# In-room imaging for radiotherapy (cone-beam CT, ExacTrac, CT-on-rails, HyperSight)

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

## TL;DR

The scanners built into or beside the radiotherapy machine that photograph the patient seconds before the beam fires, so the tumour is where the plan expects and, on the newest machines, so the plan can be redrawn to that day's anatomy.

## Summary

Image guidance moved from weekly port films to daily three-dimensional imaging in the 2000s. A kilovoltage X-ray tube and flat panel mounted at right angles to the treatment head (Varian's On-Board Imager, Elekta's XVI) rotate with the gantry to produce a cone-beam CT of the patient on the couch, which is registered to the planning CT to shift the couch; the megavoltage treatment beam and its imaging panel provide a lower-contrast alternative. Brainlab's ExacTrac uses two floor-mounted kV tubes and ceiling detectors for stereoscopic X-rays that verify position at any gantry angle, the standard for frameless brain radiosurgery. CT-on-rails places a diagnostic CT scanner in the treatment room that slides over the couch, giving full-quality images for adaptive planning; TomoTherapy's megavoltage CT and Radixact's ClearRT kV CT image with the treatment ring; Varian's HyperSight on Halcyon and Ethos acquires a CT-quality cone-beam scan in seconds; and MR-linacs replace X-rays with a magnet.

The gains are smaller margins and daily adaptation; the costs are imaging dose that accumulates over a course, time on the couch, image quality that is worse than diagnostic CT for soft tissue on most cone-beam systems, and a heavier physics QA load.

## Fields

- Kind: Technology
- Status: standard-of-care
- Last checked: 2026-09-17
- Also known as: IGRT hardware; kV imaging
- Tags: machines-wave2
- Principle: Kilovoltage or megavoltage sources and detectors on or beside the treatment gantry acquire radiographs, cone-beam CT or fan-beam CT of the patient in treatment position, which are registered to the planning images to correct position or to re-plan.
- Since: 2005
- Strengths: Daily verification of tumour position; Enables adaptive replanning; Stereoscopic X-rays suit frameless radiosurgery
- Limitations: Cone-beam soft-tissue contrast is poorer than diagnostic CT; Adds imaging dose and couch time; Heavy QA burden

## Sources

- Jaffray et al., Flat-panel cone-beam computed tomography for image-guided radiation therapy (IJROBP 2002): https://doi.org/10.1016/S0360-3016(02)02884-5
- Brainlab: ExacTrac Dynamic: https://www.brainlab.com/radiosurgery-products/exactrac/

## Connected records

- technologies: [Adaptive radiotherapy (online replanning)](https://onco.cc/technologies/adaptive-radiotherapy/), [CT (computed tomography)](https://onco.cc/technologies/ct/), [IMRT / IGRT (modern external beam)](https://onco.cc/technologies/imrt-igrt/), [MR-guided adaptive radiotherapy](https://onco.cc/technologies/mr-linac/), [Patient positioning and surface guidance systems (AlignRT, Catalyst, ExacTrac Dynamic)](https://onco.cc/technologies/patient-positioning-surface-guidance-systems/), [Radiotherapy QA phantoms and dosimeters (Sun Nuclear, IBA Dosimetry, PTW)](https://onco.cc/technologies/radiotherapy-qa-phantoms-dosimeters/), [Respiratory gating and tumour tracking systems (RPM, ABC, Calypso, Synchrony)](https://onco.cc/technologies/respiratory-gating-tumour-tracking-systems/), [Ring-gantry linacs (Halcyon, Ethos)](https://onco.cc/technologies/ring-gantry-linac/), [TomoTherapy and Radixact (helical radiotherapy)](https://onco.cc/technologies/tomotherapy/)
- cancers: [Cervical cancer](https://onco.cc/cancers/cervical/), [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: [Imaging](https://onco.cc/fronts/imaging/), [Radiation Therapy](https://onco.cc/fronts/radiation/)
- companies: [Accuray](https://onco.cc/companies/accuray/), [Brainlab](https://onco.cc/companies/brainlab/), [Elekta](https://onco.cc/companies/elekta/), [Siemens Healthineers](https://onco.cc/companies/siemens-healthineers/), [Varian (Siemens Healthineers)](https://onco.cc/companies/varian/)

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