Irradiation of Moving Target Volume

 
The most difficult task of radiation therapy is the irradiation of moving target volume. The traditional method is to determine larger margins around the target volume, which takes into account the movement of tumour and in this way it is ensured that the tumour is always in the field, and thus receives the prescribed dose. As a result, the normal tissue will receive relatively high doses.
Respiration gated radiotherapy has the potential for clinical target volume–planned target volume margin reduction, dose escalation to the target volume, and dose reduction to surrounding normal structures. In an idealized gated treatment, tumour position should be directly monitored and the delivery of radiation is only allowed when the tumour is at the correct position. This technique needs some special requirements: precise and real-time tumour localization, and prompt linac reaction to the gating signal. Internal gating utilizes internal tumour motion surrogates such as implanted fiducial markers while external gating relies on external respiratory surrogates such as makers placed on the patient’s abdomen. Respiratory gating should be applied to patient imaging during CT imaging, patient pre-treatment simulation and daily setup, in addition to the treatment delivery. Because the treatment is delivered only to a particular portion of patient breathing cycle (gating window), the patient/tumour geometry corresponding to the gating window should be used for treatment planning and patient alignment. Throughout the whole treatment course, this consistency of patient/tumour geometry should be maintained. The treatment fields will be reduced and the health tissue sparing will increase. The treatment time will be longer because the intermittent treatment.
For moving lung cancer the treatment planning and the treatment delivery can be made on the base of 4D CT imaging. The movement of target volume can be determined individually for all patients. The CT imaging can be made by respiratory gating and the image reconstruction will be performed in several phases. These image sets will be analysed in cine-mode and the definition of planning target volume can made taking into account the tumour movements. In this way we can reduce the margin around the clinical target volume. The treatment will be in traditional way, so that the treatment time is not increased as the respiratory gating technique.

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