Pencil-beam scanning and intensity-modulated proton therapy
Modern proton machines paint the tumour spot by spot with a magnetically steered pencil beam, so the dose can be shaped in three dimensions and the proton's stop-point is put to full use.
Overview
Early proton therapy used passive scattering with brass apertures and compensators for each field. Pencil-beam scanning, in routine use since the late 2000s, steers a narrow proton beam across the target layer by layer and varies its energy to set the depth, allowing intensity-modulated proton therapy that optimises all fields together. Almost all new centres use scanning only; it underpins proton arc therapy and proton FLASH research, and reduces neutron dose to the patient. Range uncertainty and sensitivity to anatomy changes remain the technique's weak points.
How it works
A scanned monoenergetic proton beam deposits most of its energy at a controllable depth (the Bragg peak); layering spots of different energies fills the target with dose and stops beyond it.
- No exit dose beyond the target
- Full 3D dose shaping
- Lower neutron dose than passive scattering
- Range uncertainty of a few millimetres
- Sensitive to motion and anatomy change
- High capital and running cost
Latest papers
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