Organ motion/internal margin

 Variations in organ motion may be small (e.g. brain), larger and predictable (e.g. respiration or cardiac pulsation), or unpredictable (e.g. rectal and bladder filling).

When treating lung tumours, the displacement of the CTV caused by respiration can be dealt with in several ways: by increasing the CTV-PTV margin eccentrically to include all CTV positions during a respiratory cycle; by using suspended respiration with a technique such as the active breathing control (ABC) device; or by delivery of radiation using gating or respiratory correlated CT scanning and treatment. Protocols for minimising effects on the CTV of variations in bladder and rectal filling are described in relevant chapters. Uncertainties from organ motion can also be reduced by using fiducial markers, and published results are available for lung, prostate and breast tumours. Radio-opaque markers are inserted and imaged at localisation using CT or MRI, and at treatment verification, using portal films, electronic portal imaging devices (EPIDs) or online cone beam CT image-guided radiotherapy (IGRT). The internal margin therefore allows for inter- and intra-fractional variations in organ position and shape which cannot be eliminated.

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