4D Computed Tomography/Respiratory Gating Treatment
A concern with the irradiation of lung cancers is that many of these tumors move with respiration. Motion poses a number of special problems, including problems with accurate target definition (moving targets may appear blurred on CT and larger in size) and increased irradiation of normal tissues (larger fields are often used to ensure that the tumor is not missed).
To improve the visualization of moving tumors, a variety of techniques have been proposed. One of the simplest is voluntary breath hold, in which the patient holds his or her breath during imaging. This is often problematic, however, in many lung cancer patients due to poor lung function.
At UCSD, a more sophisticated approach has been implemented known as 4D computed tomography (4DCT) imaging and respiratory gating. A 4D CT scan is comprised a large number of individual CT scans obtained at various phases of the respiratory cycle. This approach allows the radiation oncologist to watch the movement of the tumor with respiration. Interestingly, some tumors have considerable movement, others have only minimal movement. Unfortunately, it is not possible to predict in advance which tumors move and which do not. Thus, a 4DCT is used to evaluate all patients.
Healthy Volunteer Undergoing a 4DCT Scan. Note small box on the abdomen with infrared markers and the infrared detector at the bottom of the treatment couch. Respiration is monitored by the movement of the box and the images are sorted according to the phase of the breathing cycle.
Together with the Medical Physicist, the Radiation Oncologist uses the 4DCT data to decide whether it is best to deliver treatment only at specific phases of breathing (“gated therapy”) or with the patient freely breathing.
If gated therapy is selected, the physician and physicist must select the proper respiratory phase to deliver treatment. During treatment the patient’s respiration is monitored and the beam is turned on and off during the phases when tumor movement is minimal. Using this approach, significant reductions in the volume of normal lung tissue is achieved, potentially reducing the risk of radiation toxicity.
Monitor at Treatment Console in a patient treated with respiratory gating. The beam is turned on and off automatically based on the breathing cycle of the patient.
At UCSD, respiratory gating is performed using the Varian real-time position management (RPM) system. At simulation, a small “box” is placed on the patient’s chest/upper abdomen. Specialized cameras as used to monitor the motion of this box during respiration. This information is used to correlate the position of the lung tumor with specific phases of the respiratory cycle. At treatment, motion of the box allows the treatment beam to be turned on and off (gating) during specific phases of the breathing cycle.