External beam treatment planning. Can we deliver what we plan?

Many sources of uncertainty exist in the delivery of the prescribed dose to the specified target volume in external beam radiotherapy. Their magnitudes are compared with recommendations for the overall accuracy of dose delivery (+/- 5%). Five dose intercomparisons are reviewed, including a recent UK survey undertaken by the Institute of Physical Sciences in Medicine (IPSM). For phantom doses, standard deviations in the ratio of stated to delivered doses range from 1-3.5%, with larger uncertainties when irradiation of volumes within anthropomorphic phantoms is considered. In addition, uncertainties arise from geometrical discrepancies, and the role of portal films in their assessment is also considered. Imaging during radiotherapy will be increasingly important in the development of complex conformal treatments, where the shape of the target volume may be irregular.