Design and characterization of flattening filter for high dose rate 192Ir and 60Co Leipzig applicators used in skin cancer brachytherapy: A Monte Carlo study

Purpose: This study aimed to design optimal flattening filters for high dose rate (HDR) Ir-192 and Co-60 Leipzig applicators which are used to treat skin cancer. Materials and methods: MCNPX Monte Carlo code was used to design flattening filters for Leipzig applicators with inner diameters of 1, 2 and 3 cm. Then, their dosimetric characterizations such as dose distribution, dose profile, percentage depth dose, flatness, symmetry and homogeneity were evaluated in a 20 x 20 x 20 cm(3) water phantom and compared with those without the flattening filter. Results: The flattening filter thickness varied from 0 mm (at the edge) to the maximum values of 0.30, 1.18, and 2.41 mm for the Ir-192 Leipzig applicators of H1, H2, and H3 type, respectively. This quantity has maximum values of 0.96, 6.27, and 12.31 mm for the Co-60 double wall applicators of D1, D2, and D3 type, respectively. The dose profile flatness values for the H1, H2, and H3 Ir-192 Leipzig applicators with the optimal flattening filters were 0.76, 1.26, and 1.85%, respectively. Furthermore, the dose profile flatness values for the D1, D2, and D3 Co-60 double wall applicators with the optimal flattening filters were 1.11, 2.10 and 3.12%, respectively. The dose profile symmetry values obtained from various source-applicator combinations were less than 1.02. Compared to the applicators without flattening filter, the homogeneity values for the H1, H2, and H3 Ir-192 Leipzig applicators with the optimal flattening filters were improved 1.68, 6.51, and 13.17 times, respectively, and for the D1, D2, and D3 Co-60 double wall applicators were improved 1.23, 6.21 and 9.54 times, respectively. Conclusion: The findings revealed that the inhomogeneous dose distribution resulted from the Leipzig applicators without the optimal flattening filter at the treatment surface could be improved by insertion of optimal lead flattening filters between the sources and treatment surface.