The potential for systematic field placement errors when using asymmetric collimation for photon field junctions

During radiotherapy of breast or head and neck cancer it is often necessary to junction photon radiation fields to cover primary disease as well as the draining lymphatics. One increasingly popular technique to achieve this is a mono-isocentric technique for all abutting fields employing independent jaws. While this technique has many advantages it was found in a study of quality assurance requirements that the mono-isocentric technique was prone to some systematic errors in field placement due to incorrect calibration of the collimator read-outs and the isocentre definition of linear accelerator. It is not only important that operators are aware of these potential problems but also that quality assurance activities specific to the use of asymmetric collimation are implemented.