Physics contribution
Improved treatment planning for COMS eye plaques

https://doi.org/10.1016/j.ijrobp.2004.09.062Get rights and content

Purpose

A recent reanalysis of the Collaborative Ocular Melanoma Study (COMS) medium tumor trial concluded that incorporating factors to account for anisotropy, line source approximation, the gold plaque, and attenuation in the Silastic seed carrier into the dose calculations resulted in a significant and consistent reduction of calculated doses to structures of interest within the eye. The authors concluded that future eye plaque dosimetry should be “performed using the most up-to-date parameters available.” The reason these factors are important is attributable to the low energy 125I radiation (approximately 28 keV) that is primarily absorbed by the photoelectric process. Photoelectric absorption is quite dependent on the atomic composition of the absorbing material. Being 40% silicon by weight, the effective atomic number of Silastic is significantly greater than that of water. Although the AAPM TG43 brachytherapy formalism inherently addresses the issues of source anisotropy and geometry, its parameter that accounts for scatter and attenuation, the radial dose function g(r), assumes that the source is immersed in infinite homogeneous water. In this work, factors are proposed for 125I that correct for attenuation in the Silastic carrier and scatter deficits resulting from the gold plaque and nearby air. The implications of using 103Pd seeds in COMS plaques are also discussed.

Methods and materials

An existing TG43-based ophthalmic plaque planning system was modified to incorporate additional scatter and attenuation correction factors that better account for the path length of primary radiation in the Silastic seed carrier and the distance between the dose calculation point and the eye-air interface.

Results

Compared with homogeneous water, the dose-modifying effects of the Silastic and gold are greatest near the plaque surface and immediately adjacent to the plaque, while being least near the center of the eye. The calculated dose distribution surrounding a single 125I seed centered in a COMS 20 mm plaque was found to be consistent with previously published examples that used thermoluminescent dosimetry measurements and Monte Carlo methods. For fully loaded 12 and 20 mm plaques, calculated dose to critical ocular structures ranged from 16%–50% less than would have been reported using the standard COMS dose calculation protocol.

Conclusions

Treatment planning for COMS eye plaques that accurately accounts for the presence of the gold, Silastic and extraocular air is both possible and practical.

Introduction

The Collaborative Ocular Melanoma Study (COMS) is a multicenter investigation begun in the mid 1980s whose purpose was to evaluate the role of radiation therapy in the treatment of choroidal melanoma. The medium tumor trial of the COMS was designed to compare patient survival after randomization between enucleation and episcleral plaque therapy. A recent report indicates that survival rates for the two treatments are about the same (1).

The COMS study mandated the use of 125I, provided a standardized set of plaques, and enforced a strict set of dosimetric assumptions. The COMS plaque designs have been discussed in the literature (2, 3, 4, 5). Briefly, they consist of a 0.5-mm thick bowl-like gold alloy backing (77% gold, 14% silver, 8% copper, and 1% palladium) with a cylindrical collimating lip and a Silastic seed carrier into which the 125I seeds are loaded. The carrier offsets the seeds by 1 mm from the concave (front) surface of the plaque. The COMS dosimetric assumptions (DCOMS) require that the 125I seeds be treated as point sources, that source anisotropy and the effects of the gold backing on scatter and attenuation be ignored, that the Silastic insert of the plaque be assumed to be water equivalent, and that shielding effects of the collimating lip be ignored. A recent reanalysis of the medium tumor trial data by Krintz et al. (6) concluded that incorporating factors to account for anisotropy, line source approximation, the gold plaque, and attenuation in the Silastic seed carrier into the dose calculations resulted in a significant and consistent reduction of calculated doses to structures of interest within the eye.

