Attenuation of iodine 125 radiation with vitreous substitutes in the treatment of uveal melanoma.
OBJECTIVE
To demonstrate attenuation of radiation from iodine 125 ((125)I) to intraocular structures using liquid vitreous substitutes.


METHODS
Four candidate vitreous substitutes were tested for attenuation using empirical measurement and theoretical calculation. In vitro and ex vivo cadaveric dosimetry measurements were obtained with lithium fluoride thermoluminescent dosimeters to demonstrate the attenuation effect of vitreous substitution during (125)I simulated plaque brachytherapy. Theoretical dosimetry calculations were based on Monte Carlo simulation.


RESULTS
In a cylindrical phantom at a 17-mm depth, liquid vitreous substitutes as compared with saline showed significant reduction of radiation penetration (48% for 1000-centistoke [cSt] silicone oil [polydimethyl-n-siloxane], 47% for 5000-cSt silicone oil [polydimethyl-n-siloxane], 40% for heavy oil [perfluorohexyloctane/polydimethyl-n-siloxane], and 35% for perfluorocarbon liquid [perfluoro-n-octane]). Human cadaveric ex vivo measurements demonstrated a 1000-cSt silicone oil to saline dose ratio of 35%, 52%, 55%, and 48% at arc lengths of 7.6, 10.6, 22.3, and 28.6 mm from the plaque edge, respectively, along the surface of the globe. Monte Carlo simulation of a human globe projected attenuation as high as 57% using 1000-cSt silicone oil.


CONCLUSIONS
Intraocular vitreous substitutes including silicone oil, heavy oil, and perfluorocarbon liquid attenuate the radiation dose from (125)I. Cadaveric ex vivo measurements and Monte Carlo simulation both demonstrate radiation attenuation using 1000-cSt silicone oil at distances corresponding to vital ocular structures. Clinical Relevance Attenuation of radiation with silicone oil endotamponade in the treatment of uveal melanoma may significantly reduce radiation-induced injury to vital ocular structures.
Oliver
Scott C N
SC
Department of Ophthalmology, Jules Stein Eye Institute, University of California-Los Angeles, 100 Stein Plaza, Los Angeles, CA 90095, USA.
Leu
Min Y
MY
DeMarco
John J
JJ
Chow
Philip E
PE
Lee
Steve P
SP
McCannel
Tara A
TA
D001918:Brachytherapy; D005466:Fluorocarbons / Q000627:therapeutic use; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D008545:Melanoma / Q000532:radiotherapy*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011832:Radiation Injuries / Q000517:prevention & control*; D011837:Radiation-Protective Agents / Q000627:therapeutic use*; D011879:Radiotherapy Dosage; D012827:Silicone Oils / Q000627:therapeutic use; D013046:Specific Gravity; D014604:Uveal Neoplasms / Q000532:radiotherapy*; D055675:Viscosupplements / Q000627:therapeutic use*; D014821:Vitrectomy; D014822:Vitreous Body
D005466:Fluorocarbons; D007457:Iodine Radioisotopes; D011837:Radiation-Protective Agents; D012827:Silicone Oils; D055675:Viscosupplements; C515608:densiron; C073608:perfluorooctane