Purpose: A novel method of intraocular shielding in I-125 eye plaque brachytherapy has been proposed by using a small amount of magnetic fluid polymethlysiloxane-Fe₃O₄ (PDMS) in the presence of a magnet. Surrounding the melanoma with ferrofluid will maintain excellent tumor coverage but shield and limit excessive dose to organs at risk (OAR). To analyze the effectiveness of a magnetic shield-ferrofluid system, Monte Carlo simulations were performed to model the dose distribution and dose reduction potential.
Methods: Two types of magnetic fluid were studied. The first, PDMS, is the intended fluid that would be clinically used as a shielding material. Ferrotec© EFH Series Education Fluid (EFH) is the second fluid used due to its low cost and availability for proof-of-concept measurements. GEANT4-GAMOS Monte Carlo (MC) transport code was used. An IsoAid Advantage™ I-125 source was benchmarked with AAPM TG-43 parameters, along with the 20 mm COMS plaque geometry as described in TG-129. The torus-shaped fluid distribution was modeled in the geometry along with a 5 mm tumor height. Dose to water was calculated with A) no fluid, B) PDMS, and C) EFH fluid. Dose and percent dose reduction were calculated in directions parallel and perpendicular to the plaque.
Results: The half-value layer of PDMS and EFH were 1.3 and 3.1 mm, respectively, for I-125. Along the edge of the plaque where OARs likely exist, the PDMS fluid reduces dose by more than 70-90%. The EFH fluid produced less dose reduction, ranging from 50-70%.
Conclusion: OARs that lie near the plaque (macula, optic disc) are at risk for unavoidably high dose. The finding in this proof-of-concept study shows that a small amount (<1 ml) of magnetic fluid can reduce dose to the OAR by over 70%. This magnitude of dose reduction could significantly improve patient’s quality of life after treatment.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by an internal department fund, Spark Grant, and in addition to support from Nanomedics, LLC.