Design of flattening filters based on intracavitary absorbed dose measurements in external radiation therapy

A method has been developed for the design of flattening filters on the basis of in-vivo oesophagus measurements of absorbed dose distribution in patients given external radiation therapy. The measurements were made by means of LiF-Teflon thermoluminescent dosimeters, read out by an automatic TL-system. A single filter was designed from the resultant dose distribution of parallel opposing fields. The method was checked clinically and was found to be convenient and reliable. The filters reduced the dose variation along the midline of the patients undergoing upper mantle treatments from about +/- 20% to about +/- 5%. The same limits of variation are believed to exist through the entire mid-plane of the body, though it was not possible to prove this by direct measurements.