Delivered dose changes in COMS plaque-based ocular brachytherapy arising from vitrectomy with silicone oil replacement Hali Morrison 1,2, * , Matthew P. Larocque 1,3 , Geetha Menon 1,3 , Ron S. Sloboda 1,3 , Ezekiel Weis 4,5 1 Department of Oncology, University of Alberta, Edmonton, Alberta, Canada 2 Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta, Canada 3 Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada 4 Department of Ophthalmology, University of Alberta, Edmonton, Alberta, Canada 5 Department of Surgery, University of Calgary, Calgary, Alberta, Canada ABSTRACT PURPOSE: The purpose of the study was to determine dosimetric effects of performing concurrent I- 125 Collaborative Ocular Melanoma Study plaque brachytherapy and vitrectomy with replacement using silicone oil, previously shown to be a means of shielding uninvolved parts of the eye. METHODS AND MATERIALS: Monte Carlo simulations using MCNP6 were performed to compare the dosimetry with all eye materials assigned as water, and for the vitreous (excluding the tumor), composed of polydimethylsiloxane oil for three generic, one large tumor, and two pa- tient geometry scenarios. Dose was scored at the tumor apex, along the sclera, and within a 3D grid encompassing the eye. The assessed patient cases included vitrectomies to treat intraocular pathol- ogies; not to enhance attenuation/shielding. RESULTS: The doses along the sclera and for the entire eye were decreased when the silicone oil replaced the vitreal fluid, with a maximum decrease at the opposite sclera of 63%. Yet, absolute changes in dose to critical structures were often small and likely not clinically significant. The dose at the tumor apex was decreased by 3.1e9.4%. Dose was also decreased at the edges of the tumor because of decreased backscatter at the tumor-oil interface. CONCLUSIONS: Concurrent silicone vitrectomy was found to reduce total radiation dose to the eye. Based on current radiation retinopathy predictive models, the evaluation of the absolute doses revealed only a subset of patients in which a clinically significant difference in outcomes is ex- pected. Furthermore, the presence of the silicone oil decreased dose to the tumor edges, indicating that the tumor could be underdosed if the oil is unaccounted for. Ó 2019 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved. Keywords: Uveal melanoma; Ocular plaque brachytherapy; Silicone vitrectomy; COMS plaques; Monte Carlo; Dosimetry Introduction The most common form of intraocular cancer is uveal melanoma, mainly arising from the choroid.(1) Since the 1980s, ocular brachytherapy has been frequently used for the treatment of uveal melanomas as it has been found to have equivalent survival rates as enucleation for medium- sized tumors, while allowing the patient to retain their eye, providing cosmetic benefits and for many patients functional vision retention (2, 3). Although local tumor control rates have been found to be excellent with ocular brachytherapy, typically O90%, ra- diation toxicity and vision loss continues to be a major issue (4, 5). The primary radiation side effects that can affect visual function include active radiation retinopathy Received 15 February 2019; received in revised form 24 May 2019; accepted 31 May 2019. Conflict of interest: The authors report no proprietary or commercial interest in any product mentioned or concept discussed in this article. Financial disclosure: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Ethics approval: Health Research Ethics Board of Alberta (HREBA) Cancer Committee, Project name: ‘‘Preclinical evaluation of heteroge- neous corrections in brachytherapy treatment planning,’’ Project Number: HREBA.CC-16-0657, August 20, 2018. * Corresponding author. Department of Medical Physics, Tom Baker Cancer Centre, 1331 29 Street NW, Calgary, Alberta, T2N 4N2, Canada. Tel.: þ1-403-521-3598; fax: þ1-403-521-3327. E-mail address: hamorris@ualberta.ca (H. Morrison). 1538-4721/$ - see front matter Ó 2019 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved. https://doi.org/10.1016/j.brachy.2019.05.013 Brachytherapy - (2019) -