Stereotactic Radiosurgery

Introduction

Acoustic neuroma, also known as vestibular schwannoma, is a benign neoplasm arising from the vestibular portion of the cochleovestibular nerve. This tumor can cause a variety of sequelae, most common among which are hearing loss, tinnitus, and dizziness/imbalance. The tumor can grow to a large size and cause additional symptoms related to brain-stem compression, hydrocephalus, and effects on other cranial nerves (CNs). Extremely large tumors can be life threatening. Advances in magnetic resonance imaging (MRI) and audiologic evaluation have led more of these tumors to be detected even when there are few symptoms and the tumors are typically small. This has created a greater need for the management of these tumors over time. The main goal in this endeavor is to prevent the tumor from growing to a large size, thus decreasing the likelihood of morbidity, while also minimizing the deleterious effects of treatment. Secondary goals include the prevention of progressive CN dysfunction due to the tumor, such as loss of hearing and facial nerve dysfunction.

In order to determine the optimal treatment for a particular patient, it is necessary to understand the natural history of acoustic neuroma (growth rate and hearing changes over time) as well as the outcomes following intervention, whether that is surgery or stereotactic radiosurgery (SRS). Observation (watch and scan) has become more prevalent, adding another dimension to the decision making involved in treatment. What is the chance that a particular tumor will grow to a life-threatening size during the patient’s natural life span and require treatment? On average these tumors grow slowly (2 mm per year), but the growth rate varies widely, with about 50% showing no growth during observation and others that grow an average of 2 to 4 mm per year (up to 18 mm). Moreover, the growth rate is nonlinear over time, since some growing tumors can stop growing while other tumors that showed no growth for years may start growing again.

The advent of radiosurgery for the treatment of acoustic neuromas more than 40 years ago has significantly changed the contemporary management of acoustic neuroma. To date more than 100,000 patients with acoustic neuroma have been treated with radiosurgery, and the rate of radiosurgery treatment is growing. Currently about 50% of acoustic neuromas are treated with surgery, and the remainder are split between observation and radiosurgery. In addition to primary treatment for tumors, radiation offers a noninvasive treatment of residual tumor after microsurgery.

Stereotactic radiation treatments are available in two types. Single-fraction stereotaxy (SRS) is one and stereotactic radiotherapy, used for multifraction treatment, is the other. Both types use precise stereotactic localization of the lesion as well as conformal treatment plans that focus the radiation beam on the target tissue. The use of the Gamma Knife to treat acoustic neuroma was first reported in 1971 and, over time, this technology has been refined through several generations of devices to its current utilization of MRI for target localization and automated radiation delivery. The Gamma Knife uses numerous fixed Cobalt 60 sources and collimators (the Perfexion Unit has 192 sources and collimators) to send a focused radiation beam to the target ( Fig. 148.1 ). Several linear accelerator (LINAC)–based radiosurgery delivery systems are also available. These use advanced robotic technology, multileaf collimators, and radiographic verification of positioning, all of which are highly accurate and flexible.

Fig. 148.1, Conceptual diagram of focused beam radiation.

This chapter discusses in further detail the management of acoustic neuroma with stereotactic radiosurgery. The management considerations are based on the limited data available at the time of writing.

Key Operative Learning Points

1.
Patient selection is complex owing to the lack of reliable information on long-term tumor control rates after radiotherapy.
2.
The planning of tumor treatment must produce a highly conformal radiation field comprising the volume of the tumor and minimizing radiation to areas that are sensitive to radiation damage, including the cochlea and brain-stem.

Preoperative Period

History

1.
History of present illness
- a.
  Age and family history of longevity. The length of time during which tumor control will be needed is important.
- b.
  Hearing loss. The presence of usable hearing will be important in patient counseling.
- c.
  Dizziness. The patient may have imbalance, a floating sensation, and vertigo; all of these may be affected by treatment.
- d.
  Facial numbness—Large tumors may cause trigeminal nerve dysfunction.
- e.
  Headache
- f.
  Facial nerve weakness or facial spasms. These may indicate a possible facial nerve schwannoma or a vascular tumor involving the facial nerve.
- g.
  The presence and nature of tinnitus
2.
Past medical history
- a.
  History or family history of neurofibromatosis II (NF2)
- b.
  History of prior intracranial surgery or radiation to the area
- c.
  Chronic serious medical conditions including coronary artery disease, chronic lung disease, chronic anticoagulation, or other conditions that may make the patient a poor surgical candidate