The reason that these factors are important is attributable to the soft (low) energy of the radiation sources used in these plaques. 125I decays by electron capture to an excited state of 125Te. Characteristic X-rays in the range 27–35 keV are emitted along with 35.5 keV γ photons resulting from the decay of 125Te to its ground state. Some models of 125I seeds that contain silver markers also emit fluorescent (characteristic) X-rays at 22 and 25 keV. Titanium encapsulation absorbs liberated electrons and very low energy X-rays. In 1999, however, the National Institute of Standards and Technology (NIST) revised its calibration procedures for 125I to account for the presence of 4.5 keV titanium characteristic X-rays that do not contribute to dose in water at distances beyond 1 mm. For the purposes of this work, 125I will be considered to emit photons with an average energy of 28 keV. Another isotope that has recently been proposed for eye plaque therapy (7, 8) is 103Pd, which also decays by electron capture with the emission of characteristic X-rays in the range 20–23 keV (average energy approximately 21 keV).

For photons of these energies, the photoelectric effect is the dominant process by which energy is absorbed in common materials. Because the photoelectric mass attenuation coefficient is roughly proportional to the cube of the atomic number (Z) of the absorbing material, inhomogeneities in the immediate vicinity of an 125I or 103Pd seed can significantly affect the absorbed dose distribution. For instance, anisotropy resulting from self-absorption is severe for these seeds.

The AAPM TG43 brachytherapy formalism (9), which now forms the basis for most commercial brachytherapy planning systems, is also recommended by the COMS Manual of Procedures (10) for COMS dosimetry. Although TG43 inherently addresses the issues of source anisotropy and geometry (i.e., linear vs. point source) mentioned by Krintz et al. (6), the TG43 parameter that accounts for scatter and attenuation, the radial dose function g(r), assumes that the source is immersed in infinite homogeneous water. Calculating dose to water is an acceptable practice because the effective atomic numbers of ocular tissues, and therefore their scattering and attenuation properties, are fairly close to those of water. The effective atomic number (Zeff) is the atomic number of a hypothetical single element that would attenuate photons at the same rate as a composite material.

The COMS Silastic carrier has been reported (4) to be made of Dow Corning medical grade elastomer, MDX4-4210. Table 1 compares the fractional elemental composition by weight, density, and effective atomic number of water, eye lens, blood, and Silastic. The composition of Silastic was taken from Table 1 in Chiu-Taso et al. (4). The elemental composition of the other materials was obtained from an online NIST database (11). The Zeff was calculated using the method of Mayneord (12) as described in Khan (13) as: Zeff=(a1Z12.94+a2Z22.94+a3Z32.94++anZn2.94)1/2.94 where a1, a2, a3, … an are the fractional contributions of each element to the total number of electrons in the mixture.

Mass attenuation coefficients (μ/ρ) expressed in cm2/g (11) for monoenergetic photons corresponding to the average energies (Eavg) of 125I (approximately 28 keV) and 103Pd (approximately 21 keV) are plotted in Fig. 1 for materials with effective atomic numbers between 5 and 15. Summarized in Table 2 are some useful constants for Lucite (acrylic), water, Silastic, and gold that are readily derived from the mass attenuation coefficients: the linear attenuation coefficient (μ), the half value thickness (HVL = 0.693/μ), and the mean free path (MFP = μ−1). If a large number of photons of identical energy are incident upon an absorber of infinite thickness, then the MFP is the average, or mean distance traveled by any photon before a first collision. In 1 MFP of absorber the uncollided photon flux will be reduced to 1/e (0.37) of its original value.

Being 40% silicon (Z = 14) by weight, the effective atomic number of Silastic is close to 11, significantly higher than that of water, blood, or eye lens. For 125I photons, the linear attenuation coefficient (μ) of Silastic (approximately 1.01 cm−1) is about 2.2 times greater than that of water (0.46 cm−1), and for 103Pd, about 2.6 greater than that of water. The dose-modifiying effects of the Silastic for 125I radiation have been modeled using Monte Carlo methods by Chiu-Tsao et al. (4) and measured using thermoluminescent dosimeters (TLD) by Zerda et al. (5). They concluded that the effect of the Silastic insert and gold plaque was a dose reduction (compared with water) of about 10% at 1 cm on axis and about 15% at 2 cm and at off-axis points. The dose reduction in the Silastic insert will be even greater for 103Pd radiation.

