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Title: SU-F-T-48: Clinical Implementation of Brachytherapy Planning System for COMS Eye Plaques

Abstract

Purpose: To commission the Brachytherapy Planning (BP) system (Varian, Palo Alto, CA) for the Collaborative Ocular Melanoma Study (COMS) eye plaques by evaluating dose differences against original plans from Nucletron Planning System (NPS). Methods: NPS system is the primary planning software for COMS-plaques at our facility; however, Brachytherapy Planning 11.0.47 (Varian Medical Systems) is used for secondary check and for seed placement configurations not originally commissioned. Dose comparisons of BP and NPS plans were performed for prescription of 8500 cGy at 5 mm depth and doses to normal structures: opposite retina, inner sclera, macula, optic disk and lens. Plans were calculated for Iodine-125 seeds (OncoSeeds, Model 6711) using COMS-plaques of 10, 12, 14, 16, 18 and 20 mm diameters. An in-house program based on inverse-square was utilized to calculate point doses for comparison as well. Results: The highest dose difference between BP and NPS was 3.7% for the prescription point for all plaques. Doses for BP were higher than doses reported by NPS for all points. The largest percent differences for apex, opposite retina, inner sclera, macula, optic disk, and lens were 3.2%, 0.9%, 13.5%, 20.5%, 15.7% and 2.2%, respectively. The dose calculated by the in-house program was 1.3% highermore » at the prescription point, and were as high as 42.1%, for points away from the plaque (i.e. opposite retina) when compared to NPS. Conclusion: Doses to the tumor, lens, retina, and optic nerve are paramount for a successful treatment and vision preservation. Both systems are based on TG-43 calculations and assume water medium tissue homogeneity (ρe=1, water medium). Variations seen may result from the different task group versions and/or mathematical algorithms of the software. BP was commissioned to serve as a backup system and it also enables dose calculation in cases where seeds don’t follow conventional placement configuration.« less

Authors:
; ; ;  [1]
  1. University of Oklahoma Health Sciences Center, Oklahoma City, OK (United States)
Publication Date:
OSTI Identifier:
22642297
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; BRACHYTHERAPY; CRYSTALLINE LENS; IODINE 125; MELANOMAS; PLANNING; RADIATION DOSES; RADIATION SOURCE IMPLANTS; RETINA

