Epidemiology:
Incidence of Disease: Symptoms usually start after the age of 30.³
Etiology:
Risk & Causative Factors: "Initial growth within the internal acoustic canel causes vestibular and hearing abnormalities in up to 95% of patients with Acoustic Neuroma (AN)."¹ "Expansion into the cerebellopontine angle may lead to trigeminal symptoms; unilateral corneal reflex depression is an early sign of trigeminal involvement."¹ “An acoustic neuroma is believed to occur when there is a defect in a gene that normally prevents tumors from forming. The cause of the genetic defect is not known. However, acoustic neuroma is often linked with the genetic disorder neurofibromatosis type 2 (NF2).”³ "Exposure to high-dose ionizing radiation is the only definite environmental risk factor associated with an increased risk of developing an acoustic neuroma."²
Signs & Symptoms:
Detection and Diagnosis: "Large ANs may impinge on the cerebellum and brainstem, leading to ataxia, long tract signs, and involvement of lower cranial nerves (IX to XII)."¹ Common symptoms include, vertigo, hearing loss, and tinnitus.³ Less common symptoms include: difficultly understanding speech, dizziness, headache, loss of balance, numbness in the face or one ear, and abnormalities in vision.³
Diagnostic Procedures:
All patients with unilateral hearing loss that is not clearly cochlear in origin should undergo MRI, with special emphasis on the acoustic nerves. Acoustic neuromas will be well-delineated, solidly (or sometimes cystically) contrast enhancing masses somewhere along the eighth cranial nerve. Often, they are located in and expand the internal auditory meatus, a location in which meningiomas are not found. When located in the cerebellopontine angle rather than in the internal auditory meatus, distinguishing acoustic neuromas from meningiomas may be difficult.⁸
Histology:
​Two histologic types of tissue have been identified in acoustic tumors. Antoni A tissue consists of elongated spindle cells with a palisading pattern. Antoni B tissue has a loose spongy texture and markedly reduced cellularity. A given acoustic neuroma may contain areas with both Antoni A and Antoni B tissue. Another histologic feature characteristic of schwannomas are rows of palisading nuclei called Verocay bodies. Although the histologic appearance of acoustic tumors is fairly straightforward, they can occasionally be difficult to distinguish from meningiomas.²
Lymph Node Drainage:
​Absence of lymphatics in the brain; therefore no lymphatic drainage due to the blood brain barrier.
Metastatic Spread:
As acoustic neuromas are benign, no metastasis is known. Recurrance is rare but has been found due to incomplete resection.²
Grading:
​•Grade I - Tumor is small, occurring only within the internal auditory canal itself.
​•Grade II - Tumor extends into the fluid spaces around the brainstem.

•Grade III - Larger tumor (usually up to 2.5cm in diameter). Extends up to the brainstem.
•Grade IV - Very large tumor (up to 5cm in diameter). Compresses the brainstem, often involves the nerves of swallowing and the 5th cranial nerve (face and eye sensation).⁷

Staging:
​As a rule, acoustic neuromas are benign tumors, although rare cases of malignant acoustic neuromas have been reported in the literature. No widely accepted staging system exists for acoustic neuromas.²
Radiation Side Effects:
-Swelling and edema of the tumor may occur within the first 6-18 months following radiation treatment; therefore, the treatment cannot be deemed a success or failure until 2 years after treatment.⁴
-In single fraction doses of 18 Gy or more, facial and trigeminal nerve complications can occur (ie: facial paralysis, weakness or numbness.⁴
,
-Nausea, dizziness, and headaches can occur immediately following an SRS treatment.⁴
-Radiation can make hearing loss worse, but in most cases hearing can be preserved.⁵

Prognosis:
Rarely these benign tumors are completely eliminated by radiation therapy. Reports indicated that with SRS, long term tumor control is 93%.⁵
Treatments:
Total surgical removal of the tumor is curative, and no further theapy is necessary. Some investigators have recommended radiosurgery or fractionated stereotactic radiotherapy as treatment for small acoustic neuromas, particularly in those patients who are elderly or have comorbidities that do not make them surgical candidates. Adjuvant external irradiation may play a role in subtotally resected acoustic neuromas to decrease local recurrence. A dose of 50 to 66 Gy in 1.8 to 2.0 Gy fractions in 5 to 6 weeks is recommended.

Obervation alone may be appropriate in acoustic neuroma patients willing to undergo regular clinical and imaging follow-up, and may allow deferred treatment for some time.¹

TD5/5:
Brain: 45Gy (whole), 50Gy (2/3), 60Gy (1/3)
Brainstem (large tumors): 50Gy (whole), 53Gy (2/3), 60Gy (1/3)
Ear (acute serous otitis): 30Gy
Ear (chronic serous otitis): 55Gy⁶

Planning Photos:
acoustic_neuroma_rev.jpg
(A) A 51 year old woman presented with progressive hearing loss. An axial MRI of the brain after gadolinium administration demonstrated a left cerebellopontine angle acoustic neuroma. (B) Planning CT scan with IV contrast. The patient was treated with 2500 cGy to the 79% isodose line.⁹


References
1. Chao KS, Perez CA, Brady LW. Radiation Oncology - Management Decisions. 2nd ed. Philadelphia, PA: Lippincott, Williams & Wilkins; 2002: 153-4.
2. eMedicine. http://emedicine.medscape.com/article/882876-overview. Accessed January 15, 2010.
3. Google Health. https://health.google.com/health/ref/Acoustic+neuroma. Accessed January 13, 2010.
4. eMedicine. http://emedicine.medscape.com/article/857604-overview . Accessed January 14, 2010.
5. Acoustic Neuroma Association. Treatment Options. http://www.anausa.org/treatment_options.shtml . Accessed January 15, 2010.
6. Wikibooks. Radiation Oncology/Toxicity/Emami. http://en.wikibooks.org/wiki/Radiation_Oncology/Toxicity/Emami . Accessed January 15, 2010.
7. Brain Surgery Information Center. http://www.brain-surgery.com/acoustic.html. Accessed January 15, 2010.
8. Lenhard RE, Osteen RT, Gansler T. The American Cancer Society's Clinical Oncology. American Cancer Society. 2001: 677-678.
9. Radiation Oncology Journal. http://www.ro-journal.com/content/1/1/46/figure/F1

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