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Dosimetry of the Beta Ophthalmic Applicator Utilized in the Cook Et Al. RCT for Scar Prophylaxis in Glaucoma Trabeculectomy Surgery

A Cohen* (1), L Marsteller (1), IE Murdoch (1,2), N Joubert (3), J Wetter (3), G Maree (3), W Lutz (1); (1) Radiance Therapeutics, Inc.; (2) Institute of Ophthalmology, University College London, UK; (3) Department of Radiation Oncology, University of Cape Town, Groote Shuur Hospital, ZA.

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: We present the dosimetry of the Beta Ophthalmic Applicator (Amersham #0528ML) used in Cook’s (SA Ophth J 2018) Randomized Controlled Trial demonstrating efficacy with trabeculectomy. Beta Ophthalmic Applicators are indicated for treatment and prophylaxis of scar and inflammation, often applied in glaucoma and pterygium surgeries. We report on the 3-D dose variation within the study’s defined PTV for several legacy Beta Ophthalmic Applicators including the study applicator.

Methods: Our primary tools included Gafchromic EBT3-film, Epson 12000XL scanner, FilmQA Pro and Excel software. The Wisconsin ADCL Lab no longer calibrates Sr-90 sources but offered their generous assistance in establishing a secondary calibrated source in our lab. We have also calibrated our secondary standard differently using a 6MeV linac electron beam. The electron beam was validated as a calibration source for Sr-90 by comparing the linac beam depth dose curve to the end of its range measured with an ion chamber and independently by EBT3 film. We compared dose distributions measured with both calibration methods.

Results: The study’s PTV was considered 8mm diameter by 0.6mm depth. We present the absolute calibration, and the relative distributions within the study PTV. This demonstrates that with a maximum applied dose of 1250, the minimum dose is 500 cGy. This dose distribution is consistent with a study by Khaw (BJO 1991) that revealed doses between 500-1000 cGy inhibited fibroblast proliferation. We also present dosimetry comparisons of the study’s source against another Amersham source, a New England Nuclear source, and a Tracer Lab source.

Conclusion: Cook found beta radiation more effective than Mitomycin-C as an adjunct to trabeculectomy to prevent bleb fibrosis and failure. Cook's device has efficacious dose distribution throughout the PTV. Comparison of dosimetry demonstrates that not all legacy Beta Ophthalmic Applicators provide equivalent dose distribution within the PTV.

Download ePoster [PDF]

Funding Support, Disclosures, and Conflict of Interest: A Cohen*, Internship position supported by Radiance Therapeutics, Inc.; L Marsteller, IE Murdoch, W Lutz, stock holders in Radiance Therapeutics, Inc. N Joubert, J Wetter, G Maree, nothing to declare.

Keywords

Ophthalmic Applicators, Dosimetry, Clinical Trials

Taxonomy

TH- Brachytherapy: Dose optimization and planning

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