In the radio therapeutic treatment of patients, inability to irradiate the entire diseased area consistently and accurately can result in local failure or improper control of disease. Since 1969, a film technique developed for imaging of the complete patient radiation exposure has been applied to evaluate localization errors occurring in our radiotherapy department. The technique basically involves placing a film with good image quality, proper sensitivity and contrast in the exit-dose exposure ranges encountered in radiotherapy, under the patient during the entire radiation exposure. In the evaluation of the film a localization error was noted when portions of the diseased area were not properly included in the treatment field, or when an excessive region of normal tissue was improperly shielded. Examples from an investigation are shown whereby verification films were taken for every treatment in 5 selected cases, including 3 patients treated with extended mantle fields and 2 treated with pelvic fields. In this study, 62 out of 206 treatments showed localization errors. For the extended mantle fields in this study, if a localization error occurred, it was usually propagated throughout the treatment. The main goal in reducing errors in radiation therapy is to provide optimum treatment for the patient. In the past, much work has been done to reduce errors in dosimetry. It is clear from the results described that a major effort must also be made to minimize localization errors.
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