Monte Carlo dosimetry for and eye plaque brachytherapy
Points of interest for eye plaque dosimetry, given in the center of eye reference frame (scale in centimeters) for a right eye. The eye is idealized as a sphere of radius . The (planar) air interface, which is transverse to the optic axis, is shown. The dashed line indicates the extent of the scoring grid, described in Sec. II. While a plaque positioned between the posterior pole and equator is shown here, plaques in other positions, e.g., centered on the posterior pole and on the equator nasal and temporal to the eyeball, are also simulated.
Ratio of the doses along the plaque's central axis for a single seed at the center of a 12 or plaque with a Modulay or gold backing and water insert to the doses with water (no plaque) for (a) and (b) .
Ratio of the doses along the plaque's central axis for a single seed in a Silastic insert (backings as indicated) at the center of a 12 (dashed lines) or (solid lines) plaque to the doses for the same seed in water (no backing or insert) for (a) and (b) .
Comparison of single seed and multiseed results along the plaque's central axis: Ratio of the doses for seeds in a Modulay/Silastic plaque to the doses for the same seeds in water (no plaque) with no interseed effects for different plaque sizes and (a) and (b) seeds. Unless otherwise noted, the results are for multiseed simulations.
Effect of collimation due to the plaque: Ratio of the doses for a Modulay/Silastic plaque with seeds to the doses for the same seed configuration in water (no plaque, no interseed effects) vs. transverse position at and for various depths .
Isodose contours in the , plane for a Modulay/Silastic plaque with seeds. The dose is scored in voxels and is set to zero in voxels intersecting the plaque, resulting in the ridges seen in the upper part of the figure.
Comparison of plaque central axis depth-dose curves for and for various plaque sizes (Modulay plaque, Silastic insert).
The effect of an air interface for : The ratio of the doses with an air interface to the doses with no air vs the distance from the air interface, , for a Modulay/Silastic plaque. In (a), the plaque is on the posteriorpole and results are for and various transverse positions . In (b), the plaque is on the equator and the results are given for different depths . The label “all seeds” indicates that the results are for a fully loaded plaque (24 seeds); otherwise, there is one seed in the plaque’s central slot. The line marked “Zerda” is the best-fit line of de la Zerda et al. (Ref. 15).
The effect of an air interface for : The ratio of the doses with an air interface to the doses with no air vs the distance to the air interface, , for a Modulay/Silastic plaque. The plaque positions and labeling are as in Fig. 8.
The effect of a cylinder (of diameter ) of bone about the eye. The ratio of doses with bone to the doses with no bone vs the position along the plaque’s central axis for a Modulay/Silastic plaque fully loaded with either or seeds. The plaque is centered on the posterior pole or on the equator temporal to the eyeball.
Dose deposited in a phantom made of homogenized eye or lens material relative to dose deposited in a water phantom for one or seed in the central slot of a Modulay/Silastic plaque vs position along the plaque's central axis.
Composition and densities of certain eye tissues and bone used in simulations, taken from ICRU Report 46 (Ref. 39) and ICRP Report 23 (as quoted in Ref. 40).
Timing for simulations of a fully loaded Modulay/Silastic plaque performed on a single Woodcrest core, scoring in either or voxels in water. The times to achieve 2% statistical uncertainty on the dose at either (i) the tumor apex (prescription point, ) or (ii) the point on the plaque’s central axis where the dose is half the prescription dose are given. The statistical uncertainty on the dose at the point at the opposite side of the eye from the plaque is given as a percentage of the local dose and of the prescription dose .
Comparison of the dose at points of interest for a (fully loaded with 13 seeds) plaque on the equator temporal to the eyeball. The columns labeled “Water” indicate the dose for the seeds in water with no plaque present and no interseed effects, while the label “MS” indicates the Modulay/Silastic combination. The results when the air interface is present are also included as percentages of the prescription dose, . Doses to the optic disk are included for the plaque on the equator nasal to the eyeball (“nasal”) and centered on the equator above or below the eyeball (“above or below”). The air kerma strength per seed required to obtain a prescription dose of at the tumor apex in for model 6711 seeds and for model 200 seeds (“Water” configuration) is indicated where . The statistical uncertainties are less than 0.34% for and 0.50% for .
Comparison of dose at points of interest for a plaque midway between the posterior pole and equator temporal to the eyeball, as shown in Fig. 1. The labeling and uncertainties are as in Table III, and again the treatment time is taken to be 100 for .
Comparison of the dose rates (in cGy/h) at points of interest obtained using BrachyDose to those of Astrahan (Ref. 17) using Plaque Simulator . The results are for a plaque midway between the posterior pole and equator temporal to the eyeball and a plaque centered on the equator temporal to the eyeball. Both plaques are filled with model 6711 seeds with . The statistical uncertainties on the BrachyDose results are less than 0.34%.
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