Vestibular Schwannomas Acoustic Neurinoma

The indications for the radiosurgical option evolved over the years. First we considered only the elderly and medically infirm who would not tolerate a long surgical procedure. The next group of patients - those with tumors in their only hearing ear or with bilateral tumors - were referred later as a result of the recognition of a perceived superior hearing preservation with radiosurgery. Similarly, facial nerve preservation may be difficult with surgery for recurrent tumors, so these were considered for radio-surgery. Finally, it became a primary choice for those who refused microsurgery in order to avoid the inconvenience and interruption of their daily life by open surgery. The latter group is undoubtedly influenced by the plethora of information available on the Internet both from other patients and from institutions.

Those patients with brainstem compression symptoms usually benefit from at least partial removal of the mass. Therefore, they should undergo open surgery rather than hoping for a slow decompression by eventual shrinkage of the tumor. The arbitrary upper limit of 3.5 cm maximum diameter is born out of the observation that a larger tumor cannot be irradiated with a therapeutically effective single dose without unacceptable cranial nerve neuropathies. There are, of course, anecdotal successes even amongst these large tumors.

The aim of radiosurgery is different from that of open surgery. It does not aim to remove the lesion, merely to control its growth. This control may be defined as "no growth compared with the pre-treatment size", or the looser definition advocated by some, where the criterion is "the absence of need for further intervention"; the latter is open to biased interpretation. The control rate is reported to be about 97-98%, with an actual reduction in size in 23-55% [21].

Involution of the tumor occurs slowly, through several years (Fig. 8.2). Considering the rates of residual and recurrent tumors seen even in the best published surgical series, not to mention the case material submitted for radiosurgery, tumor control with radiosurgery is at least as good.

Complications of radiosurgery can be immediate and late. In the first 24 hours, nausea and headache were seen, particularly in the early days of radiosurgery when larger radiation doses and less precise dose planning were used. The combination of conformal planning, delivering no more than 12-15 Gy peripheral dose, and perioperative steroid cover eliminated these side-effects. Late cranial neuropathies are dependent on peripheral dose and the size of the tumor. The latter is an important factor, as with larger tumor size a longer section of the nerves receives toxic dose. On the other hand, intra-canalicular tumors may also pose a challenge. They are more difficult to delineate precisely, and the relative imprecision of MRI imaging may lead to complications. In these cases fusion with CT scan is particularly helpful.

Hearing preservation is reported in 30-75%, and as the fading of hearing is gradual, the result is dependent on length of follow-up. In our own material (in Sheffield) concerning unilateral vestibular schwannomas (VS), using the Gammaplan planning software, no higher than 15 Gy peripheral dose and at least 5 years' follow-up, hearing preservation was achieved in 75% of patients (submitted for publication). Over-enthusiastic reduction in the delivered dose may be counterproductive: losing tumor control would lead to morbidity through increased tension on the cranial nerves. One must not forget that, after all, most VS are originally diagnosed as a result of progressive hearing loss.

In considering tumor control and the management of patients after radiosurgery, it is important to understand the changes that occur with time after the treatment. Early on, around 1 year, the tumor is frequently larger. It is important not to rush into surgery as subsequently the tumor may well involute and shrink. At that time the cranial nerves may be at their most fragile, rendering the operation higher risk. Failure of tumor control should not be declared until 4-5 years after radiosurgery owing to the slow effect of the treatment.

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