VERIFICATION OF SET-UP DEVIATIONS IN PATIENTS WITH BREAST-CANCER USING PORTAL IMAGING IN CLINICAL-PRACTICE

Aim: The aim of this study was to determine set-up deviations during i rradiation of patients with breast cancer using Electronic Portal Imag ing (EPI). Patients and Method: In order to monitor the intrafractiona l set-up deviations multiple portal image readings were carried out on 5 patients with breast cancer. Moreover interfractional set-up deviat ions were determined by the acquisition of daily EPIs in a total of 20 patients. Results: The results of intrafractional set-up deviation ba sed on 130 EPIs revealed a maximum lateral and longitudinal (cranio-ca udal) deviation of 2 mm (range: -10 mm to 8 mm) as well as 12 degrees in rotation (range: -2 degrees to 2 degrees). The interfractional set- up deviations in 20 breast cancer patients during a treatment series o f 25 fractions showed mean standard deviations of 5 mm in lateral and longitudinal direction, respectively. Only in few cases deviations up to maximally 24 mm were observed. The mean standard deviation of the r otational error was 2 degrees and reached a maximum of 6.5 degrees. Co nclusion: These results show that intrafractional set-up deviations in breast cancer patients are negligible in clinical practice. They can be attributed to random errors due to patient movement and breathing. The set-up deviations during a treatment series can be differentiated in systematic and random errors. Patient fixation and immobilization i s crucial in minimizing random errors. Taking into account 10 mm safet y margins as used in our department around the ''clinical target volum e'' (CTV), set-up errors outside of this volume (PTV) were exceptional . The enlarged PTV definition of 15 mm for lateral and cranio-caudal f ield margins, respectively as used in this study covers 99% of the CTV in all patients.