Proton and heavy-ion beam therapy physics

6 problems

• Effective range extension at the distal falloff

  Partially resolved

  The last few millimetres of a proton beam are the most damaging per unit dose, which effectively pushes the treated region deeper than planned. By how much is not agreed.

• Nuclear fragmentation cross sections for therapy beams

  Open

  Heavy ions break up when they hit nuclei in tissue, creating lighter fragments that deposit dose past the tumour. The break-up probabilities are not measured well enough.

• Millimetre in vivo verification of particle range

  Partially resolved

  Doctors cannot see where the particles actually stop inside the patient, so they treat with extra margin. A reliable way to measure the stopping point during treatment is missing.

• Which ion species is optimal for therapy?

  Open

  Beyond protons, beams of helium, carbon, oxygen and neon are possible, each with a different balance of sharpness, biological effect and unwanted fragments. Which one is best for which tumour is unsettled.

• Variable biological effectiveness of proton beams

  Contested

  Clinics assume protons are a fixed 10 percent more damaging than x-rays, but laboratory data show the factor rises near the end of the particle range. How much it rises in patients is unknown.

• Patient stopping-power accuracy below one percent

  Partially resolved

  The plan converts a CT scan into a map of how fast particles slow down. Converting image numbers into that map introduces an error of a few percent, which becomes a few millimetres of range error.