Citation Formats

Ferreira, C, Islam, M, Ahmad, S, and De La Fuente Herman, T. SU-F-T-48: Clinical Implementation of Brachytherapy Planning System for COMS Eye Plaques. United States: N. p., 2016. Web. doi:10.1118/1.4956183.
Ferreira, C, Islam, M, Ahmad, S, & De La Fuente Herman, T. SU-F-T-48: Clinical Implementation of Brachytherapy Planning System for COMS Eye Plaques. United States. doi:10.1118/1.4956183.
Ferreira, C, Islam, M, Ahmad, S, and De La Fuente Herman, T. 2016. "SU-F-T-48: Clinical Implementation of Brachytherapy Planning System for COMS Eye Plaques". United States. doi:10.1118/1.4956183.
@article{osti_22642297,
title = {SU-F-T-48: Clinical Implementation of Brachytherapy Planning System for COMS Eye Plaques},
author = {Ferreira, C and Islam, M and Ahmad, S and De La Fuente Herman, T},
abstractNote = {Purpose: To commission the Brachytherapy Planning (BP) system (Varian, Palo Alto, CA) for the Collaborative Ocular Melanoma Study (COMS) eye plaques by evaluating dose differences against original plans from Nucletron Planning System (NPS). Methods: NPS system is the primary planning software for COMS-plaques at our facility; however, Brachytherapy Planning 11.0.47 (Varian Medical Systems) is used for secondary check and for seed placement configurations not originally commissioned. Dose comparisons of BP and NPS plans were performed for prescription of 8500 cGy at 5 mm depth and doses to normal structures: opposite retina, inner sclera, macula, optic disk and lens. Plans were calculated for Iodine-125 seeds (OncoSeeds, Model 6711) using COMS-plaques of 10, 12, 14, 16, 18 and 20 mm diameters. An in-house program based on inverse-square was utilized to calculate point doses for comparison as well. Results: The highest dose difference between BP and NPS was 3.7% for the prescription point for all plaques. Doses for BP were higher than doses reported by NPS for all points. The largest percent differences for apex, opposite retina, inner sclera, macula, optic disk, and lens were 3.2%, 0.9%, 13.5%, 20.5%, 15.7% and 2.2%, respectively. The dose calculated by the in-house program was 1.3% higher at the prescription point, and were as high as 42.1%, for points away from the plaque (i.e. opposite retina) when compared to NPS. Conclusion: Doses to the tumor, lens, retina, and optic nerve are paramount for a successful treatment and vision preservation. Both systems are based on TG-43 calculations and assume water medium tissue homogeneity (ρe=1, water medium). Variations seen may result from the different task group versions and/or mathematical algorithms of the software. BP was commissioned to serve as a backup system and it also enables dose calculation in cases where seeds don’t follow conventional placement configuration.},
doi = {10.1118/1.4956183},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: Novel plaques are used to treat iris melanoma at the Mayo Clinic Rochester. The plaques are a modification of the Collaborative Ocular Melanoma Study (COMS) 22 mm plaque design with a gold alloy backing, outer lip, and silicone polymer insert. An inner lip surrounds a 10 mm diameter cutout region at the plaque center. Plaques span 360{sup o}, 270{sup o}, and 180{sup o} arcs. This article describes dosimetry for these plaques and others used in the treatment of anterior eye melanomas. Methods and Materials: The EGSnrc user-code BrachyDose is used to perform Monte Carlo simulations. Plaques and seeds aremore » fully modeled. Three-dimensional dose distributions for different plaque models, TG-43 calculations, and {sup 125}I (model 6711) and {sup 103}Pd (model 200) seeds are compared via depth-dose curves, tabulation of doses at points of interest, and isodose contours. Results: Doses at points of interest differ by up to 70% from TG-43 calculations. The inner lip reduces corneal doses. Matching plaque arc length to tumor extent reduces doses to eye regions outside the treatment area. Maintaining the same prescription dose, {sup 103}Pd offers lower doses to critical structures than {sup 125}I, with the exception of the sclera adjacent to the plaque. Conclusion: The Mayo Clinic plaques offer several advantages for anterior eye tumor treatments. Doses to regions outside the treatment area are significantly reduced. Doses differ considerably from TG-43 predictions, illustrating the importance of complete Monte Carlo simulations. Calculations take a few minutes on a single CPU, making BrachyDose sufficiently fast for routine clinical treatment planning.« less
  • Purpose: A recent reanalysis of the Collaborative Ocular Melanoma Study (COMS) medium tumor trial concluded that incorporating factors to account for anisotropy, line source approximation, the gold plaque, and attenuation in the Silastic seed carrier into the dose calculations resulted in a significant and consistent reduction of calculated doses to structures of interest within the eye. The authors concluded that future eye plaque dosimetry should be 'performed using the most up-to-date parameters available.' The reason these factors are important is attributable to the low energy {sup 125}I radiation (approximately 28 keV) that is primarily absorbed by the photoelectric process. Photoelectricmore » absorption is quite dependent on the atomic composition of the absorbing material. Being 40% silicon by weight, the effective atomic number of Silastic is significantly greater than that of water. Although the AAPM TG43 brachytherapy formalism inherently addresses the issues of source anisotropy and geometry, its parameter that accounts for scatter and attenuation, the radial dose function g(r), assumes that the source is immersed in infinite homogeneous water. In this work, factors are proposed for {sup 125}I that correct for attenuation in the Silastic carrier and scatter deficits resulting from the gold plaque and nearby air. The implications of using {sup 103}Pd seeds in COMS plaques are also discussed. Methods and materials: An existing TG43-based ophthalmic plaque planning system was modified to incorporate additional scatter and attenuation correction factors that better account for the path length of primary radiation in the Silastic seed carrier and the distance between the dose calculation point and the eye-air interface. Results: Compared with homogeneous water, the dose-modifying effects of the Silastic and gold are greatest near the plaque surface and immediately adjacent to the plaque, while being least near the center of the eye. The calculated dose distribution surrounding a single {sup 125}I seed centered in a COMS 20 mm plaque was found to be consistent with previously published examples that used thermoluminescent dosimetry measurements and Monte Carlo methods. For fully loaded 12 and 20 mm plaques, calculated dose to critical ocular structures ranged from 16%-50% less than would have been reported using the standard COMS dose calculation protocol. Conclusions: Treatment planning for COMS eye plaques that accurately accounts for the presence of the gold, Silastic and extraocular air is both possible and practical.« less
