Proton Radiotherapy Verification and Dosimetry Applications

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Background

How Radiotherapy works 

Damaging the DNA within cancer cells will stop them growing or they may even die .– radiotherapy deposits high levels of energy in the cancerous cells to cause their DNA to become damaged.  When the cells die the body breaks them down and gets rid of the waste chemicals.  Normal cells in the radiotherapy region may also be damaged but they can usually repair themselves. 

Most external radiotherapy uses high-energy x-ray or gamma beams (photons) with energies between a few 100 keV and tens of MeV.  The energy deposited in  tissue as an x-ray or gamma beam passes through falls off approximately exponentially from the surface.  To ensure the maximum energy is deposited at the tumour site, the beam is rotated and shaped; so that over the entire treatment session the majority of the photon energy is deposited at the tumour site.

For general information on radiotherapy visit cancerhelp.cancerresearchuk.org/about-cancer/treatment/radiotherapy

 

Proton Radiotherapy

Proton therapy uses beams of protons – these are the elementary particles in the atomic nucleus that carry a positive electrical charge. They release some energy as they slow down passing through the tissue. However, there is a peak of energy released towards the end of their journey and this can be manipulated to be at the target – the cancer. This peak is called the Bragg Peak as protons release most of their energy when they stop rather than when they are travelling through the tissue. 

Proton therapy can increase the likelihood of killing cancer cells by giving a higher dose of radiation straight to the tumour, while reducing damage to healthy tissue between the charged particle radiation source and the tumour, and sets a finite range for tissue damage after the tumour has been reached. The dose curve below illustrates how dose and depth penetration may be manipulated, through modifying the proton's energy, so that the maximum dose at the Bragg Peak is localised at the tumour site.   

Dose Curve

The capabilities of proton therapy over conventional photon therapy are illustrated through the dose density distribution models for a nasopharyngeal (throat) tumour.  The coloured regions show that some radiation dose is deposited other a large volume of the brain in the case of photons (x-rays), but much less for the better targeted protons.

 

Proton therapy is particularly beneficial for tumour sites near other critical structures or organs – for example, ocular tumors (uveal melanoma), skull base and paraspinal tumours (chondrosarcoma and chordoma).  The other area of treatment is cancer occurring in young children, where the increased dose to surrounding healthy tissue can result in an additional long-term cancer risk.

 

Proton Therapy in the UK

People who have melanoma of the eye can have proton therapy at the NHS Clatterbridge Centre for Oncology, Wirral, Merseyside

See:  www.clatterbridgecc.nhs.uk/patients/treatment/protontherapy

Currently the NHS will pay for people to go abroad if it is considered that they will benefit from  proton treatment for cancers in other parts of the body.

See: www.specialisedservices.nhs.uk/service/proton-beam-therapy

In December 2011, the UK Department of Health confirmed that proton therapy will be made available for patients in the UK. There will be  two new treatment centres, in London and Manchester. Patients should be able to have treatment at these centres from 2017.

 

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