Collaborative Ocular Melanoma Study Plaques Research Articles

Monte Carlo dosimetry for and eye plaque brachytherapy with various seed models

PurposeDose distributions are calculated for various models of and seeds in the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). The sensitivity to seed model of dose distributions and dose distributions relative to TG‐43 are investigated.MethodsMonte Carlo simulations are carried out with the EGSnrc user‐code BrachyDose. Brachytherapy seeds and eye plaques are fully modeled. Simulations of one seed in the central slot of a 20 mm Modulay (gold alloy) plaque backing with and without the Silastic (silicone polymer) insert and of a 16 mm fully loaded Modulay/Silastic plaque are performed. Dose distributions are compared to those calculated under TG‐43 assumptions, i.e., ignoring the effects of the plaque backing and insert and interseed attenuation. Three‐dimensional dose distributions for different and seed models are compared via depth‐dose curves, isodose contours, and tabulation of doses at points of interest in the eye. Results are compared to those of our recent BrachyDose study for COMS plaques containing model 6711 or 200 seeds [R. M. Thomson et al., Med. Phys. 35, 5530–5543 (2008)].ResultsAlong the central axis of a plaque containing one seed, variations of less than 1% are seen in the effect of the Modulay backing alone for different seed models; for the Modulay/Silastic combination, variations are 2%. For a 16 mm plaque fully loaded with seeds, dose decreases relative to TG‐43 doses are 11%–12% (19%–20%) and 14%–15% (20%) at distances of 0.5 and 1 cm from the inner sclera along the plaque's central axis, respectively. For the same prescription dose, doses at points of interest vary by up to 8% with seed model. Doses to critical normal structures are lower for all seed models than for with the possible exception of the sclera adjacent to the plaque; scleral doses vary with seed model and are not always higher for than for .ConclusionsDose decreases relative to doses calculated under TG‐43 assumptions vary slightly with seed model (for each radionuclide). Dose distributions are sensitive to seed model; however, variations are generally no larger than the magnitudes of other systematic uncertainties in eye plaque therapy.

WE-D-BRB-01: Eye Plaque Dosimetry: Report of the AAPM Therapy Physics Committee Task Group No. 129

Purpose: The AAPM Eye Plaque Dosimetry Task Group 129 presents an update of the dosimetry calculations for the Collaborative Ocular Melanoma Study (COMS) plaques. Results from a multi‐center comparison using several brachytherapytreatment planning systems (BTPSs) are presented. Method and Materials: Dose distributions around 16‐mm diameter COMS plaques loaded with I‐125 (model 6711) or Pd‐103 (model 200) were determined using three TG43‐based BTPSs (Pinnacle v8.0d, BrachyVision v6.1 and v8.1, PlaqueSimulator v5.3.7), and two Monte Carlo codes (MCNP5 and BrachyDose). TG‐43 plans assumed an unbounded homogeneous medium. Monte Carlo plaque simulations included the Modulay (gold alloy) backing, Silastic (silicone polymer) insert, and interseed interactions. Doses along tumor central axis (−1 to 20 mm) and to defined critical structures such as fovea, optic disc, lens center, and lacrimal gland center in a standard eye model were calculated. Results: Agreement among all TG‐43 based plans on the central axis was within ±2% for both point‐ and line‐source approximations. However, for off‐axis points, BrachyVision v 6.1 had a truncation error in coordinates, which resulted in dose deviation of about 5% relative to other plans. As expected, the doses at off‐axis points were lower for the line source approximation than the point source approximation. The largest deviations were found at the lacrimal gland center, where the line source model resulted in 10% and 20% less dose than the point source model for I‐125 and Pd‐103 sources, respectively. Monte Carlo simulations predicted dose values are about 20–30% lower than the average of TG‐43 plan values, due to full plaque geometry; with the exception of few off‐axis points up to 90% lower. Conclusions: This multi‐center comparative analysis of BTPSs dose results indicated the importance of careful selection of TG‐43 parameters, source model assumptions, and the BTPS coordinate resolution limits, and the value of complete Monte Carlo calculations.

Monte Carlo dosimetry for and eye plaque brachytherapy

A Monte Carlo study of dosimetry for eye plaque brachytherapy is performed. BrachyDose, an EGSnrc user code which makes use of Yegin's multi-geometry package, is used to fully model I125 (model 6711) and Pd103 (model 200) brachytherapy seeds and the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). Three-dimensional dose distributions in the eye region are obtained. In general, dose to water is scored; however, the implications of replacing water with eye tissues are explored. The effect of the gold alloy (Modulay) backing is investigated and the dose is found to be sensitive to the elemental composition of the backing. The presence of the silicone polymer (Silastic) seed carrier results in substantial dose decreases relative to water, particularly for Pd103. For a 20mm plaque with a Modulay backing and Silastic insert, fully loaded with 24 seeds, the dose decrease relative to water is of the order of 14% for I125 and 20% for Pd103 at a distance of 1cm from the inner sclera along the plaque's central axis. For the configurations of seeds used in COMS plaques, interseed attenuation is a small effect within the eye region. The introduction of an air interface results in a dose reduction in its vicinity which depends on the plaque's position within the eye and the radionuclide. Introducing bone in the eye's vicinity also causes dose reductions. The dose distributions in the eye for the two different radionuclides are compared and, for the same prescription dose, Pd103 generally offers a lower dose to critical normal structures. BrachyDose is sufficiently fast to allow full Monte Carlo dose calculations for routine clinical treatment planning.

