Radiotherapy and radiosurgery both use beams of X-rays. These damage the DNA of the target tumour, but they also damage healthy tissue before and beyond the tumour as the beam passes through the body.
The limitations of radiotherapy
The advantages of proton therapy
Proton therapy uses protons (hydrogen ions) instead of X-rays. These have considerably less impact on the path to the tumour, deposit a high dose in the tumour and then almost nothing beyond.
Potentially serious and distressing side effects which are common in some conventional radiotherapy treatments are greatly reduced.
Proton therapy can treat tumours where X-rays cannot be used because they would damage or destroy sensitive tissue beyond.
The danger of the radiation therapy itself causing cancers later in life is much less. For this reason proton therapy centres initially prioritised treatment of children, but proton therapy is now available to adults too.
The number of days required for treatment can be much less (this varies with different cases and between centres).
Proton therapy can safely deliver higher doses of radiation to stubborn, deep-seated tumours than is possible with conventional radiotherapy
Whilst X-rays can be aimed quite accurately, the intensity of the beam dies away only gradually as it penetrates the body and passes out the other side.
How does proton therapy differ from conventional radiotherapy?
Unike X-ray photon beams, the range of a proton beam can be controlled precisely. It delivers a high energy peak at the selected target, but beyond that the energy level drops very sharply.
The reduced burden on surrounding healthy tissue enables the number of sessions ('fractions').to be reduced. The radiation dose absorbed by healthy tissue is only 1/3 to 1/5 that of X-ray therapy, allowing an increased effective dose to be administered.
Worldwide, all proton facilities currently planned or under construction will use scanning technology. Using this method, the beam scans the tumour in a grid-like fashion with the utmost precision, with up to 10,000 target points in the tumour.
The use of proton scanning technology means that proton therapy can now sterilise small areas anywhere in the body.
Tumours in the head/neck area, where proton therapy reduces damage to saliva glands and minimises dry mouth problems after therapy
Prostate carcinoma. When used as the sole treatment, proton scanning achieves the same favourable dose distribution as combined brachytherapy/X-ray radiation. This is important to maximise effective dose to the tumour whilst reducing risk of damage to continence and erectile function.
Tumours of the brain and base of skull, including those close to optic and auditory nerves, pituitary gland, brain stem and cranial nerves.
In principle, the physical and biological properties of protons allow the treatment of any tumour previously treated with X-ray radiation.
Lung and liver tumours. Preliminary indications from the Loma Linda centre in the USA seem to indicate that proton radiation for bronchial carcinomas (lung cancer) is even superior to surgery.
Ocular tumours. This is one of the oldest applications of proton therapy since X-rays would damage the optic nerve and brain behind the tumour.
Local recurrence and individual metastases. In selected cases, proton therapy is the only way to effect partial or total removal in all localizations in the body. It is the most common option for metachronous multiple liver metastases (following carcinomas of the colon), which are not suitable for X-ray therapy because of the risk of liver damage.
Tumours of the abdomen and pelvis, where adverse effects of proton therapy are reduced by 67% to 80% compared to X-ray therapy.
Spinal tumours and metastases. Proton therapy allows the radiation dose to the spinal cord to be drastically reduced, even at the center of the radiation field.
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Various techniques and proprietary technologies have been developed in order to minimise damage to healthy tissue. These include more accurate targeting, 'gating' to time the beam to meet the tumour during body movements and treating from many different angles.
However the same basic problem is inherent in all external X-ray based treatments.
This means that the dosage is much more concentrated in the desired area, so that:
For this reason many treatments are given (up to 35) on diffferent days, to allow healthy tissue time to recover between treatments.
How accurate is proton therapy?
Unfortunately the tumour can also recover, which can reduce the chance of a cure.
Proton therapy is especially suitable for tumours where X-ray treatment is difficult or impossible due to associated side effects. This includes, but is not limited to: