{"entity":{"id":"intensity-modulated-proton-therapy","kind":"technology","name":"Pencil-beam scanning and intensity-modulated proton therapy","aka":[],"tldr":"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.","status":"established","asOf":"2026-09-17","links":[{"label":"Wikipedia","url":"https://en.wikipedia.org/wiki/Proton_therapy"}],"tags":["radiation-wave1"],"related":[],"cancers":["childhood-cancers","head-and-neck","brain-tumours"],"sections":["radiation"],"technologies":["proton-therapy","proton-arc-therapy","flash-rt"],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":["bragg-peak","linear-energy-transfer","relative-biological-effectiveness"],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":["proton-therapy"],"notes":[],"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.","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","Distal LET/RBE is not in the optimisation unless a variable-RBE model is turned on"],"since":2008},"route":"/technologies/intensity-modulated-proton-therapy/","neighbours":{"cancer":[{"id":"brain-tumours","kind":"cancer","name":"Brain and spinal cord tumours (all types)","route":"/cancers/brain-tumours/"},{"id":"childhood-cancers","kind":"cancer","name":"Childhood cancers (all types)","route":"/cancers/childhood-cancers/"},{"id":"head-and-neck","kind":"cancer","name":"Head and neck squamous cell carcinoma","route":"/cancers/head-and-neck/"},{"id":"stage-iii-unresectable-nsclc","kind":"cancer","name":"Unresectable stage III non-small-cell lung cancer","route":"/cancers/stage-iii-unresectable-nsclc/"}],"section":[{"id":"radiation","kind":"section","name":"Radiation Therapy","route":"/fronts/radiation/"}],"technology":[{"id":"carbon-ion-synchrotrons","kind":"technology","name":"Carbon-ion synchrotrons and facilities","route":"/technologies/carbon-ion-synchrotrons/"},{"id":"flash-rt","kind":"technology","name":"FLASH radiotherapy","route":"/technologies/flash-rt/"},{"id":"proton-arc-therapy","kind":"technology","name":"Proton arc therapy","route":"/technologies/proton-arc-therapy/"},{"id":"proton-therapy","kind":"technology","name":"Proton therapy","route":"/technologies/proton-therapy/"},{"id":"proton-therapy-systems","kind":"technology","name":"Proton therapy machines: cyclotrons, synchrotrons and single-room systems","route":"/technologies/proton-therapy-systems/"}],"term":[{"id":"bragg-peak","kind":"term","name":"Bragg peak","route":"/terms/bragg-peak/"},{"id":"linear-energy-transfer","kind":"term","name":"Linear energy transfer (LET)","route":"/terms/linear-energy-transfer/"},{"id":"relative-biological-effectiveness","kind":"term","name":"Relative biological effectiveness (RBE)","route":"/terms/relative-biological-effectiveness/"}],"pairing":[{"id":"proton-vs-imrt","kind":"pairing","name":"Caution: proton therapy vs IMRT","route":"/pairings/proton-vs-imrt/"}],"idea":[{"id":"idea-bio2-let-rbe-ab-selects-protons","kind":"idea","name":"Use LET, RBE and alpha/beta to decide when protons beat IMRT","route":"/ideas/idea-bio2-let-rbe-ab-selects-protons/"}],"paper":[{"id":"paper-paganetti-proton-rbe-ijrobp-2014","kind":"paper","name":"Paganetti 2014: proton RBE is not a fixed 1.1","route":"/key-papers/paper-paganetti-proton-rbe-ijrobp-2014/"}]}}