In addition to the Silastic carrier, the gold (Z = 79) backing, lip, and neighboring seeds are examples of nearby objects whose elemental compositions are also very different from water (Zeff approximately 7.4). Dose enhancement (compared with water) close to a plaque from gold L-shell fluorescent X-rays (L1 = 14.4 keV, L2 = 13.7 keV, and L3 = 11.9 keV), and the modifiying effects of the gold backing on scattered 125I radiation have been discussed extensively in the literature (14, 15, 16, 17, 18, 19).

Attempts to account for collimation of primary radiation by the gold backing and lip of the plaque in treatment planning have been reported by Astrahan et al. (3), Chiu-Tsao et al. (4), and Zerda et al. (5). With the HVL of 125I radiation in pure gold being about 0.01 mm, transmission through the 0.5 mm thick gold alloy plaque can be considered to be zero. There will occur, however, a penumbral region, as illustrated in Fig. 2, whose properties depend on the proximity and orientation of the seeds with respect to the collimating lip. For the low energy of 125I radiation, charged particle equilibrium can be assumed in the penumbral region (unlike a megavoltage beam) and transmission penumbra at the terminus of the lip can be ignored for practical purposes. The geometrical relationship between a seed and the lip, however, can create significant geometric penumbra because the active length of a seed (approximately 3 mm) is comparable to the height of the lip (approximately 3 mm). To address geometric penumbra, Astrahan et al. (3) implemented a general purpose line-of-sight approach that calculates the fraction of a linear source that is visible above the lip horizon (see Fig. 2). Zerda et al. (5) used TLD in a solid water phantom to measure planar dose distributions for a single 125I seed centered in a 20 mm plaque and then used a modified Fermi-Dirac function to model the off-axis penumbral characteristics in both the transverse and longitudinal planes of the seed. This approach is derived from the two-dimensional (2D) methods used to characterize teletherapy beams. In that situation, there is a single source, a single collimation system, and a fixed geometric relationship between the source and collimator. While potentially accurate, their method is inconvenient for eye plaques because it requires additional measurements or correction factors to account for all of the possible seed locations, orientations, and shapes of plaques. There now exist many different models of 125I seeds. Plaques come in many shapes and sizes, some with notches in the lip for a snug fit to the optic nerve. Alternatively, Fluhs et al. (20) have described a measuring system that mechanically scans the entire three-dimensional (3D) dose rate distribution surrounding an eye plaque using a small volume (1 mm3) plastic scintillator detector. This 3D dataset is used directly in their treatment planning process.

In this report, the dominant dosimetric reductions and enhancements relative to water produced by the Silastic carrier, the gold plaque, and air in front of the eye will be explored. Correction factors will be proposed that may be used with the TG43 formalism. The more complex dosimetric effects arising from interactions between adjacent seeds and extraocular inhomogenities such as the lead-lined eye patches worn by patients and orbital bone will be ignored. These inhomogenities presumably cause insignificant dosimetric changes and are better suited to Monte Carlo methods.

Chiu-Tsao et al. (4) used Monte Carlo methods to model a single 125I seed (model 6711) centered in the Silastic carrier of a 20 mm COMS plaque immersed in homogeneous water. In a follow-up study, Zerda et al. (5) used TLD to measure planar dose distributions in a solid water head phantom with an eye-air interface for the same source geometry. They concluded that the central axis dose reduction compared with homogeneous water is about 10% at 1 cm, and 15% at 2 cm and at off-axis points. Chiu-Tsao et al. (4) also noted that removal of the gold backing from the plaque “did not make a measureable difference in the dose reduction results” at 1 cm. This surprising observation warrants a closer look at their data and the underlying physics. The data points, less error bars, from Fig. 5, Fig. 6 in Chiu-Tsao et al. (4) are redrawn here on an expanded vertical scale in Fig. 3. An extra point has been added in Fig. 3 at 0 mm for curve fitting purposes. These data were obtained by Monte Carlo methods and represent the ratio of dose for the Silastic insert alone, the gold backing alone, and the Silastic-plus-gold combination immersed in infinite water (i.e., no eye-air interface) to the dose in homogeneous water, plotted as a function of distance along the plaque central axis, which is also the seed transverse axis.

