Radiotherapy

Notes based on a talk given in Cambridge by Kate Burton. Reported by Chris Richards, AMNET. Published by kind permission of AMNET.

Kate specialises in the treatment of patients with brain tumours and tumours of the spinal cord at Addenbrookes Hospital.

A brief history of radiotherapy

The history of radiotherapy goes back to 1885 when x-rays were first discovered and used as a treatment option and in 1899 there was the first reported cure due to radiotherapy. In 1922 radiation oncology was recognised as a medical discipline. By the 1950s radiotherapy had started to look as it does now with the use of linear accelerators which allow a much higher and better directed dose of x-ray radiation for treatment.

Fewer side-effects

There are a number of myths associated with radiotherapy and Kate assured us that the side effects are not as extreme as they used to be. In earlier treatments it was not known how much radiation was required and dosage was measured in relation to the redness of the skin (erythema), but this has been found to be higher than is actually required to treat and the skin effects are far less now dosage is calculated more accurately. Radiotherapy can cause nausea and sickness but this is generally only when the stomach area is being irradiated.

What is radiotherapy?

Radiotherapy is using radiation to treat, and in oncology this is usually to treat cancer, but radiotherapy is also used to treat benign tumours such as acoustic neuromas, meningiomas and pituitary tumours. These need to be treated because they occur in the skull which is a fixed size cavity. Even a small tumour can cause problems through pressure on the brain and tumours may be difficult to remove surgically because of their position close to important structures in the brain.

How does it work?

Radiotherapy works by damaging the DNA within the tumour cells and destroying their ability to reproduce. Tumour cells are normally radiation sensitive and die whereas normal cells are able to recover and repair themselves after radiation. Recent advances in medical imaging has resulted in lowering doses which means even less damage to normal cells.

The team-based approach

The radiotherapy team consists of the oncologist, the medical specialist, the medical physicist who is responsible for ensuring the machines are working properly and that people receive the correct dosage of radiation, radiographers of whom there are about 50 in the department in Addenbrookes (many part-time) and a similar number of oncology nurses who also give chemotherapy and palliative care support.

What do we mean by radiotherapy?

The term radiotherapy encompasses many different treatments. Radiotherapy can be given externally and this is known as External Beam Treatment (EBRT) when the beam is passed into the body from outside, or it can be given internally (Brachytherapy) when it can be placed into body cavities or inserted into the body using catheters. This approach is rarely used for brain tumours.

Single treatment radiotherapy or fractionated?

Radiotherapy can be given as a single treatment and this is known as radiosurgery SRS, or as a number of treatments when it is known as fractionated. The machine used to deliver the radiotherapy is called a linear accelerator and it produces high energy x-ray photons which can be delivered in a very accurate way. The machine only produces radiation when it is actually switched on so there is little radiation hazard associated with these machines.

Fractionation means what exactly?

Fractionation is the dividing up of the radiation dose and the degree to which this is done depends on the type and size of the tumour, the location of the tumour and the surrounding normal tissue, and the treatment intent – whether this is radical or palliative. Brain tumours are normally treated with between 30 and 33 fractions which are given daily, five days a week, over six weeks. This dosage means there is a higher chance of the tumour cells being hit by the radiation when they are most vulnerable ie when they are dividing, and it also gives the normal cells a chance to recover between treatments, and a higher total dose can be given than in just a single treatment.

Beneficial advances

Recent advances now mean that they are able to use three dimensional conformal radiation therapy which allows a 3D picture of the tumour to be created and beams are precisely directed so as to avoid treating normal tissue as much as possible. Intensity Modulated Radiation Therapy (IMRT) is a specialised form of radiotherapy which allows the dose to be ‘shaped’ to the tumour. This is particularly good for tumours which are close to critical organs such as the spinal cord or very sensitive areas of the brain.

The radiotherapy pathway

The Radiotherapy Pathway followed by patients (outlined below) includes firstly consultation with the oncologist, immobilisation, imaging and planning and then treatment.

Immobilisation and frame-making:  Immobilisation is necessary as the patient is required to keep very still while treatment is carried out and they need to get back into exactly the same place each day so it is only the tumour which is treated. Immobilisation is ensured by using plastic shells or masks or stereotactic head frames. The plastic frames are made from a mould of the head taken using Plaster of Paris bandages. These are wrapped around the head leaving eyes, nose and mouth free and then allowed to set which takes 20-30 minutes. A mould of the head can then be made and a Perspex mask produced using this.

The stereotactic head frame can also be used and this has a mouth bite which is made using dental impression material and the frame holds the head still during treatment. Although producing these immobilisation instruments can be a little uncomfortable, staff try to make it as pleasant as possible and through all these processes patients can have a choice of music playing and often can have a friend or relative with them.

Imaging the brain:  Imaging involves getting a very detailed image of the tumour and two types of scan are used for this.

The CT scan shows the density of bone and the brain tissue and also how much air is present in the brain. This allows the oncologist to work out the dose required based on what structures it will need to pass through and their density. However a CT scan cannot ‘see’ the tumour.

