¹²⁵I eye plaque dose distribution including penumbra characteristics

Abstract

The two main purposes of this work are (1) to determine the penumbra characteristics for 125I eye plaque and the relative influence of the plaque and eye-air interface on the dose distribution, and (2) to initiate development of a treatment planning algorithm for clinical dose calculations. Dose was measured in a newly designed solid water eye phantom for an 125I (6711) seed at the center of a 20 mm COMS eye plaque using thermoluminescent dosimeter (TLD) "cubes" and "minichips" inside and outside the eye, in the longitudinal and transverse central planes. TLD cubes were used in most locations, except for short distances from the seed and in the penumbra region. In the presence of both the plaque and the eye-air interface, the dose along the central axis was found to be reduced by 10% at 1 cm and up to 20% at 2.5 cm, relative to the bulk homogeneous phantom case. In addition, the overall dose reduction was greater for larger off-axis coordinates at a given depth. The penumbra characteristics due to the lip collimation were quantified, particularly the dependence of penumbra center and width on depth. Only small differences were observed between the profiles in the transverse and longitudinal planes. In the bulk geometry (without the eye-air interface), the dose reduction due to the presence of the plaque alone was found to be 7% at a depth of 2.5 cm. The additional reduction of 13% observed, with the presence of eye-air interface (20% combined), can be attributed to the lack of backscattering from the air in front of the eye. The dose-reduction effect due to the anterior air interface alone became unnoticeable at a depth of 1.1 cm (1.5 cm from the eye-air interface). An analytic fit to measured data was developed for clinical dose calculations for a centrally loaded seed. The central axis values of the dose rates multiplied by distance squared, Dr2, were fitted with a double exponential function of depth. The off-axis profile of Dr2, at a given depth, was parametrized by a modified Fermi-Dirac function to model both the penumbra characteristics due the plaque lip collimation and the effect of oblique filtration by silastic.

Modified COMS Plaques for 125I and 103Pd Iris Melanoma Brachytherapy

“…The outer collimating lip (Iris-180, Iris-270, Iris-360, and COMS plaques) constrains radiation to the eye and reduces doses to neighboring tissues. Dose decreases due to the outer lip are consistent with the thermoluminescent dosimeter (TLD) measurements of de la Zerda et al (10). The Modulay segment joining the inner and outer lips on the Iris-180 and Iris-270 plaques reduces doses to tissues beyond the treatment area.…”

“…Doses at critical points of interest in the eye can differ by 30% or more in comparison with TG-43 calculations (8). An air interface reduces doses by up to 10% (8,10).…”

A comprehensive dosimetric comparison between 131Cs and 125I brachytherapy sources for COMS eye plaque implant

“…first studied the impact of the silastic carrier and the gold plaque using Monte Carlo (MC) technique and thermoluminescent dosimeters (TLD). Later, de la Zerda et al (11) also measured the dose distribution with TLDs. All studies showed that the silastic carrier and the gold plaque could reduce the dose by up to 10% at a depth of 1.0 cm on the plaque axis compared with a homogeneous water medium.…”

Comparison of dose calculation methods for brachytherapy of intraocular tumors

“…In 2003, the American Brachytherapy Society recommended prescribing to the tumor apex for all mediumsized choroidal melanomas, even those Ͻ5 mm in height. 12 In the 1990s, Chiu-Tsao et al 13,14 and de la Zerda et al 15 reported thermoluminescent dosimeter ͑TLD͒ measurements and Monte Carlo ͑MC͒ radiation transport simulations of the dose distributions in an eye phantom for a single 125 I and 103 Pd source in a COMS-plaque. These groups observed central-axis dose reductions of ϳ10% and ϳ16% for 125 I and 103 Pd, respectively, and off-axis dose reductions up to 30%.…”