The linear attenuation coefficient μ represents the probability per photon per unit path length that a photon interaction will occur. For the energies of interest in this work, μ can be simplified to the sum of two components: μ = (τ + σ), where τ is the photoelectric linear absorption coefficient and σ is the total Compton linear attenuation coefficient. For simplicity, the photoelectric process will be considered as purely absorptive and the Compton process considered as partly absorptive and partly scattering. Plotted in Figs. 4a, b, and c are the relative importance of the total Compton and photoelectric processes in water, Silastic and gold as a function of photon energy. The two processes have roughly equal probabilities of occurrence at 26 keV in water, 40 keV in Silastic, and 480 keV in gold. In the range 10–30 keV, photoelectic absorption accounts for virtually all of the interactions in gold and the majority of interactions with Silastic. Only in water (or water equivalent mixtures) does Compton scattering play a significant role.

For incident-photons of low energy (hν0), Compton scattering transfers only a small fraction of the total energy to the recoil electron and the angular distribution of the differential Compton scattered photons per unit solid angle is approximately symmetric around φ = 90 degrees as illustrated in Fig. 5. The energy of a Compton scattered photon (hν′) can be calculated as: hν′ = hν0 (1 / [1 + α(1 − cos φ)]), where α = (hν0 / m0c2) and φ is the angle of photon scatter. The quantity m0c2 = 511 keV is the rest energy of an electron. For photons in the energy range of 125I and 103Pd plaque therapy, Compton scattering at φ approximately 90 degrees results in an energy loss per interaction of 2%–5% of the incident energy (see Table 3). The small energy loss per Compton interaction means that, even in water where the number of Compton interactions is significant, the dose delivered by Compton interactions will be small compared with the dose delivered by photoelectric absorption. Furthermore, the probability of subsequent interactions for the scattered photon will be nearly the same as for the incident photon. Dale (21) used Monte Carlo methods to explore this in more detail.

Now, consider an 125I source surrounded by infinite water. About half of the emitted primary photons will undergo photoelectric absorption as their first interaction with the surrounding water. Most of these interactions will occur within one MFP (approximately 2 cm, roughly the diameter of an eye!) of the source. The other half will experience a Compton scattering interaction with an average directional change of about 90 degrees. Because the energy loss per Compton interaction is small, about half of the once-scattered photons will undergo photoelectric absorption as their next interaction, and half will experience a second Compton interaction. Of the twice-scattered photons, about half will undergo photoelectric absorption as their next interaction, and half will experience a third Compton interaction. Less than 7% of the originally emitted photons will survive four interactions with the surrounding water, and those photons that do survive the first four interactions will have delivered very little absorbed dose. Since these low energy photons cannot scatter very many times before they are photoelectrically absorbed, the dosimetric effects of inhomogenities (e.g., the gold backing and extraocular air) that produce changes in backscatter are likely to be range limited phenomena. Inhomogeneities that directly affect photoelectric attenuation (e.g., primary attenuation in the Silastic carrier) should have a more universal influence.

