From the Dosimetry menu select Dose Calc. Options. In the options submenu enable both Ray Trace Linear Attenuation and Substitute Silicone Oil for Vitreous. Note: the example below, plaque #3 (P3) is the currently "active" plaque context, so the listed set of dosimetric options applies specifically to plaque #3. These options might have different states for other plaque contexts.

When calculating the attenuation of radiation as it passes through the eye, the entire eye, by default, is assumed to be water equivalent. Enabling Ray Trace Linear Attenuation tells Plaque Simulator to take into account the subtle differences in tissue density and effective atomic number (Z_eff) for the currently "active" plaque context. For example, the lens has nearly the same density as water but has a slightly greater Z_eff which makes a small difference for I-125 and Pd-103 plaques because most attenuation for these radionuclides is photoelectric. If the vitreous humor will be replaced by silicone oil, enable this option to correct for the greater attenuation coefficient of silicone oil compared to normal vitreous. These corrections are performed by ray tracing the primary radiation path through a 3D inhomogeneous voxel model of the eye with resolution as fine as 0.2 mm. Voxels within the eye model are classified as sclera, cornea, tumor, lens, nerve, aqueous humor, vitreous humor, water and air with appropriate Z_eff and densities. In order to calculate the effects of oil substitution in the vitreous, ray tracing must be enabled. This means that all differences in density and Z_eff compared to water for the tissues and volumes being modeled will also be included in the calculations.

The densities and Z_eff for most eye tissues and volumes are found in Plaque Simulator's Preference Settings's Model pane in the Eye control group. The linear attenuation coefficient for silicone oil is found in the Dose Constants tab of each radionuclide's physics file and may need to be customized for whichever silicone oil variant you are using. Ray tracing is applicable only to gamma sources and is disabled by default because it slows the calculations a bit and the dosimetric effects are small.

When silicone oil is substituted for normal vitreous, the greatest ratio of dose reduction compared to normal vitreous will occur at locations where the primary radiation emitted by the sources travels the furthest distance through the oil because there is little scatter. Depending upon the shape of the tumor base and elevation of the tumor apex, and assuming the plaque lies directly below the tumor base, the maximum ratio of dose reduction should occur near the diametrically opposite side of the eye from the plaque. Radiation as it passes through tumor and other non-vitreous materials will be attenuated at same the rate as whatever is usual for those tissues. Dose reduction resulting from Si oil substitution will be essentially due to simply the primary radiation path length through the vitreous chamber, and the primary paths will be inversely related to the height and volume of the tumor. For example, large tumors will occupy a greater volume of the vitreous chamber compared to small tumors. Therefore, large tumors reduce the available volume of viteous humor for which Si oil can be substituted, and hence reduce the opportunity for additional attenuation compared to normal vitreous.

In this example the tumor base has chord dimensions of 12 x 12 mm and an apex elevation of 8 mm.

The left pane represent normal vitreous and the right pane Si oil substitution. Observe that attenuation in the lens is greater than normal vitreous owing primarily to its greater Z_eff.

Dose to the nerve surface is reduced from 73.67 Gy to 70.65 Gy (about 3 Gy or 4%). Dose at the posterior pole (ie near the fovea) is reduced from 39.21 Gy to 31.89 Gy (about 7.3 Gy or 19%). Dose at the opposite side of the eye is reduced from 11.88 Gy to 7.09 Gy (about 4.8 Gy or 40%). Observe that the greatest ratio of dose reduction (40%) does indeed occur at the far side of the eye, but the absolute reduction in dose is less than 5 Gy because the "inverse-square law" has already substantially reduced the absolute dose.

We really dont care about dose reduction to the vitreous volume itself. Other than the lens, almost all living tissue is located in a thin layer lining the inner sclera. For regions of the retina (and nerve) close to the plaque rim, primary radiation passes almost entirely through only sclera or tumor, so there is little or no opportunity for significant attenuation in oil. The maximum absolute reduction, compared to a normal environment, evidently occurs at some intermediate retinal distance from the plaque rim.

To study dose to the retinal surface, we will use a circumferential dose profile curve that originates at a point on the inner sclera opposite the plaque center, passes through the posterior pole of the eye and terminates at the inner sclera adjacent to the center of a plaque (red curve in the figure below). In this example the circumferential profile covers a distance of 34.56 mm of arc. We could also consider a linear profile (a 22 mm length red vector in the figure below) that originates at the inner sclera adjacent to the plaque center and crosses the eye diametrically to the point opposite the plaque center, but such a "central axis" profile is not particularly useful except perhaps to study dose as a function of elevation within the tumor.

The plot below compares circumferential dose profiles for normal vitreous (grey highlight) vs Si oil substitution (gold highlight).The maximum absolute dose reduction occurs between about 6 and 13 mm of arc from the rim of the plaque, but is only a few Gy.

