radiation-wave5

The spot where a proton or carbon-ion beam dumps most of its energy and then stops, so tissue behind the tumour gets almost no dose.

Protons stop inside the tumour; modern X-ray beams (IMRT) still exit through healthy tissue. That physical difference only matters if it changes cure or late harm enough to justify the cost, which randomised adult trials have not settled.

How densely a radiation track dumps energy along its path. Sparse tracks (X-rays, most of a proton beam) are easier to repair; dense tracks (the proton's stop-point, carbon ions, alpha particles) punch clustered holes in DNA.

The number used to convert proton gray into X-ray gray (1.1) is a convention. The true relative effect is higher where the beam stops, and it depends on the tissue and the size of each dose.

The first multicentre randomised trial of protons versus IMRT for localised prostate cancer found no difference in bowel, urinary or sexual quality of life and the same cancer control at five years.

In the first multicentre randomised comparison of protons and IMRT for localised prostate cancer, bowel, urinary and sexual quality of life and five-year cancer control were the same with either technique.

A randomised trial of protons versus photons for non-metastatic breast cancer, asking whether protons spare the heart and lung when the breast and nodes need radiation.

How many grays of ordinary X-rays you would need to match the damage from one gray of this radiation. Protons are treated as 1.1 times as damaging as X-rays; that number is a convention, and the true value is higher where the beam stops.

Protons cost more than shaped X-ray beams. They are worth it when the extra punch where the beam stops, and the missing exit dose, widen the gap between killing the tumour and harming late-reacting tissue: an alpha/beta and RBE question, not a machine question.