Physical Examination

1.
Otologic evaluation
- a.
  Tympanic membrane
- b.
  Tuning fork
2.
Neurologic evaluation
- a.
  Facial nerve evaluation—The House-Brackmann grading scale is typically used.
- b.
  Other CNs
  - 1)
    Trigeminal nerve
    - a)
      Facial sensation
    - b)
      Corneal reflexes
- c.
  Cerebellar evaluation—dysmetria
- d.
  Balance evaluation
  - 1)
    Gait and tandem gait
  - 2)
    Romberg test
3.
Audiologic evaluation
- a.
  Pure tones
- b.
  Speech discrimination scores
- c.
  Tympanogram
4.
Vestibular evaluation
- a.
  Search for pathologic nystagmus
- b.
  Bedside testing of the vestibuloocular reflex (VOR)—head impulse test
- c.
  Videonystagmogram—This test may be performed if there are concerns regarding the status of contralateral vestibular function or the presence of other vestibular disorders.

Imaging

MRI with gadolinium including high-resolution images through the internal auditory canals (IACs). Alternatively, MRI without gadolinium with high-resolution submillimeter images through the IAC (fast imaging employing steady-state acquisition (FIESTA)/constructive interference steady state (CISS) sequences) can image the IACs and cerebellopontine angle adequately but may miss other intracranial lesions.
Computed tomography (CT) of the brain with contrast may be used if MRI cannot be performed due to metal implants or other issues, but its sensitivity is much lower than that of MRI.

Indications for Radiation

1.
The considerations regarding the need to provide treatment for acoustic neuroma with radiation or surgery include tumor size, tumor growth rate, patient’s symptoms, patient’s age, and medical comorbidities. Because there is considerable uncertainty as to the optimal treatment for any given situation, the patient’s and the treating physician’s preferences and biases may play a major role in determining whether to treat and how.
2.
Most tumors that have been determined to need active treatment
- a.
  SRS is ideally suited for small to medium-size tumors.
- b.
  Surgery is generally preferred in younger patients, since the rate of long-term (20 to 30 years) tumor control following SRS is not established and the chance of developing a radiation-induced complication is theoretically greater in a young versus an older person.
3.
Patients who are medically unfit for surgery
4.
Patients who are fit for surgery but choose stereotactic radiation
5.
Tumors that have recurred after prior surgery
6.
Some tumors that have recurred after prior stereotactic radiation have been retreated with stereotactic radiation.
7.
Patients with bilateral acoustic neuromas associated with NF2 may also choose the radiation option, although tumor recurrence rates are higher and hearing preservation rates lower than those associated with sporadic tumors.

Contraindications

1.
Many centers may consider tumors with an axial dimension larger than 3 cm as too large for radiation, but there are reports of a few centers treating such large tumors.
2.
The presence of significant symptomatic brainstem compression usually requires surgical decompression with various degrees of tumor removal.
3.
Patients in their thirties or younger will have to consider the issue of very long term tumor control with radiotherapy and long-term radiation-induced complications.

Preoperative Preparation

1.
The head frame (fixator) must be applied.
- a.
  Gamma Knife system—The Leksell head frame is put in place via head stabilization pins applied under local anesthesia ( Fig. 148.2 ). This technique is characterized by high-resolution targeting and registration of the lesion and very stable head/target positioning (1-mm accuracy).
- b.
  LINAC system—The face mask is custom-made for the patient ( Fig. 148.3 ). In LINAC, the head is registered to the frame through a mask/frame system imaged together with a CT scan (2-mm cuts) followed by fusion of an MRI to the CT to target the lesion. During treatment the head is stabilized by the mask and radiographs are taken after each table and gantry movement to adjust for any movement of the head within the mask system.
- c.
  The Gamma Knife system is the most accurate, and is preferred when the greatest accuracy is required (i.e., high dose to a small target). For most other lesions, there is no practical difference between these approaches.
2.
The MRI or CT is performed with the head frame or mask in position to allow the image to be referenced to the fixator.
- a.
  The imaging data are imported into the targeting system for use in treatment planning.

Operative Period

Anesthesia

None

Positioning

Supine: The patient is positioned in the radiotherapy bay and the head frame or face mask is fixed to the system.

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