THE VALUE OF SETUP PORTAL FILMS AS AN ESTIMATE OF A PATIENTS POSITIONTHROUGHOUT FRACTIONATED TANGENTIAL BREAST IRRADIATION - AN ONLINE STUDY

Purpose: To determine if portal setup films are an accurate representa tion of a patient's position throughout the course of fractionated tan gential breast irradiation. Methods and Materials: Thirteen patients u ndergoing external beam irradiation for T1-T2 infiltrating ductal carc inoma of the breast following excisional biopsy and axillary dissectio n were imaged using an on-line portal imaging device attached to a 6 M V linear accelerator, Medial and lateral tangential fields were imaged and a total of 139 fractions, 225 portal fields, and 4450 images were obtained, Interfractional and intrafractional variations for anatomic al parameters including the central lung distance (CLD), central flash distance (CFD), and inferior central margin (ICM) were calculated fro m these images, A pooled estimate of the random error associated with a given treatment was determined by adding the interfractional and int rafractional standard deviations in quadrature, A 95% confidence level assigned a value of two standard deviations of the random error estim ate, Central lung distance, CFD, and ICM distances were then measured for all portal setup films, Significant differences were defined as oc curring when the simulation-setup difference was greater than the 95% confidence value. Results: Differences between setup portal and simula tion films were less than their 95% confidence values in 70 instances indicating that in 90% of the time these differences are a result of r andom differences in daily treatment positioning. Conclusions: In 90% of cases tested, initial portal setup films are an accurate representa tion of a patients daily treatment setup. Copyright (C) 1997 Elsevier Science Inc.