Precision of determining compliance with prescribed fields from conventional portal films

Three hundred and seventy measurements of field placement errors (FPEs) have been made by a total of 16 observers on 20 prescription-treatment film pairs taken during routine radiotherapy for cancer of the prostate. Analysis of the distributions of the measured FPEs has yielded the precision of the measurement under a variety of conditions. We report here the influence on the precision of determining FPEs of the following factors: the clinical duties of the observers, the quality of the treatment film, the relative magnification of prescription and treatment films, and whether double-exposure techniques were employed.