# Patient positioning and surface guidance systems (AlignRT, Catalyst, ExacTrac Dynamic)

Source: https://onco.cc/technologies/patient-positioning-surface-guidance-systems/  
OnCo record `patient-positioning-surface-guidance-systems` (Technology). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Cameras that watch the patient's skin in three dimensions, robotic couches and moulded shells that hold the body still, so each radiotherapy dose lands where the plan says without tattoos and with the beam paused if the patient moves.

## Summary

Radiotherapy accuracy depends on putting the patient in the same position every day. Immobilisation devices (thermoplastic masks for head and neck and brain, vacuum bags, breast boards, indexed couch tops from Orfit, Qfix and CIVCO) provide the coarse fix. Surface-guided radiotherapy systems then project structured light onto the skin and reconstruct the surface from ceiling cameras many times a second: Vision RT's AlignRT, C-RAD's Catalyst+ and Sentinel, Brainlab's ExacTrac Dynamic (which fuses surface and thermal imaging with stereoscopic X-rays) and Varian's IDENTIFY. The surface is registered to the planned surface to guide set-up, monitor intra-fraction motion and gate the beam, which allows tattoo-free set-up, deep-inspiration breath-hold for left breast cancer, and open-face masks for brain radiosurgery. Six-degree-of-freedom robotic couches correct residual rotations.

The skin is a surrogate: for tumours deep in the body or lying loosely under the surface (prostate, lung, liver) surface position does not guarantee target position, so cone-beam CT or fiducial tracking remains necessary. The systems add capital cost and commissioning effort, and their accuracy depends on camera calibration and on how much skin is exposed.

## Fields

- Kind: Technology
- Status: established
- Last checked: 2026-09-17
- Tags: machines-wave2
- Principle: Structured-light cameras reconstruct the patient's skin surface in real time and register it to the reference surface from planning, driving set-up corrections, motion monitoring and beam gating; rigid immobilisation and robotic couches supply the coarse and rotational corrections.
- Since: 2000
- Strengths: Tattoo-free set-up and continuous motion monitoring; Enables breath-hold and open-face masks; No added radiation dose
- Limitations: Skin is only a surrogate for deep targets; Camera calibration and exposed skin limit accuracy; Additional capital and commissioning

## Sources

- Vision RT: AlignRT: https://www.visionrt.com/product/alignrt/
- C-RAD: Catalyst+: https://c-rad.se/
- Al-Hallaq et al., AAPM Task Group 302: surface-guided radiotherapy (Medical Physics 2022): https://doi.org/10.1002/mp.15532

## Connected records

- technologies: [C-arm medical linear accelerators (TrueBeam, Versa HD and others)](https://onco.cc/technologies/c-arm-linac/), [IMRT / IGRT (modern external beam)](https://onco.cc/technologies/imrt-igrt/), [In-room imaging for radiotherapy (cone-beam CT, ExacTrac, CT-on-rails, HyperSight)](https://onco.cc/technologies/in-room-imaging-systems/), [Respiratory gating and tumour tracking systems (RPM, ABC, Calypso, Synchrony)](https://onco.cc/technologies/respiratory-gating-tumour-tracking-systems/), [Stereotactic radiosurgery (Gamma Knife, CyberKnife, linac SRS)](https://onco.cc/technologies/radiosurgery-srs/), [Surface-guided radiotherapy (SGRT)](https://onco.cc/technologies/surface-guided-radiotherapy/)
- cancers: [Breast cancer (all types)](https://onco.cc/cancers/breast-cancer/), [Head and neck squamous cell carcinoma](https://onco.cc/cancers/head-and-neck/), [Non-small-cell lung cancer](https://onco.cc/cancers/nsclc/), [Prostate cancer](https://onco.cc/cancers/prostate/)
- terms: [Brain metastases (intracranial disease)](https://onco.cc/terms/brain-metastases/)
- fronts: [Devices & Physical Therapies](https://onco.cc/fronts/devices/), [Radiation Therapy](https://onco.cc/fronts/radiation/)
- companies: [Brainlab](https://onco.cc/companies/brainlab/), [C-RAD](https://onco.cc/companies/c-rad/), [Varian (Siemens Healthineers)](https://onco.cc/companies/varian/), [Vision RT](https://onco.cc/companies/vision-rt/)

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