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Source Modelling for Eye Plaque Dosimetry: A Planning Study

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N Papanikolaou

N.T. Papanikolaou*, J Roring , S Stathakis , N Kirby , D Saenz , K Rasmussen , University of Texas HSC SA, San Antonio, TX


SU-I-GPD-T-44 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Choroidal melanomas have been treated with great success for several decades using brachytherapy following the guidelines established by COMS. In this study, we report on the effect of the specification of the iodine source in the planning system and its effect in the dose calculation.

Methods: We have modeled the COMS eye plaques in the pinnacle planning system (Philips) for a homogeneous, water-like medium. The Isoaid iodine source was also modeled in Pinnacle following the TG43 guidelines. Two models were established: one for a point source and one for a line source. We then developed plans for each of the plaque sizes using: point source model, a line model with all the sources aligned (parallel to each other), and a line model with all the sources rotated to match the true orientation of each source on the respective plaque.

Results: We developed plans for all the eye plaque sizes in our inventory (10mm up to 22mm) using a prescription depth of 3, 6 and 9mm to deliver 85Gy over a five day implant. Consequently, we had three plans per plaque size for each of the prescription depths. The seeds for each plan were arranged as point, linear parallel, and linear rotated sources. Differences on average of 4% in activity per seed were found between the point sources and properly oriented line sources to deliver the prescribed dose. The parallel arrangement versus properly oriented arrangement of linear sources showed up to 2% difference in activity.

Conclusion: We have shown that there are differences in the activity required to deliver the prescribed dose when one accounts for the linear nature of the source and the orientation of each seed. The differences are up to 4.8% in source strength and can change up to 1% with plaque size or prescription point (apex).

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