Photons, which have no mass or charge, are highly penetrating and deliver their dose throughout the volume of tissue irradiated. Most of the radiation is delivered at a depth of 5 to 30mm and then gradually loses energy before reaching its target (see below left). This means that healthy tissues both before and after the tumour suffer collateral damage which can lead to significant side effects for the patient.
An alternative to radiotherapy based on photons is the use of protons, subatomic particles (see above right) that are both heavy and charged. Protons are generated using a cyclotron and can be readily controlled to deliver their dose at a precise point, penetrating to a depth of 300mm. Travelling very fast, protons enter the skin and deposit a small dose on their way to the target. The absorbed dose increases gradually as the protons slow down, and suddenly rises to a peak when they are ultimately stopped (see left), a phenomenon known as the Bragg peak.
Researchers at the University of Pennsylvania have sought to answer the question: Is radiotherapy based on protons safer than traditional therapy based on photons? Their findings were published recently in JAMA Oncology and included data from almost 1500 patients with 11 types of cancer – including breast cancer – and showed that those who received proton therapy experienced far fewer side effects than those treated with traditional radiation based on photons. Importantly, the two treatment methods performed equally well in treating the cancer and preserving life.
Traditional photon therapy for lung cancer (top and bottom left) delivers radiation to the tumour and surrounding healthy tissues, while proton therapy (top and bottom right) focuses the majority of the radiation on the tumour (© Trials, 2016). The major drawback of proton therapy is that it is more expensive than traditional radiotherapy, requiring a dedicated cyclotron. South Africa has one such facility, built in the 1980s, while there are 31 hospitals in the USA that have spent millions of dollars building proton therapy centres. The National Cancer Institute is currently funding clinical trials to see whether the benefits of protons justify the expense.