The movements in the target area caused by breathing have to be taken into account when planning radiotherapy for the chest (lung and breast cancer) in particular.

This is done by selecting safety zones which are large enough to ensure that the tumour does not move out of the radiation field while radiation is being administered.

As a result, however, larger areas of healthy lung or the heart are sometimes radiated as well. In respiratory triggered radiotherapy (gating) the entire breathing cycle is included in a CT made while the radiation is being planned.

When the radiotherapy is planned, the phases of the breathing cycle which are best for administering the radiation can be selected.  Radiation is then only applied during the phases in which the tumour is in a certain position.

This technique enables the safety zones for radiation to be reduced, thus affecting less of the healthy lung and the heart in particular.