Protons and cons

It’s Asco week in Chicago: the biggest meeting of clinical oncologists in the world. McCormick Place convention centre, the largest in the US, is filled to its 2.6 million square foot capacity with people talking about cancer. And one of the hottest topics being discussed is something called proton therapy, a possible new tool in the anti-cancer arsenal.

But does it actually work? Given that two private proton therapy machines have already been built in the UK, and that two more NHS machines are on their way at a cool £250 million, you’d think we’d know. The sad truth is that, for many cancers, the medical jury is still out as to how helpful these machines actually are.

Proton therapy is a new kind of radiotherapy. Radiotherapy has been successfully used to treat cancer for decades. In essence, it’s a way of using targeted energy to destroy tumours, with standard radiotherapy machines using light particles (photons) as their mode of attack.

The problem with using photons is that they’re extremely high energy. Unfortunately, the same high energy that allows them to destroy tumours also means that they travel deep into the body, harming whatever comes into their path.

Fortunately the photons’ energy diminishes as they penetrate deeper into the body and so, with careful planning, most of the force is spent on the tumour and less hits the surrounding healthy tissue. Nonetheless, radiotherapy can still cause side effects to the healthy tissue. Depending on where the tumour is — for example, in the brain — the side effects can potentially be so harmful that doctors need to think carefully to determine if the radiotherapy will cause more harm than good.