SRS for acoustic schwannoma

 What is an acoustic schwannoma ?

A benign tumour arising from Schwann cells enclosing the VIIIth cranial nerve. Schwann cells are not nerve cells themselves, but are responsible for protection and insulation of nerve cells. The VIIIth  nerve is the vestibulocochlear nerve, which is responsible for hearing and balance.

Coronal MRI images of acoustic schwannoma at the left CP angle ( arrow)

Is an acoustic schwannoma also called a vestibular schwannoma?

Yes. This tumour arises from the vestibulocohlear nerve. It is also called an acoustic neurinoma.

What symptoms does a patient suffering from an acoustic schwannoma feel:

·      Decreased hearing.

·      Tinnitus i.e. hearing a sound in the ear.

·      Loss of balance

·      Headache

·      In large tumours:

o   facial paralysis

o   impaired taste

o   symptoms arising from pressure on brain stem or other cranial nerves, such as deviation of tongue, difficulty in swallowing.

Axial images of acoustic schwannoma of left CP angle

Can acoustic schwannoma / vestibular schwannoma be treated with stereotactic radiosurgery (SRS)?

Stereotactic radiosurgery is an effective method of treating vestibular schwannomas, controlling the tumour in more than 90% of patients and hearing in more than 80 % of patients. The tumour does not disappear and in most patients either decreases slowly or remains the same size.

What is stereotactic radiosurgery?

Radiosurgery is the treatment of a lesion in the brain using X or gamma rays in a sharply focused manner, in a single or limited number of sessions, while ensuring strict attention to positioning and immobilization of the head.

How do you treat acoustic schwannomas using radiosurgery?

I treat patients with acoustic schwannoma using a technique called frameless radiosurgery. The treatment is delivered on a Novalis Tx linear accelerator, a very precise radiosurgery delivery platform installed at Indraprastha Apollo Hospital, New Delhi.

The patient’s head is immobilized in a thermoplastic cast, that molds into the shape of the head, and helps to hold it accurately in the same position. A special Brain Lab cast is used for radiosurgery. This is followed by scanning of the patients brain, using a CT scan as well as an MR, with  detailed imaging of the brain and neighbouring structures such as the middle and inner ear. These images are then fused to allow information regarding the anatomy to be extracted in detail. 

An acoustic schwannoma (green), and organs at risk, brainstem (purple), cochlea (sea-green), optic nerves and chiasm and eyes (red), in 3D

The lesion and surrounding normal structures are delineated by me and my team, following which a team of medical physicists creates multiple radiosurgery plans, targeting the lesion in a focused manner. The best plan is then chosen and a quality assurance test performed to assess whether the chosen plan can accurately be delivered on the linear accelerator.

Following this, the patient receives premedication with steroids to counter the effects of some swelling that may happen following radiosurgery, and is then transported to the Novalis Tx suite. The treatment is painless  and lasts about 40  to 60 minutes. The radiotherapy is delivered using  a sophisticated radiation plan comprising non co planar fields and arcs.

Acoustic Schwannoma (left) outlined in red,
in axial and coronal CT images;
SRS dose represented by colourwash,
closely conforms to the edges of the lesion.

Though headache, nausea and dizziness are possible immediate side effects of radiosurgery, they seldom occur.

Attention to detail, during planning and delivery of radiosurgery, is critical 

 What special techniques are used to preserve hearing ?

Hearing is preserved using sophisticated radiosurgery planning that restricts the dose to the cochlea , i.e. the inner ear and a specific component of the cochlea, the modiolus.

What is the usual dose schedule used in treating an acoustic schwannoma?

The most common schedule is a single fraction of radiosurgery, delivering 11-13 Gy to the edge of the lesion (and a higher dose centrally). Larger lesions may however be treated using more protracted schedules, comprising 3, 12 or 28 sessions.

Better outcomes for patients with brain metastases

Brain metastases most commonly arise from lung and breast cancers. Others common causes of brain metastases are renal cell carcinoma and malignant melanoma. The impact of brain metastases cannot be overstated, in view of the risk of losing functional independence, mental abilities as well as a patient’s sense of self.

With improvement in imaging methods, especially magnetic resonance imaging ( MRI) scanning of the brain, the chances of identifying brain metastases in asymptomatic patients has increased, at the same time resulting in improvement of results of treatment due to the detection of smaller metastases.

MRI Image demonstarting 2 brain metastases.

Identification of certain molecular subtypes of lung cancer, and the development for drugs targeting  abnormal pathways such as Her 2 neu gene in breast cancer and EGFR or Alk pathway in  non small cell lung cancer has also resulted in an improvement of outcome for patients suffering from tumours carrying these pathways.

Radiotherapy is the treatment of tumours using high energy X rays or charged particles. It is an essential component of the treatment of brain metastases. Traditionally, the treatment of brain metastases has been giving radiotherapy to the whole brain.

A sophisticated, high precision form of radiotherapy called stereotactic radiosurgery (SRS) is the administration of a very high dose of radiotherapy in a single session, the dose ranging from 15 Gy to 24 Gy versus the usual conventional daily dose of radiotherapy, 1.8 – 2 Gy. When added to whole brain radiotherapy, this method of treatment improves the survival of patients and also decreases the chances of the treated brain lesion regrowing.

Stereotactic Radiosurgery plan of a patient with 2 brain metastases.

 One of the concerns of whole brain radiotherapy is its impact on the mental abilities of the patient receiving this form of treatment. In view of this, there is a school of thought that advocates treating patients with SRS alone, omitting whole brain radiotherapy. Patients who receive SRS alone have a better chance of preserving their cognition; though with the caveat they also have a higher chance of requiring treatment for brain metastases in other parts of the brain. This treatment can be in the form of repeat SRS or also whole brain radiotherapy.

Another way of preserving cognition is administering radiation to the whole brain while avoiding high doses to the hippocampus, a structure in the brain, responsible for creating new memories. Though it is quite possible that the hippocampus is not the only structure responsible for cognition, studies have proven that patients treated with techniques that avoid dose to the hippocampus can reduce the chance of deterioration in memory due to radiotherapy.

Hippocampal avoidance whole brain radiotherapy to prevent cognitive decline.  

     

While this is still under investigation, in certain situations, hippocampal avoidance radiotherapy may be combined with a simultaneous high dose to the metastatic lesions in the brain.

Whole brain radiotherapy with hippocampal avoidance & simultaneous boost to brain metastases.

 

These sophisticated technqiues, SRS and hippocampal avoidance radiotherapy add quality to life of cancer patients suffering from brain metastases. This is due to better control of metastases as well as reduction in side effects of radiotherapy.