Improving quality assurance for assembled COMS eye plaques using a pinhole gamma camera

A quality assurance system has been designed to verify the location and strength of seeds loaded in a brachytherapy eye plaque. This system consists of (1) a pinhole camera in conjunction with a Lumisys ACR-2000i computed radiography (CR) unit to image the location and measure the relative strength of the seeds with autoradiography, and (2) a source strength jig with a survey meter to estimate the total activity of the seeds in the plaque. Five holders of different sizes were made for fixation of the COMS (Collaborative Ocular Melanoma Study) plaques (12, 14, 16, 18, and 20 mm) in the camera. The plaque-to-pinhole distance (dpp) has been optimized to be 30 mm to give approximately uniform intensity on the CR image for uniformly loaded COMS plaques. The pinhole-to-detector distance (dpd) can be kept at either 30 mm for 1:1 scale, or at larger distances for higher magnification. For a 1:1 scaling and pinhole diameter of 0.345 mm, useful images are obtained with time-activity product (mCi sec) ranging from 5 to 250 mCi sec. Within this range, the pinhole system is able to differentiate seed activities of >10%. The resulting pinhole autoradiograph is able to (1) confirm the correct number of seeds loaded in the plaque, (2) verify the proper sitting of the seeds in the silastic carrier and the plaque, (3) verify the relative activity distribution of the seeds loaded in the plaque, and (4) potentially evaluate the integrity of the seed. The source strength measurement system is able to measure the total strength of seeds in the plaque ranging from 10 to 80 mCi with an uncertainty of 5%.

Sci-Sat AM(2): Brachy-03: Monte Carlo dosimetry for I-125 and Pd-103 eye plaque brachytherapy.

A Monte Carlo study of dosimetry for eye plaque brachytherapy was performed. BrachyDose, an EGSnrc user-code which makes use of Yegin's multi-geometry package, was used to fully model Iodine-125 (model 6711) and Palladium-103 (model 200) brachytherapy seeds and the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). Three-dimensional dose distributions in the eye region were obtained. In general, dose to water was scored, however the implications of replacing water with eye materials was explored. The effect of the gold alloy (Modulay) backing was investigated and the dose was found to be sensitive to the elemental composition of the backing. The presence of the silicon polymer (Silastic) seed carrier resulted in substantial dose decreases relative to water, particularly for Pd-103. For the Modulay backing and Silastic insert combination in a 20 mm plaque, the dose decrease relative to water is of the order of 12% for I-125 and 20% for Pd-103 at a distance of 1 cm from the central seed along the plaque's central axis. For the configurations of seeds used in COMS plaques, interseed attenuation is a small effect within the eye region. The introduction of an air interface results in a dose reduction in its vicinity which depends on the plaque's position within the eye and the source type. The dose distributions in the eye for the two different sources were compared and, for the same prescription dose, Pd-103 generally offers a lower dose to critical normal structures.

Design and dosimetric considerations of a modified COMS plaque: The reusable “seed‐guide” insert

The Collaborative Ocular Melanoma Study (COMS) developed a standardized set of eye plaques that consist of a 0.5 mm thick bowl-like gold alloy backing with a cylindrical collimating lip. A Silastic seed carrier into which 125I seeds are loaded was designed to fit within the backing. The carrier provides a standardized seed pattern and functions to offset the seeds by 1.0 mm from the concave (front) surface of the carrier. These Silastic carriers have been found to be difficult to load, preclude flash sterilization, and are a source of dosimetric uncertainty because the effective atomic number of Silastic is significantly higher than that of water. The main dosimetric effect of the Silastic carrier is a dose-reduction (compared to homogeneous water) of approximately 10%-15% for 125I radiation. The dose reduction is expected to be even greater for 103Pd radiation. In an attempt to improve upon, yet retain as much of the familiar COMS design as possible, we have developed a thin "seed-guide" insert made of gold alloy. This new insert has cutouts which match the seed pattern of the Silastic carrier, but allows the seeds to be glued directly to the inner surface of the gold backing using either dental acrylic or a cyanoacrylate adhesive. When glued directly to the gold backing the seeds are offset a few tenths of a millimeter further away from the scleral surface compared to using the Silastic carrier. From a dosimetric perspective, the space formerly occupied by the Silastic carrier is now assumed to be water equivalent. Water equivalency is a desirable attribute for this space because it eliminates the dosimetric uncertainties related to the atomic composition of Silastic and thereby facilitates the use of either 125I and/or 103Pd seeds. The caveat is that a new source of dosimetric uncertainty would be introduced were an air bubble to become trapped in this space during or after the surgical insertion. The presence of air in this space is modeled and the dosimetric impact discussed. Another unintended consequence of water equivalency is that some fluorescent x rays emitted from the gold backing can now reach the eye. These very low energy x rays were virtually eliminated by absorption in Silastic. When loaded with 125I seeds the modified plaque appears to produce dose distributions that are almost the same as those of the original COMS plaque and the maximum dosimetric uncertainty introduced by an air bubble is about 2%. Dose distributions calculated for a modified plaque loaded with 103Pd seeds show that dose to healthy ocular structures distal to the tumor apex can be reduced compared to 125I. Clearly, it is faster and easier to glue seeds into the reusable gold seed-guide insert than it is to load the COMS-Silastic carrier.

Improved treatment planning for COMS eye plaques

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 (125)I 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 (125)I that correct for attenuation in the Silastic carrier and scatter deficits resulting from the gold plaque and nearby air. The implications of using (103)Pd seeds in COMS plaques are also discussed. 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. 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 (125)I 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. Treatment planning for COMS eye plaques that accurately accounts for the presence of the gold, Silastic and extraocular air is both possible and practical.