and for infirm patients with tumors <1 cm who would be poor candidates for surgery or radiation. In up to 57% of AN cases, no further tumor growth occurs after diagnosis; in about 8% of these cases, tumor regression is noted.⁹ A little more than half of patients will experience further hearing loss.⁹

Unfortunately, there are no prediction rules for determining who is most at risk for tumor growth and hearing loss. A recent study found that conservative management was a cost-effective approach for tumors <1.5 cm in any age group,¹⁰ provided there was no increase in complications from continued tumor growth. This, then, is a third group of patients for whom watchful waiting might be an option.¹⁰ Follow-up MRI can be used as a surveillance tool.^8,9,11

Microsurgery. This option is the oldest and best-studied treatment for AN. Micro-surgery appears to provide the best tumor control, although morbidity and mortality remain risks. A systematic review by Yamakami in 2003 showed that microsurgery completely removed 96% of ANs, with tumor recurrence, mortality, and major disability rates of 1.8%, 0.63%, and 2.9%, respectively.¹² More recent reviews have shown mortality rates of approximately 0.1%.¹³ Surgery usually involves removal of cranial nerve VIII, with the risk of damage to cranial nerve VII. Nerve-sparing procedures are available. Cerebral spinal fluid leaks and meningitis are occasional adverse events. The experience of the surgical team can affect outcome, including complications and cost.¹⁴

Stereotactic radiosurgery. Through the use of sophisticated imaging devices and 3-dimensional treatment-planning computers, stereotactic irradiation allows much more specific targeting of the AN, with significantly less radiation delivered to surrounding healthy tissues.¹⁵ Dynamic beam shaping and intensity modulation provide flexibility and enable delivery of much higher radiation doses to the tumor, resulting in greater control rates and decreased complications. There are 3 delivery technologies: Gamma Knife, proton beam, and specially modified linear accelerators.

Whereas older studies did not provide sufficient evidence to support the use of low-dose over high-dose radiation for long-term control of ANs,¹⁶ a more recent systematic analysis by Yang et al seems to indicate that patients treated with the lower dose (12.5 Gy) did equally as well with better preservation of hearing.¹⁷

Applying the evidence in practice
A large randomized controlled trial comparing these treatment options has yet to be done and, indeed, would be difficult to conduct due to the small number of AN cases, varying surgical expertise among centers, the different treatment goals inherent in the 3 therapies, and the risk involved in each. Evidence to date generally indicates that observation is appropriate for small intracanalicular tumors (<1 cm) in the elderly, medically infirm, or asymptomatic patients who understand and opt for this management approach. For tumors ≥3 cm, evidence supports microsurgery as optimal management.^13,18 Tumors falling between these extremes pose the real challenge.

Over the last 10 years, numerous studies have demonstrated good tumor control with either microsurgery or radiosurgery, but with varying degrees of hearing preservation and permanent nerve injury to the facial and trigeminal nerve. There is also a concern for malignant transformation of AN after radiosurgery, with 8 cases reported in the literature.^8,13,19

Three evidence-based studies in the last 6 years have compared the 2 interventions. In 2002, Nikolopoulos et al reviewed 111 studies and concluded there was insufficient evidence to support one approach over the other.¹⁸ Pollock conducted a prospective cohort study in 2008 that showed superior outcomes in facial movement, hearing preservation, and Health Status Questionnaire subscales for patients undergoing stereotactic radiosurgery.¹³ This study was limited to nonfractionated radiosurgery, and follow-up varied from 12 to 62 months.

In 2009, a Norwegian prospective study of 91 patients reported better facial nerve and hearing outcomes from radiosurgery for medium and small tumors. This study was well performed, but it looked at only a small, non-randomized population.²⁰ The same author in 2005 had found that, from the patient perspective, cranial nerve function and overall outcomes were better in the radiosurgery group.²¹

Stereotactic radiosurgery does confer lower risks for acute treatment complications than microsurgery, and therefore can be advantageous for patients who are older, infirm, require anticoagulant therapy, are otherwise poor candidates for surgery or anesthesia, or have serviceable hearing and opt for a more conservative approach. Other advantages of stereotactic radiosurgery over microsurgery are its lower cost and its preferred use in patients with permanent hearing loss in the unaffected ear. These advantages of stereotactic radiosurgery may, however, be offset in the long term by cranial neuropathy and eventual hearing loss, which can be comparable to the experiences of patients after microsurgery.⁸