Reproducibility of field alignment in radiation therapy. A large-scale clinical experience.

A specially designed cassette which gives an excellent quality of portal films has been used to determine the set-up accuracy and uncertainties in treatment alignment during full-course radiation therapy on patients treated for some common malignant diseases. An analysis of a comparison between simulator films and portal films is also presented. For various diagnoses treatment-to-treatment positioning varied with an average standard deviation of 3.5 mm despite the use of laser alignment and good patient fixation.