a critical evaluation of internal organ immobilisation techniques

this paper aims to evaluate the range of techniques available to minimise both interfraction and intrafraction errors. the main interfraction errors are due to changes in volume of the rectum and bladder. intrafraction errors are mainly due to respiration and to a lesser extent cardiac motion. there are various methods of minimising internal organ motion that attempt to permit reduction of the internal margin around the clinical target volume (ctv).techniques such as rectal balloon insertion and breathing control are evaluated to determine their role in reduction of margins for improved conformal radiotherapy. the paper concludes that rectal balloons have been shown to permit limited reduction of internal margins and morbidity levels. breathing control has not increased reproducibility, but has allowed for reduction in lung morbidity. reduction of margins can only be recommended when using breathing control in conjunction with daily ctv relocalisation.although these techniques do have a role to play at the moment, it appears that attempting to maintain a static environment within the highly mobile patient is fraught with difficulties and we must accept that there is always going to be motion. rather than attempting to control the position of the tumour, future developments such as adaptive radiotherapy and tomotherapy may account for the movement.