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Image-Guided RadioTherapy (IGRT):
- The goal of IGRT is to provide the tools necessary to manage both interfractional and intrafractional motion to improve the accuracy of treatment delivery. That is, the goal of IGRT is to be able to keep track of tissues and tumors in spite of the fact that they may experience a great range of natural movement (both between treatment fractions and even during a fraction) for the purpose of amending the plan, patient and machine position for more accurate dose delivery.
- Intrafractional movement may be caused by the cardiac system, organ filling and peristalsis, the function of the respiratory system, and the movement of the skeletal muscle.
- Retrospective Gating in 4DCT - uses low pitch helical scans recording data and then binning data with respect to the respiratory cycle. Data is collected for all phases of the respiratory cycle and then separated by software. This can be useful in establishing Maximal Intensity Projections (MIPs).
- Prospective Gating in 4DCT - records data during only one phase of the respiratory cycle meaning that lower doses are given because images are only taken during a particular phase.
- Conventional Approaches for Combating Intrafractional Motion:
- Increase the margins to take into account the motion during treatment (i.e. create an Internal Target Volume, ITV).
- Difficulties in Respiratory Radiotherapy Treatments:
- Irregular breathing patterns.
- Sudden breathing changes (caused by, for example, coughing).
- Mismatched gated CT data with patient treatment.
- Increased treatment times (reduced patient throughput).