Figure 6 is a highly simplified interaction model of a low energy isotropic point source located at the center of an infinite sphere of water. Symmetry allows the sphere, and therefore photon interactions within the sphere, to be collapsed onto a 2D planar surface for the purposes of this discourse. The sphere, illustrated as a circle, is divided into front and rear hemispheres that are divided by a bisecting plane of symmetry depicted by the dashed horizontal line. The average direction of emitted photons with respect to this plane of symmetry are represented by the arrows. Half of the primary photons are emitted into the front hemisphere, the other half into the rear hemisphere. Now, consider only the primary photons emitted into the rear hemisphere. For 125I, as mentioned above, about half of these photons will undergo photoelectric absorption within a couple of cm of the source as their first interaction with the surrounding water. The other half will experience a Compton scattering interaction. The average Compton scattering angle is assumed to be close to 90 degrees in this model. Since the energy of these once-scattered photons will be almost the same as the primary photons, about half will undergo photoelectric absorption as their next interaction within a couple of cm of the first interaction. The once-scattered photons that are not photoelectrically absorbed will experience a second Compton interaction with, once again, an average scattering angle of roughly 90 degrees. Of these twice-scattered photons, half will be directed back toward the front hemisphere, the other half will proceed deeper into the rear hemisphere. This admittedly simple model predicts that roughly 12.5% of the photons originally emitted into the rear hemisphere will scatter back toward the front hemisphere. Of these once backscattered photons, a similar fraction will probably double back once again and return to the rear hemisphere. As was pointed out above, it is unlikely that many photons will survive more than four interactions before they are absorbed. The net result predicted by our model is that about 11% of the photons originally emitted into the rear hemisphere may actually wind up being absorbed in the front hemisphere. Applying the same conceptual model to 103Pd, for which the probability of photoelectric absorption in water is about 70%, scatter from the rear into the front hemisphere should be closer to 5%. Due to symmetry, the same fraction of photons originally emitted into the front hemisphere will scatter back to the rear hemisphere, and an equilibrium condition will exist between the two hemispheres.

When the source is entirely surrounded by infinite water we have what is commonly referred to as full scatter geometry. Imagine now that the rear hemisphere consists entirely of air instead of water. Under these conditions very few photons emitted into the rear hemisphere will scatter back into the front hemisphere. Our model predicts that about 11% of the absorbed dose in each hemisphere can be attributed to photons that were originally emitted into the opposite hemisphere. If there is no backscatter from the rear hemisphere, dose in the front hemisphere should be about 11% less than it would be for the comparable full scatter geometry. Weaver (14) reported that with air backing an array of four 125I seeds the measured dose at 15 mm depth in a polystyrene phantom was reduced by 9% compared with the full scatter geometry. Luxton et al. (15) measured a 10%–14% dose reduction in an acrylic phantom compared with full scatter over the range 10–18 mm in front of a single 125I seed. Cygler et al. (17) calculated a dose reduction of about 10% at 10 mm in water for air backing compared with water backing for a single 125I source.

Figure 7 conceptualizes interactions in the vicinity of the same low energy isotropic point source of Fig. 6, with the exception that it is now surrounded by the gold backing of an eye plaque on one side and water and air on the other side. When a gold backing is present, our model assumes that all photons emitted into the rear hemisphere immediately encounter the high Z gold where they are all photoelectrically absorbed. If all photons emitted into the rear hemisphere are absorbed in the gold, then none of those photons will scatter back into the front hemisphere, a situation nearly identical to that described above in which the rear hemisphere consisted of air. Dose in the front hemisphere should therefore also be about 11% less than it would be for the homogeneous full scatter geometry. The dosimetric effects of various types of backings including gold and silver have been reported by several investigators (14, 15, 16, 17, 18, 19). Weaver (14) reported that a sheet of gold backing reduced dose by 9% compared with full scatter in polystyrene at 15 mm in front of an array of four 125I seeds. Luxton et al. (15) measured a 7%–10% reduction in acrylic compared with full scatter over the range 10–18 mm in front of a single 125I seed that was backed by a nominally 15 mm diameter gold plaque. Cygler et al. (17) measured diode response reductions of about 7%–9% over the range 10–20 mm in water for a single 125I seed with gold backing compared with water backing. Although the data in Fig. 3 are noisy, over the range 6–16 mm the average ratio of dose for the gold plaque alone to dose in homogeneous water is about 0.87 (13% reduction).

