Lung malignancies may undergo significant shift due to breathing; this ensures the need for large ITV expansions to account for intrafraction motion. This can be overcome with two techniques: 4D CT and respiratory gating.
- 4D CT allows accurate determination of the ITV and reduction (hopefully!) of treatment volumes.
- Gating allows further reduction in the ITV by only delivering treatment when the tumour is in a particular phase of respiration.
To image the movement of a tumour within the lung, there are several techniques:
- Simple fast acquisition CT of the patient; this does not take into account tumour motion and this must be estimated based on the position of the tumour; the typical PTV expansion is 2 - 3 cm in the superior-inferior margin
- Slow CT or 'poor man's 4DCT'; acquired slowly during free breathing; this generates respiratory 'artefact' due to motion during CT acquisition; this allows an estimation of the tumour movement.
- 4D CT, where CT through every slice is acquired at different phases of respiration.
The tumour should be visualised at inspiration, mid exhalation, exhalation, and mid inhalation as it may be in different positions at each of these time points. The combination of these movements allows for an ITV to be created from the CTVs around the tumour at each phase of respiration. The PTV expansion is then 0.5 - 1 cm depending on the departmental set up error.
Respiratory gating attempts to reduce the ITV further by only delivering treatment during a set phase of the respiratory cycle. It requires:
- 4D CT to track tumour movement
- Tracking of respiratory motion during treatment
- Internal tracking involves imaging of the chest during treatment to identify the tumour position or a fiducial marker placed within the tumour
- External tracking involves detection of respiration through an external device that is used to determine when the tumour is in the desired position
Internal tracking requires online kV imaging, usually from multiple detectors placed within the bunker, to image the tumour position. These in turn control the beam 'on' and 'off' settings based on the tumour position.
External tracking (e.g. Varian RPM) uses a device on the patient's chest that is imaged with a non-ionising source. The device moves with respiratory motion.
Internal tracking is more accurate, more expensive and exposes the patient to more radiation and more risk of pneumothorax if markers are inserted. External tracking is less accurate but also less expensive, easier to implement, and safer.