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Targeting a Moving Lung Tumour
Breathing causes organs to move
How do you treat a moving target with an X-ray beam?
Especially when it is a lung tumour moving while the patient breathes? Often the treatment targets are small and the planned dose distributions have steep gradients with potentially very high doses delivered in a small number of fractions (Figure 1a).
Ben Cooper, PhD
Qualified Medical Physics Specialist,
Chief Medical Physics Specialist, Medical Physics and Radiation Engineering, Canberra Health Services
If there are patient motion or set-up errors, there is a geometric error between the target and the beam. This leads to a geographic miss – i.e. part of the target is underdosed and some normal tissue receives a higher than planned dose (Figure 1b).
The most significant motion in the thoracic and abdominal region is caused by respiration. Respiration is characterized as largely involuntary (i.e. automatic) but also with some voluntary control (e.g. holding one’s breath). The range of target motion due to respiration varies between patients. Each patient should be individually assessed to design the best means of motion management – during planning and treatment.
The extent of lung motion
In a trial carried out on 20 patients, 3-D tumor motion was measured. The results indicated that there was a different path it moved during inhalation and exhalation. The measurements were obtained from a real-time 3-D fluoroscopic recording of gold seeds implanted in the tumour. One-third of the cases showed cardiac motion affected trajectories of 1-4 mm.
However, the anatomical complexity of each case precluded the possibility of developing an accurate general mathematical model that could describe tumour motion. Figure 2. summarizes the 21 tumour trajectories measured in the study.
Options for better tumour targeting
Researchers have attempted a number of different methods to solve the problem of treating a moving target. Each technique has different pros and cons.
The targeting methods that are either in clinical use or are in the process of clinical implementation are:
- Enlarge the treatment volume to cover tumour motion;
- Restrict tumour motion (using active breath control methods);
- Use X-ray beam gating (turn on/off during breathing cycle)
- Measure and monitor motion during treatment
Figure 3. theoretically demonstrates motion diagrams of an ‘aqua’ coloured ellipsoid tumour for:
(a) the full extent the tumour moves
(b) a mid-ventilation tumour target,
(c) with abdominal compression,
(d) using breath hold and X-ray beam gating and
(e) tracking and targeting the tumour.
The volume of normal tissue unnecessarily irradiated with X-rays is much larger in option (a) than option (e) for target tracking
Enlarge the treatment volume
The initial proposal to allow for tumour motion was a simplistic approach. The prescribed treatment volume was made sufficiently large to cover the full extent of the tumour motion during the X-ray treatment. This ‘sledge-hammer’ method ensures that the X-ray beam would always hit the moving target.
Part (a) in Figure 3., illustrates an example of multiple positions of the “elliptical” shaped tumour as it moves up and down while the patient breathes. The overall area covered by the tumour (for all the aqua coloured ellipsoids) are enclosed within an extended (red) rectangle which describes the size and shape of the volume to be X-ray irradiated.
Irradiating so much normal lung tissue included in the ‘red’ volume is not ideal.
Because of the maximum dose placed on the normal tissue treated in the enlarged target volume, it limits the maximum dose that the tumour can be given.
In an attempt to improve this technique, the method of defining the tumour dimensions, extent, location, and the full extent the tumour encompasses during respiratory motion, a ‘four-dimensional’ result is derived from computed tomography (4D-CT) scan images. The treatment volume is calculated for either:
(a) Full extent of tumour movement (Figure 3(a))
3-D images are computer created from X-ray CT scans. Multiple planar scans recorded during the breathing cycle at each small intervals along the length of the patient. The recording of the patient’s breathing cycle is added to the 3-D scan to make it a 4-D CT scan. Corrections can be made to the image to reduce blurring caused by voluntary and involuntary patient movement.
Inclusion of the patient’s respiratory motion data in the 4D-CT provides improved images for:
- the size and shape of the aqua coloured tumour; and
- the extent of the tumour movement (Figure 3(a)).
Major disadvantages to this method are:
- normal lung tissue is unnecessarily treated; and
- the treatment volume may be over-estimated.
(b) Mid-ventilation CT scan (Figure 3(b))
The mid-ventilation CT scan technique uses a single, “well-chosen” CT scan from the 4D-CT set (Figure 3b). This acts as a representative reference average position of the mobile lung tumor target during the breathing cycle.
Restrict tumour motion
The department’s lung cancer working party should decide whether motion compensation by breath-hold will be used for treatment delivery or not. This affects the size allowed for treatment volume margins. Breath-hold techniques can potentially allow smaller target margins which lead to less normal tissue irradiation.
The patient goes through the ‘hold breath’ procedure during the planning and treatment sessions. Both procedures include control of respiratory motion signals and image guidance. Radio-opaque marker is often implanted to view and verify the breath-hold and tumor targeting.
(a) Deep inspiration breath-hold (DIBH) method
Similar to the technique used for breast cancer treatments, the deep inspiration breath-hold (DIBH) technique can be used for lung cancer radiation therapy. The patient modifies their normal breathing pattern, forcibly expels air, and then slowly take a deep breath and hold during treatment delivery (Figure 4.).
The DIBH technique increases the lung volume with air lowering the average lung density, reducing normal lung tissue dose, and, importantly, significantly reduces tumor target motion during the breath-hold. DIBH reduces the relative volume of lung receiving a critical 25 Gy dose by 30%.
(b) Active Breathing Control (ABC)
Active breathing control (‘ABC’) is a treatment technique designed to minimize tumor target motion during lung treatments. The air flows into the patient’s lungs via a computer-controlled breathing tube system. At a pre-determined phase of the respiratory cycle (usually full inspiration), the ABC stops the airflow and forces the patient to hold breath during the X-ray beam exposure (Figure 5.).
However, patients often find it difficult to comfortably hold their breath during the full length of the X-ray beam exposure and the ABC procedure has to be completed again during the rest of the exposure.
As can be appreciated from this article, enlarging the treatment volume to encompass the movement of the lung tumour, provides unnecessary treatment of normal lung tissue. And restricting tumour motion by the breath-hold method is also less than optimum in delivering an accurate dose that’s precisely targeted on the tumour during the whole X-ray exposure.
More recent research and development for targeting the moving tumour with an accurate dose, are described in the next article.
Ben Cooper PhD, 27 July 2020
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