Interestingly, metallic backings do not simply eliminate backscatter from originating behind the plaque. As is also depicted in Fig. 7, with a gold backing, L-shell fluorescent X-rays resulting from the photoelectric absorption of 125I photons in the gold are emitted back into the front hemisphere. These X-rays have a mean energy of about 13 keV, which means they have a MFP in water of around 2 mm. One would therefore expect most of these fluorescent X-rays to be aborbed within about 6 mm of the plaque. Cygler et al. (17) have pointed out that most of these fluorescent X-rays will never escape the 0.5 mm thick gold shield because their MFP in gold (3E-4 cm) is less than half that of the 28 keV photons (1.6E-3 cm) responsible for their creation. If the number of gold fluorescent X-rays entering the region just in front of the plaque exceeds the number of photons lost from that same region by lack of backscatter, a dosimetric enhancement compared with the full scatter condition will occur within that region. In Fig. 3, looking at the data for gold alone, a small dose enhancement is seen in the first few mm from the source axis. Luxton et al. (15) measured a 2% enhancement compared with full scatter at 2 mm in front of a single 125I seed backed by a gold plaque. Cygler et al. (17) measured diode response enhancements of up to 7% at 2 mm in water for a single 125I seed with gold backing compared with water backing. Meli and Motakabbir (19) measured a diode response enhancement of only about 1% at 2 mm depth in water for gold backing compared with water backing.

Figure 7 also reminds us that photons that enter the air located in the front hemisphere can be considered lost as a source of scatter. The result will be a reduction of dose in the water near the water-air interface compared with the full scatter geometry.

In Fig. 8 our source is now embedded in the Silastic carrier of a COMS eye plaque. Silastic has a higher effective atomic number (Z = 10.7 vs. 7.4) and slightly higher density (ρ approximately 1.1 vs. 1.0) than water resulting in greater attenuation in the Silastic than would occur in an equal path length of water. For 28 keV photons the probability of photoelectric interaction in Silastic is about 75% (compared with approximately 50% in water) so a greater proportion of that attenuation will be photoelectric. Primary photons emitted into the front hemisphere must pass through a minimum of 1 mm of Silastic. It will be shown later in this report that for a fully loaded COMS plaque, the mean linear path length through Silastic (MLPS) toward the apex of a medium height tumor is approximately 1.1 mm, and about 2.2 mm at the tumor base. Taking a median value in tumor for the MLPS of 1.7 mm (see Fig. 13), attenuation in the Silastic carrier would be about e−μx = e−1.01 * 0.17 = 0.842 vs. e−0.46 * 0.17 = 0.925 of water. For 125I, the ratio of attenuation in Silastic to attenuation in water should be about 0.842 / 0.925 = 0.91. For 103Pd, attenuation in the Silastic carrier will be greater than for 125I. The ratio to water will be about e−1.96 * 0.17 / e−0.75 * 0.17 = 0.717 / 0.880 = 0.81.

Primary photons emitted into the rear hemisphere must also pass through Silastic. If we assume an average rearward path in Silastic of 1 mm the ratio of attenuation in Silastic to attenuation in water will be about e−1.01 * 0.1 / e−0.46 * 0.1 = 0.904 / 0.955 = 0.946 for 125I, and about 0.886 (e−1.96 * 0.1 / e−0.75 * 0.1) for 103Pd. Furthermore, some of the photons that scatter from the rear hemisphere back into the front hemisphere must pass through the nominally 2.4 mm thick carrier a second time, resulting in an additional attenuation factor of about e−1.01 * 0.24 / e−0.46 * 0.24 = 0.874 for 125I compared with water, and e−1.96 * 0.24 / e−0.75 * 0.24 = 0.749 for 103Pd compared with water. This works out to a roughly 1% scatter deficit between the hemispheres.

Compared with homogeneous water, the Silastic carrier alone can be expected to reduce absorbed dose in the front hemisphere by about 9% owing to primary attenuation in the carrier and by about another 1% owing to reduced scatter from the rear hemisphere. If air is present in the front hemisphere, the dose could be reduced by several percent more near the air interface. Chiu-Tsao et al. (4) calculated and measured about a 10% dose reduction at 1 cm for the carrier alone.

