Respiratory gating and tumour tracking systems (RPM, ABC, Calypso, Synchrony)
Hardware that copes with tumours that move when the patient breathes: belts and cameras that switch the beam on only in part of the breathing cycle, devices that hold the breath, and implanted beacons or X-ray tracking that let the beam follow the tumour.
Overview
Lung, liver, pancreatic and breast targets move by a centimetre or more with each breath. Gating systems monitor a surrogate, a reflective block or belt on the chest (Varian's RPM and its successor RGSC), a spirometer (Dyn'R SDX) or the surface itself, and enable the beam only within a chosen window; 4D-CT during planning maps the motion. Breath-hold devices such as Elekta's Active Breathing Coordinator close a valve at a set lung volume so the tumour is treated in a reproducible position, with surface guidance often replacing the device. Tracking goes further: Varian's Calypso reads the position of three implanted electromagnetic transponders ten times a second and can stop the beam or drive the couch; Accuray's Synchrony on CyberKnife and Radixact fits a model between surface markers and periodic X-ray images of fiducials or the tumour itself, then moves the robot or the collimator so the beam follows; MR-linacs gate on live cine MRI of the tumour with no implant.
The trade-offs are treatment time (gated beams may be on for a third of each cycle), the invasiveness of fiducial or transponder implantation, surrogate drift when internal and external motion decouple, and, for tracking, the complexity of the model and the QA it demands.
- Ablative dose, 1-5 fractions
- Converging beams
How it works
External surrogates, spirometry or implanted electromagnetic and radio-opaque markers report the position of a moving target; the beam is enabled only within a motion window, the breath is held at a set volume, or the beam is steered to follow the target.
- Smaller margins around moving tumours
- Breath-hold reduces heart dose in breast cancer
- Tracking keeps the beam on the target without gating delays
- Gating lengthens treatment
- Fiducials and transponders need implantation
- External surrogates can drift from internal motion
Latest papers
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