Proton Therapy: A New Option in the Treatment of Acoustic Neuroma

Male doctor sits at desk with computers

Luke Pater, MD, a UC Health radiation oncologist. Photo by UC Academic Health Center Communications Services.

The new Cincinnati Children’s / UC Health Proton Therapy Center in Liberty Township, Ohio, provides an important option for people who have been diagnosed with an acoustic neuroma, a benign, slow-growing tumor that originates in a delicate area near the inner ear. Because proton therapy is able to deliver a precisely shaped beam of radiation to a precisely defined depth, it can play a role in the treatment of acoustic neuromas, which can impact hearing, balance and facial muscles.

Luke Pater, MD, a radiation oncologist at UC Health and a member of the Brain Tumor Center at the University of Cincinnati Gardner Neuroscience Institute and UC Cancer Institute, answers questions about the treatment of acoustic neuromas with proton therapy.

Q: What exactly is an acoustic neuroma and what symptoms does it cause?
A: An acoustic neuroma, also called vestibular schwannoma, is a benign tumor that usually grows slowly. These tumors originate from the cells lining the eighth (vestibulocochlear) cranial nerve. Patients most commonly present with sensorineural hearing loss in one ear and possibly vertigo (dizziness) or ringing in the ears. Other less common presenting symptoms include numbness or weakness of the face.

Q: What causes acoustic neuromas, and can they be prevented?
A: Patients diagnosed with neurofibromatosis type 2 are predisposed to acoustic neuromas. However, the majority of patients have no known risk factors.

Q: What treatments for acoustic neuroma have traditionally been offered to patients?
A: Patients diagnosed with an acoustic neuroma may undergo observation for signs or symptoms of tumor growth. Treatment options, if appropriate, involve 1) surgical removal of all or part of the tumor or 2) radiotherapy.

Q: Why is an acoustic neuroma a challenging tumor to treat?
A: Acoustic neuromas are challenging because of their close proximity to critical structures. First, the tumor arises from the eighth cranial nerve, which is involved in hearing as well as balance and spatial orientation. As the tumor grows, it typically surrounds this critically important nerve. Second, the tumor is close to the brainstem, the fifth cranial nerve (which is involved with facial sensation) and the seventh cranial nerve (involved with facial movement). These structures are related to both the symptoms the tumor causes as well as the risks associated with treatment.

Q: How does proton therapy differ from traditional radiotherapy?
A: Proton radiotherapy differs from photon (traditional) radiotherapy primarily in the decreased amount of radiation exposure to normal tissue. With traditional radiotherapy, the majority of photon particles pass through the target area and continue into normal tissue, thus delivering more radiation than necessary to bystander tissues. This can be associated with an increase in side-effects and risks. Proton particles, by comparison, come to a stop as they deposit their dose of radiation, thereby minimizing exposure to normal tissue.

Illustration showing the percision of proton therapy

An illustration of the difference between x-rays and protons in the treatment of a lung cancer. Whereas x-rays (photons) pass through the lesion and into the heart, proton beams come to a stop inside the tumor tumor.

Q: Is the rate of hearing preservation greater with proton therapy than with traditional radiotherapy?
A: At this time we do not have evidence that hearing preservation following treatment with proton therapy is superior to that of treatment with traditional radiotherapy. The dose to the tumor is the same in both treatment modalities. As a result, there is similar likelihood that both treatments will impact the eighth cranial nerve within the tumor as well as the nearby cochlea.

Q: If patients choose to undergo radiotherapy, should they select proton therapy if that option is available?
A: Choosing the best treatment strategy will vary from one patient to another and will depend on several factors. While proton radiotherapy has benefits in some scenarios because of its decreased radiation dose to bystander tissues, some tumors are currently more appropriately treated with photon radiotherapy than proton radiotherapy. Each individual case should be reviewed by a radiation oncologist.

Q: What are the risks associated with acoustic neuroma treatment with proton therapy?
A: The risks from proton radiotherapy are similar to those from photon radiotherapy. The main risk is the loss of any remaining hearing. A less common risk is a loss of function of the nearby fifth and seventh cranial nerves. The very slight risk of a secondary cancer caused by radiation exposure is lower with proton radiotherapy because of the decrease in overall radiation exposure. The magnitude of that decrease in acoustic neuromas is not quantified at this time. Minimal data currently exists for this comparison.

— Cindy Starr

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