High-resolution conformal radiation therapy demands that consultant radiation oncologists accurately localise the tumour and define the tumour contour. They can then maximise the dose to the tumour while minimising the dose to the surrounding healthy tissue.
Unfortunately, physiological functions, such as normal respiration, can change the tumour position during treatment. Traditionally, this fact required the use of a larger treatment volume to compensate for tumour movement, with associated increases in the amount of normal, healthy tissue being irradiated.
4D planning and respiratory gating in radiation therapy allows us to monitor respiration quickly and easily, without sacrificing accuracy and patient comfort.
The system uses a sophisticated video monitor to characterise the patient’s breathing pattern. The pattern is obtained by tracking the motion of the lightweight marker box placed on the patient’s abdomen.
Through video image analysis and signal processing, the system identifies both the full range of chest wall motion during respiration and the normal pattern of that motion. By correlating this data with the motion of the tumour, we can create a treatment plan that ‘gates’ the treatment beam, meaning that the treatment machine switches on only when the tumour falls within the required positions.
This system allows the patient to breathe naturally and remain comfortable, although in some cases it may be necessary to coach the patient into a more even breathing pattern or have them hold their breath for short periods of time.
This method of treatment delivery is not necessary in all tumours, and is most helpful in body sites where breathing can potentially cause a large amount of motion of the internal organs, such as thoracic and abdominal tumours like primary lung cancer, pancreatic or stomach cancers. Often, a 4D planning scan is done to assess the breathing pattern and organ/tumour motion in such cases, but respiratory gating is not always required.