In Fig. 9 our point source is now inserted in a complete COMS plaque including both the gold backing and the Silastic carrier. In this case, we might speculate that the total dose reduction at any distance could be calculated as the product of the ratios plotted in Fig. 3 for the gold shell alone and the Silastic alone. The reason this does not work is that the gold fluorescence X-rays must now pass through the Silastic carrier before they can contribute to the water dose. At 13 keV about 94% of interactions in Silastic will be photoelectric, with a MFP of less than 1 mm. Very few of the gold fluorescence X-rays will escape the Silastic carrier, so the data for gold alone are no longer valid. The data for the Silastic alone are also invalid as they included backscatter, which is no longer present.

Compared with homogeneous water, the combination of gold plaque and Silastic carrier should reduce the absorbed dose in the front hemisphere by about 9% owing to photoelectric attenuation of primary radiation in the carrier, and by another 11% owing to lack of backscatter from the rear hemisphere. If air is present in the front hemisphere, the dose could be reduced by several percent more near the air interface. Looking at the data for Silastic plus gold (solid triangles) in infinite water in Fig. 3, we see a noisy, but decreasing ratio that appears to bottom out at approximately 0.8, about 16 mm from the source, yielding the anticipated 20% dose reduction. Considering the simplicity of the model presented in this conceptual discourse, it is remarkable how well it predicts the published measurements and far more sophisticated Monte Carlo calculations.

Section snippets

Methods

The instantaneous dose rate from any individual seed at a point (r,φ) can be expressed according to the TG43 formalism (9) as: D(r,φ)TG43=Sk*Λ*g(r)*[G(r,φ)/G(r0r0,φ0)]*F(r,φ) where r = distance between point p and the seed center, φ = the angle from the seed axis, Sk = air kerma strength of source in U (1 U = 1 cGy cm2 h-1), Λ = dose rate constant at 1 cm in cGy h−1U−1, g(r) = the radial dose function, G(r,φ) = the geometry factor, and F(r,φ) = the anisotropy function.

An existing TG43-based

Results

Plotted in Fig. 13 are isoMLPS calculated on a plane bisecting fully loaded COMS 12 and 20 mm plaques. The MLPS is minimal near the center of the eye with an approximate magnitude of 1 mm. The minimum possible path length through Silastic is 1 mm because the seeds are oriented tangent to, but offset 1 mm from, the front surface of the carrier. The MLPS increases with off-axis distance and as one approaches the surface of the plaque. Immediately in front of the plaque the gradient is steep,

Discussion

Chiu-Tsao et al. (4) confirmed that fluorescent X-rays originating from the gold backing are absorbed in the Silastic seed carrier by demonstrating the absence of dose enhancement in front of the plaque, but their conclusion that “the effect of the Silastic insert alone was found to be closely the same as the Silastic/gold combination” was unexpected. The dominant effect of the Silastic carrier alone should be to isotropically remove more photons through photoelectric absorption than would have

Conclusion

In their reanalysis of the COMS medium tumor trial data, Krintz et al. (6) concluded that “the amount of reduction in the dose to structures of interest could be clinically significant, so future eye plaque dosimetry should be performed using the most up-to-date parameters available.” Dose calculations for 125I seeds in COMS eye plaques that accurately account for the linear source geometry, anisotropy, collimation by the plaque backing and lip, geometric penumbra, scatter reduction due to the

References (29)

  • S.-T. Chiu-Tsao et al.

    Dosimetry for 125I seed (model 6711) in eye plaques

    Med Phys

    (1993)
  • A. Zerda et al.

    125I seed eye plaque dose distribution including penumbra characteristics

    Med Phys

    (1996)
  • R. Nath et al.

    Dosimetry of interstitial brachytherapy sources: Recommendations of the AAPM radiation therapy committee task group no

    43. Med Phys

    (1995)
  • COMS manual of procedures, Ch 12—radiation therapy, version 2/99, PB95–179693

    (1995)
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