# Pencil-beam scanning and intensity-modulated proton therapy

Source: https://onco.cc/technologies/intensity-modulated-proton-therapy/  
OnCo record `intensity-modulated-proton-therapy` (Technology). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

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.

## Summary

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.

## Fields

- Kind: Technology
- Status: established
- Last checked: 2026-09-16
- Tags: radiation-wave1
- Principle: 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.
- Since: 2008
- Strengths: No exit dose beyond the target; Full 3D dose shaping; Lower neutron dose than passive scattering
- Limitations: Range uncertainty of a few millimetres; Sensitive to motion and anatomy change; High capital and running cost

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Proton_therapy

## Connected records

- cancers: [Brain and spinal cord tumours (all types)](https://onco.cc/cancers/brain-tumours/), [Childhood cancers (all types)](https://onco.cc/cancers/childhood-cancers/), [Head and neck squamous cell carcinoma](https://onco.cc/cancers/head-and-neck/)
- fronts: [Radiation Therapy](https://onco.cc/fronts/radiation/)
- technologies: [FLASH radiotherapy](https://onco.cc/technologies/flash-rt/), [Proton arc therapy](https://onco.cc/technologies/proton-arc-therapy/), [Proton therapy](https://onco.cc/technologies/proton-therapy/)

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