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İÜ Cerrahpaşa Tıp Fakültesi Radyasyon Onkolojisi AD, İstanbul The purpose of this study is to use two opposed tangential fields collimated with one dimentional asymmetric collimator to irradiate breast and give information about the asymmetric collimation dosimetry. In our department, tangential breast irradiation is given with 4 MV linear accelerator which has one dimentional asymmetric collimator. Asymmetric collimators have the advantages of set-up reproducibility and the elimination of beam divergence, however it has disavantages like the limitation of wedge sizes. To study the effect of asymmetric collimation on out-put values, we performed out-put measurement at various symmetric and half-blocked asymmetric fields on solid water phantom at dmax using 0.6 cc PTW ionization chamber and PTW Unidose dosimeter. It was observed that the set-up time when asymmetric collimator was used was shorter than the set-up time with other blocks. As the off-axis distance increased there was an increase in the out-put of asymmetric fields compared to symmetric fields. This difference between symmetric and asymmetric fields ranges between 1.6%-3.3%. Measured and calculated maximum out-put values for asymmetric fields were found to be 1.1% and it was in the acceptable range of 2%. As a result before starting tangential breast irradiation with asymmetric fields necessary dosimetric measurements have to be performed and these values have to be used to correct the dose. Keywords : asymmetric collimation, tangential breast irradiation, out-put