On-line measurement of field placement errors in external beam radiotherapy.

An on-line electronic portal imaging system was used in a quantitative measurement of field placement errors in external beam radiotherapy. Data from 30 patients, giving a total of 150 portals with 66% in the thoracic region and 34% in the abdominal region, were analysed. Displacements from the prescription delineated by physicians were measured. The displacements included errors to block malposition, field malposition, field malrotation and patient malposition, and these were classified in two categories of displacements, those associated with the portal boundary and those with anatomic structures. Except for one rotational error, only translational errors were found. Displacements greater than 1 cm amounted to 17% with respect to the portal boundary and between 10% and 46% with respect to anatomic landmarks within the portal. From the results of the study, a margin of 1 cm around the tumour is barely sufficient if a 5% accuracy in dose delivery is desired.