The plot below compares the central axis linear dose profiles for normal vitreous (grey highlight) vs Si oil substitution (gold highlight). The maximum absolute dose reduction occurs between about 6 and 13 mm from the inner sclera at the plaque center, but is less than about 10 Gy.

These retinal-dose-area histograms compare dose for the entire retinal surface. The Si oil substitution rediuces absolute dose to the optic disc (magenta), fovea (beige) and macular (orange) regions by up to about 10 Gy.

When silicone oil is substituted for normal vitreous, the greatest ratio of dose reduction compared to normal vitreous will occur at locations where the primary radiation emitted by the sources travels the furthest distance through the oil because there is little scatter. Depending upon the shape of the tumor base and elevation of the tumor apex, and assuming the plaque lies directly below the tumor base, the maximum ratio of dose reduction should occur near the diametrically opposite side of the eye from the plaque. Radiation as it passes through tumor and other non-vitreous materials will be attenuated at same the rate as whatever is usual for those tissues. Dose reduction resulting from Si oil substitution will be essentially due to simply the primary radiation path length through the vitreous chamber, and the primary paths will be inversely related to the height and volume of the tumor. For example, large tumors will occupy a greater volume of the vitreous chamber compared to small tumors. Therefore, large tumors reduce the available volume of viteous humor for which Si oil can be substituted, and hence reduce the opportunity for additional attenuation compared to normal vitreous.

In this example the tumor base has chord dimensions of 12 x 12 mm and an apex elevation of 3 mm. With the COMS plaques, prescription at 5 mm elevation is required to assure base and 2mm PTV coverage (Note that Eye Physics plaques are designed to provide more homogeneous coverage across the concave face of the plaque compared to COMS plaques and thus do not require Rx at 5mm in order to assure Rx dose coverage of the tumor base and PTV. Prescribing at a lower elevation reduces dose everywhere in the eye outside of the tumor according to the "inverse-square law".)

The left pane represents normal vitreous and the right pane Si oil substitution. Observe that attenuation in the lens is greater than normal vitreous owing primarily to its greater Z_eff.

Dose to the nerve surface is reduced from 41.04 Gy to 39.26 Gy (about 1.8 Gy or 4%). Dose at the posterior pole (ie near the fovea) is reduced from 21.84 Gy to 17.22 Gy (about 4.1 Gy or 19%). Dose at the opposite side of the eye is reduced from 6.62 Gy to 3.93 Gy (about 2.7 Gy or 40%). Observe that the greatest ratio of dose reduction (40%) does indeed occur at the far side of the eye, but the absolute reduction in dose is less than 3 Gy because the "inverse-square law" has already substantially reduced the absolute dose.

We really dont care about dose reduction to the vitreous volume itself. Other than the lens, almost all living tissue is located in a thin layer lining the inner sclera. For regions of the retina (and nerve) close to the plaque rim, primary radiation passes almost entirely through only sclera or tumor, so there is little or no opportunity for significant attenuation in oil. The maximum absolute reduction, compared to a normal environment, evidently occurs at some intermediate retinal distance from the plaque rim.

To study dose to the retinal surface, we will use a circumferential dose profile curve that originates at a point on the inner sclera opposite the plaque center, passes through the posterior pole of the eye and terminates at the inner sclera adjacent to the center of a plaque (red curve in the figure below). In this example the circumferential profile covers a distance of 34.56 mm of arc. We could also consider a linear profile (a 22 mm length red vector in the figure below) that originates at the inner sclera adjacent to the plaque center and crosses the eye diametrically to the point opposite the plaque center, but such a "central axis" profile is not particularly useful except perhaps to study dose as a function of elevation within the tumor.

The plot below compares circumferential dose profiles for normal vitreous (purple highlight) vs Si oil substitution (blue highlight).The maximum absolute dose reduction occurs between about 6 and 13 mm of arc from the rim of the plaque, but is less than about 5 Gy.

The plot below compares the central axis linear dose profiles for normal vitreous (purple highlight) vs Si oil substitution (blue highlight). The maximum absolute dose reduction occurs between about 6 and 13 mm from the inner sclera at the plaque center, but is less than about 5 Gy.

These retinal-dose-area histograms compare dose for the entire retinal surface. The Si oil substitution reduces absolute dose to the optic disc (magenta), fovea (beige) and macular (orange) regions by up to about 5 Gy.

In this example the tumor base has chord dimensions of 12 x 12 mm and an apex elevation of 3 mm. This EP plaque has nearly the same concave face dimension (16.17 mm) as the COMS 16 mm plaque used in the previous example. Prescription at 4 mm elevation delivers equivalent base and PTV coverage as the COMS 16 mm plaque does with Rx at 5mm.