Magnetic Resonance Imaging (MRI) shows the tumour but does not indicate any densities. The treatment can be planned using a computer programme which combines these two images so that the size of the tumour can be seen alongside the density of the structures surrounding it. This is a new technology which is not yet available in all centres. With these images the oncologist is able to ‘draw’ round the tumour on each slice of the MRI scan and the radiotherapy doses can be calculated to be directed on the tumour from a number of directions.

The aim is to deliver an even, high dose to the target with minimal dosage to normal tissue and because this is fractionated then it is delivered in small enough doses that it will be tolerated by important structures such as the brain stem.

Treatment involves the patient being carefully positioned and immobilised. The ‘beams’ are only on for about a minute but the whole treatment session usually takes 20-25 minutes. Patients do not feel or see anything and will only hear the bleeping of the monitor. All treatments are computerised, the machines are carefully checked every day and while treatment is being carried out patients are observed using close circuit TV.

Side effects:  depend on the location of the tumour in the brain and whether it is close to any critical structures, the size of the treated volume of the tumour and the patients’ general condition.

Side effects which may occur during the treatment period or immediately after are usually minimal and most patients can continue working through the treatment. They may experience tiredness and maybe a mild inflammation of treated skin. Hair loss only occurs in the treated area and normally is not permanent. There may be some nausea but this is usually controlled with medication. Later side effects some time after treatment may include an induced malignancy – the risk of this is thought to be about 1% for every 10 years of life after treatment. There may be a risk of dysfunction of the pituitary gland if this was included in the treatment area and a minimal risk of damage to nerves that are in the treated region such as hearing, balance and facial nerves – this risk is thought to be less than 1%. Toxicity does appear to be greater when treating patients who have vestibular schwannoma as a result of NF2.

The aim of radiotherapy

The aim is to stop the tumour growing bigger – it will not take it away but it will make it inactive and should stop any symptoms from getting worse. When deciding which patients should be offered radiotherapy there is a lack of evidence to show whether surgery or radiotherapy is a better treatment. It is suggested that radiotherapy should be offered to: all patients diagnosed with a grade 2 or grade 3 meningioma; where there is any tumour left following surgery and further surgery is not recommended; or in the case of tumours which come back after surgery.

Radiotherapy alone following radiological diagnosis may be offered for tumours which are surgically inaccessible because they are close to or involving critical structures and to patients with other medical problems who may not be suitable for surgery.

Outcomes – good news!

With complete surgical excision 80% are still under control at 10 years and with subtotal excision and radiotherapy in benign tumours there is 80% control at 10 years. For radiotherapy alone there is a wide variation in reported outcome but it is suggested to be 50% at 10 years.

New treatments

Proton beam therapy:  Radiotherapy can cause side effects. Doctors are looking at new ways of giving radiotherapy to reduce these side effects, while still treating the cancer. This is especially important when you are having treatment to a delicate area of the body, such as the spine.

One of the newer ways of giving radiotherapy uses a different type of beam called a proton beam. protons collect energy as they slow down and travel through the body. They then release this energy at their target point – the tumour. This means they can deliver a higher dose of radiation straight to the cancer, but only to a very small area around it. So there is less likelihood of damage to nearby healthy tissue, particularly vital tissues and organs behind the tumour, such as the spinal cord or brain stem.

Proton beam radiotherapy machines in the UK are only able to treat cancer of the eye. The machines are not able to treat cancers deeper in the body. Some countries in Europe and the USA are testing and using proton beam radiation for deeper cancers. If you think this treatment may be suitable for you, talk to your own specialist and ask their advice.

Notes on Vestibular Schwannoma/Acoustic Neuroma

These are benign tumours which grow from cells lining the eighth cranial nerve (the nerve of hearing). They are generally very slow growing and form about 8% of primary brain tumours. They can be diagnosed when they are very large or when they are very small.

Management:  Management varies depending on the size of the tumour. It may be surveillance, known as ‘watch and wait’, surgery or external beam radiotherapy. This radiotherapy may take the form of single treatment radiosurgery, also known as Gamma Knife, in which the radiotherapy dose is given in one treatment or fractionated conformal stereotactically guided radiotherapy, as desccribed above.

Hearing loss:  Evidence about hearing loss with radiotherapy is mixed. It may be reduced during therapy as the tumour tends to increase in size during treatment but then improves. There is some evidence that hearing levels may drop again later after 1-5 years

Outcome:  Reported outcomes for treatment of vestibular schwannoma tumours treated with radiotherapy (Fuss et al 2000) suggest that there is 95% control at 5 years, also that 46.5% of tumours show some shrinkage. There was useful hearing preservation in 85% of vestibular schwannoma patients and 100% pre treatment normal facial nerve function was salvaged.

NB. Vestibular Schwannoma is the medical term for acoustic neuroma tumours (usually benign) which affect the auditory nerve.

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