Computers in Biology and Medicine 123 (2020) 103878 Available online 5 July 2020 0010-4825/© 2020 Elsevier Ltd. All rights reserved. Design and characterization of fattening flter for high dose rate 192 Ir and 60 Co Leipzig applicators used in skin cancer brachytherapy: A Monte Carlo study Parvin Ghobadi a , Bagher Farhood a, * , Mahdi Ghorbani b, ** , Mehran Mohseni a a Department of Medical Physics and Radiology, Faculty of Paramedical Sciences, Kashan University of Medical Sciences, Kashan, Iran b Biomedical Engineering and Medical Physics Department, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran A R T I C L E INFO Keywords: Skin cancer High dose rate brachytherapy 192 Ir 60 Co Leipzig applicator Flattening flter ABSTRACT Purpose: This study aimed to design optimal fattening flters for high dose rate (HDR) 192 Ir and 60 Co Leipzig applicators which are used to treat skin cancer. Materials and methods: MCNPX Monte Carlo code was used to design fattening flters for Leipzig applicators with inner diameters of 1, 2 and 3 cm. Then, their dosimetric characterizations such as dose distribution, dose profle, percentage depth dose, fatness, symmetry and homogeneity were evaluated in a 20 � 20 � 20 cm 3 water phantom and compared with those without the fattening flter. Results: The fattening flter thickness varied from 0 mm (at the edge) to the maximum values of 0.30, 1.18, and 2.41 mm for the 192 Ir 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 60 Co double wall applicators of D1, D2, and D3 type, respectively. The dose profle fatness values for the H1, H2, and H3 192 Ir Leipzig applicators with the optimal fattening flters were 0.76, 1.26, and 1.85%, respectively. Furthermore, the dose profle fatness values for the D1, D2, and D3 60 Co double wall applicators with the optimal fattening flters were 1.11, 2.10 and 3.12%, respectively. The dose profle symmetry values obtained from various source-applicator combinations were less than 1.02. Compared to the applicators without fattening flter, the homogeneity values for the H1, H2, and H3 192 Ir Leipzig applicators with the optimal fattening flters were improved 1.68, 6.51, and 13.17 times, respectively, and for the D1, D2, and D3 60 Co double wall applicators were improved 1.23, 6.21 and 9.54 times, respectively. Conclusion: The fndings revealed that the inhomogeneous dose distribution resulted from the Leipzig applicators without the optimal fattening flter at the treatment surface could be improved by insertion of optimal lead fattening flters between the sources and treatment surface. 1. Introduction Skin cancer is a common human malignancy. Three main types of this cancer include: basal cell carcinoma (BCC), squamous cell carci- noma (SCC) and melanoma. Non-melanoma skin cancers (BCC and SCC) are global problems and their annual incidence is 2–3 million cases [1–3]; nevertheless, if diagnosed early, these types of skin cancer can be mostly controlled or treated effectively with radiotherapy [4–6]. There are some radiotherapy techniques for treatment of non-melanoma skin cancers comprising superfcial X-rays, orthovoltage X-rays, electron beam therapy and high dose rate (HDR) brachytherapy [5,7,8]. HDR brachytherapy for non-melanoma skin cancers can be per- formed by 192 Ir and 60 Co sources. Different 192 Ir-based HDR remote after-loading systems are available [9]. The source used in these systems ( 192 Ir) has a half-life of 73.81 days and its average photon energy (in air) is 0.380 MeV. Recently, 60 Co-based HDR remote after-loading systems have been suggested as an alternative to 192 Ir-based HDR systems. This radionuclide ( 60 Co) has a half-life of 5.27 years and emits two gamma rays per disintegration (1.17 and 1.33 MeV) with a mean energy of 1.25 MeV [10–13]. Leipzig brachytherapy applicators have been designed for surface radiotherapy treatments such as skin cancer treatment; and utilization of * Corresponding author. ** Corresponding author. E-mail addresses: bffarhood@gmail.com (B. Farhood), mhdghorbani@gmail.com (M. Ghorbani). Contents lists available at ScienceDirect Computers in Biology and Medicine journal homepage: http://www.elsevier.com/locate/compbiomed https://doi.org/10.1016/j.compbiomed.2020.103878 Received 15 May 2020; Received in revised form 21 June 2020; Accepted 21 June 2020