Somayeh Nourollahi - M.Sc.in Medical Physics, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
Mahmoud Allahverdi - Associate Professor, Medical Physics Dept., Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
Mahbod Esfahani - M.Sc.in Medical Physics, Radiotherapy Dept., Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
Mehdi Aghili - Assistant Professor, Radiotherapy Dept., Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran

Introduction: The complex geometry of breast and also lung and heart inhomogeneities near the planning target volume (PTV) result in perturbations in dose distribution. This problem can result in overdosage or underdosage in the match region of the three treatment fields. The purpose of this study is to create a homogeneous dose distribution in the match region between the supraclavicular and tangential fields, utilizing Gafchromic EBT film for dosimetry. Material and Methods: In this study, a slab phantom was designed with lung- and heart-equivalent inhomogetenities. Our measurements were carried out using Gafchromic EBT film. Irradiations were performed using the 6 MV photon beam of a Varian 2100 Clinac linear accelerator. Three-dimensional treatment planning and dose calculations were carried out using the RT Dose Plan software. The conventional method with source-to-skin distance match and a 3D method with source-to-axis distance match under different combinations of parameters such as gantry, collimator and couch rotations as well as asymmetric fields were utilized. Results: The results demonstrated differences between the 3D and conventional techniques. The best results in the match region were observed using the 3D method with asymmetric technique (maximum area of overlap = 0.43 cm2, depth of overlap = 3.55 cm, and average width of overlap = 0.75 cm). Discussion and Conclusion: This study shows that the EBT film is a suitable tool for two-dimensional distribution dosimetry and relative dose measurements. The study shows difficulties in achieving a homogenous dose distribution in the match region of supraclavicular and tangential fields and also demonstrates the effects of setup parameters on matching of treatment fields.

Breast Radiotherapy, Dose Inhomogenity, EBT Film Dosimetry, Match Region