Evaluation of the dose distribution of half-beam abutted fields in head and neck cancer by film dosimeter and computer based planning system [Baş-boyun bölgesi kanserlerinde yari{dotless}m demet bitişik alan doz dagi{dotless}li{dotless}mlari{dotless}ni{dotless}n film dozimetrisi ve bilgisayarli{dotless} planlama sistemi ile degerlendirilmesi]

OBJECTIVES The aim of this study is to determine the dose inhomogenity in abutted fields-formed by using single isocentre and asymmetric collimators by film dosimetry and TPS in the radiotherapy of HN cancer. METHODS Using different SSD's for supraclavicular and lateral fields asymmetric collimators were set to 0 mm, 1 mm, 2 mm position and treatment planning was done for 45 different situations. Films were irradiated, dose distribution along the junction were determined. RESULTS 2 mm and 1 mm overlap and gap produced 22% and 13% overdose, 21% and 15% underdose respectively. 0 mm the dose nonuniformity is about 2%. The difference in dose profiles along the junction obtained from films and TPS wasn't statistically significant. TPS and linear accelerator were in accordance with each other. Besides the penumbra and scattered radiation also contributed to the uniformity of dose distribution along the junction point. CONCLUSION Single-isocentre technique produces a dose similar to the prescribed tumor dose along the field junction. The dose distribution is homogeneous at 0 mm and treatments were delivered with accuracy. © 2013 Association of Oncology.