Patients with cancer are less likely to develop another primary cancer if their radiotherapy is carried out with protons rather than photons, a large US study shows.

But the absolute level of second primary cancers diagnoses is low, even with photon radiotherapy, at 1.55 per 100 patient-years, researchers say.

There is also nothing to choose between two widely used forms of photon radiotherapy (intensity-modulated radiotherapy and 3-dimensional conformal radiotherapy) as far as risk of causing a second cancer is concerned, allaying fears that intensity-modulated radiotherapy might put patients at more risk.

Radiation oncologists from Stanford University, California, tracked outcomes for more than 450,000 adults and children in the US National Cancer Database who were diagnosed with cancer between 2004 and 2015 and given radiotherapy for a primary cancer that had not metastasised.