  • A quality assurance system has been designed to verify the location and strength of seeds loaded in a brachytherapy eye plaque. This system consists of (1) a pinhole camera in conjunction with a Lumisys ACR-2000i computed radiography (CR) unit to image the location and measure the relative strength of the seeds with autoradiography, and (2) a source strength jig with a survey meter to estimate the total activity of the seeds in the plaque. Five holders of different sizes were made for fixation of the COMS (Collaborative Ocular Melanoma Study) plaques (12, 14, 16, 18, and 20 mm) in themore » camera. The plaque-to-pinhole distance (d{sub pp}) has been optimized to be 30 mm to give approximately uniform intensity on the CR image for uniformly loaded COMS plaques. The pinhole-to-detector distance (d{sub pd}) can be kept at either 30 mm for 1:1 scale, or at larger distances for higher magnification. For a 1:1 scaling and pinhole diameter of 0.345 mm, useful images are obtained with time-activity product (mCi sec) ranging from 5 to 250 mCi sec. Within this range, the pinhole system is able to differentiate seed activities of >10%. The resulting pinhole autoradiograph is able to (1) confirm the correct number of seeds loaded in the plaque, (2) verify the proper sitting of the seeds in the silastic carrier and the plaque, (3) verify the relative activity distribution of the seeds loaded in the plaque, and (4) potentially evaluate the integrity of the seed. The source strength measurement system is able to measure the total strength of seeds in the plaque ranging from 10 to 80 mCi with an uncertainty of 5%.« less
  • Purpose: (1) To measure absolute dose distributions in eye phantom for COMS eye plaques with {sup 125}I seeds (model I25.S16) using radiochromic EBT film dosimetry. (2) To determine the dose correction function for calculations involving the TG-43 formalism to account for the presence of the COMS eye plaque using Monte Carlo (MC) method specific to this seed model. (3) To test the heterogeneous dose calculation accuracy of the new version of Plaque Simulator (v5.3.9) against the EBT film data for this seed model. Methods: Using EBT film, absolute doses were measured for {sup 125}I seeds (model I25.S16) in COMS eyemore » plaques (1) along the plaque's central axis for (a) uniformly loaded plaques (14-20 mm in diameter) and (b) a 20 mm plaque with single seed, and (2) in off-axis direction at depths of 5 and 12 mm for all four plaque sizes. The EBT film calibration was performed at {sup 125}I photon energy. MC calculations using MCNP5 code for a single seed at the center of a 20 mm plaque in homogeneous water and polystyrene medium were performed. The heterogeneity dose correction function was determined from the MC calculations. These function values at various depths were entered into PS software (v5.3.9) to calculate the heterogeneous dose distributions for the uniformly loaded plaques (of all four sizes). The dose distributions with homogeneous water assumptions were also calculated using PS for comparison. The EBT film measured absolute dose rate values (film) were compared with those calculated using PS with homogeneous assumption (PS Homo) and heterogeneity correction (PS Hetero). The values of dose ratio (film/PS Homo) and (film/PS Hetero) were obtained. Results: The central axis depth dose rate values for a single seed in 20 mm plaque measured using EBT film and calculated with MCNP5 code (both in ploystyrene phantom) were compared, and agreement within 9% was found. The dose ratio (film/PS Homo) values were substantially lower than unity (mostly between 0.8 and 0.9) for all four plaque sizes, indicating dose reduction by COMS plaque compared with homogeneous assumption. The dose ratio (film/PS Hetero) values were close to unity, indicating the PS Hetero calculations agree with those from the film study. Conclusions: Substantial heterogeneity effect on the {sup 125}I dose distributions in an eye phantom for COMS plaques was verified using radiochromic EBT film dosimetry. The calculated doses for uniformly loaded plaques using PS with heterogeneity correction option enabled were corroborated by the EBT film measurement data. Radiochromic EBT film dosimetry is feasible in measuring absolute dose distributions in eye phantom for COMS eye plaques loaded with single or multiple {sup 125}I seeds. Plaque Simulator is a viable tool for the calculation of dose distributions if one understands its limitations and uses the proper heterogeneity correction feature.« less
  • In this work we dosimetrically evaluated the clinical implementation of a commercial Monte Carlo treatment planning software (PEREGRINE, North American Scientific, Cranberry Township, PA) intended for quality assurance (QA) of intensity modulated radiation therapy treatment plans. Dose profiles calculated in homogeneous and heterogeneous phantoms using this system were compared to both measurements and simulations using the EGSnrc Monte Carlo code for the 6 MV beam of a Varian CL21EX linear accelerator. For simple jaw-defined fields, calculations agree within 2% of the dose at d{sub max} with measurements in homogeneous phantoms with the exception of the buildup region where the calculationsmore » overestimate the dose by up to 8%. In heterogeneous lung and bone phantoms the agreement is within 3%, on average, up to 5% for a 1x1 cm{sup 2} field. We tested two consecutive implementations of the MLC model. After matching the calculated and measured MLC leakage, simulations of static and dynamic MLC-defined fields using the most recent MLC model agreed to within 2% with measurements.« less