The sources in this EP plaque utilize two strengths. The stronger seeds are positioned along the peripheral rim of the plaque in the manner of a classic "Manchester System" (aka Paterson-Parker) brachytherapy treatment using Ra needles of nearly a century ago. The Parterson-Parker source placement system was developed to deliver a uniform dose (+/- 10%) to a plane or volume. Tables were provided giving the dose distribution for given Ra needle activities and implant geometries. Total source activity was distributed between the periphery and core of the implant according to various rules with up to 2/3 of the total activity located in the periphery for small area implants. This plaque model was prototyped using 3D printing. The collimating slots that mount its peripheral seeds are a bit shallower than the slots that mount its core seeds and create a "fan-beam" effect. This difference in seed offset from the sclera adds a bit of "virtual" intensity modulation which can sometimes avoid having to use seeds with physically different strengths.

The left pane represents normal vitreous and the right pane Si oil substitution. Observe that attenuation in the lens is greater than normal vitreous owing primarily to its greater Z_eff

Dose to the nerve surface is reduced from 24.74 Gy to 24.14 Gy (about 0.6 Gy or 2.5%). Dose at the posterior pole (ie near the fovea) is reduced from 15.22 Gy to 12.49 Gy (about 2.7 Gy or 18%). Dose at the opposite side of the eye is reduced from 5.94 Gy to 3.68 Gy (about 2.3 Gy or 38%). Observe that the greatest ratio of dose reduction (38%) does indeed occur at the far side of the eye, but the absolute reduction in dose is less than 2.5 Gy because the "inverse-square law" has already substantially reduced the absolute dose.

We really dont care about dose reduction to the vitreous volume itself. Other than the lens, almost all living tissue is located in a thin layer lining the inner sclera. For regions of the retina (and nerve) close to the plaque rim, primary radiation passes almost entirely through only sclera or tumor, so there is little or no opportunity for significant attenuation in oil. The maximum absolute reduction, compared to a normal environment, evidently occurs at some intermediate retinal distance from the plaque rim.

To study dose to the retinal surface, we will use a circumferential dose profile curve that originates at a point on the inner sclera opposite the plaque center, passes through the posterior pole of the eye and terminates at the inner sclera adjacent to the center of a plaque (red curve in the figure below). In this example the circumferential profile covers a distance of 34.56 mm of arc. We could also consider a linear profile (a 22 mm length red vector in the figure below) that originates at the inner sclera adjacent to the plaque center and crosses the eye diametrically to the point opposite the plaque center, but such a "central axis" profile is not particularly useful except perhaps to study dose as a function of elevation within the tumor.

The plot below compares circumferential dose profiles for normal vitreous (grey highlight) vs Si oil substitution (gold highlight). Observe that the maximum scleral dose now occurs near the rim of the plaque rather than at its center, and this maximum is about 70 Gy less than what was required by the equivalent COMS plaque used in the previous example. In this way, tumor base and retinal PTV Rx dose coverage are extended out to the perimeter of the plaque without having to Rx to as great altitude above the plaque center as was required to compensate for dose "roll-off" near the perimeter of the COMS plaque. The maximum absolute dose reduction from Si oil substitution occurs between about 3.7 and 8.5 mm of arc from the rim of the plaque, but is less than about 3 Gy. Significant dose reduction in this region is seen for both normal and Si oil vitreous. This dose reduction is primarily the result of fan-beam collimation of the closest seed at the plaque rim.

The plot below compares the central axis linear dose profiles for normal vitreous (grey highlight) vs Si oil substitution (gold highlight). Observe the dose "build-up" in the first mm as the fan beams from the nearby seeds overlap. This effect positions the maximum dose in the tumor rather than the underlying sclera as was the case for the COMS plaque. Si oil substitution actually slightly increases dose within the tumor because the Rx point is located in Si oil about 1 mm above the tumor apex! Note that dose reduction from Si oil substitution of the vitreous is only possible at distances beyond the Rx point whenever the Rx point is located in oil and thus is itself subject to the increased attenuation of the oil! The maximum dose reduction from Si oil substitution occurs between about 7 and 13 mm from the inner sclera and is less than about 5 Gy.

These retinal-dose-area histograms compare dose for the entire retinal surface. The Si oil substitution reduces absolute dose to the optic disc (magenta), fovea (beige) and macular (orange) regions by up to about 3 Gy.

In this example we see that plaque design, source collimation and source strength intensity modulation can potentially spare some locations such as the nerve and fovea to a much greater extent than can vitreous substitution with Si oil when used in conjunction with a COMS plaque, and even further sparing would be possible if Si oil were used in conjunction with the EP plaque, but the absolute magnitude of such additional dose reduction resulting from Si oil substitution is very small. The fundamental observation is that the Si oil provides insignificant dose reduction to regions in close proximity to the plaque where dose is inherently high, and insignificant dose reduction far from the plaque where the "inverse-square law" has already substantially reduced dose. There is an intermediate donut-like "band" surrounding the rim of the plaques where there probably exists a small dose reduction between about 3 and 10 Gy which might be enough to keep dose under nominal normal tissue tolerances for a narrow range of tumor dimensions and apex heights, but to take advantage of that potential a lot more regarding normal tissue tolerance will likely be required.