Eye Plaque Research Articles

Perturbation of TG‐43 parameters of the brachytherapy sources under insufficient scattering materials

In the recommendations of Task Group #43 from American Association of Physicists in Medicine (AAPM TG43), methods of brachytherapy source dosimetry are recommended, under full scattering conditions. However, in actual brachytherapy procedures, sources may not be surrounded by full scattering tissue in all directions. Clinical examples include high‐dose‐rate (HDR) brachytherapy of the breast or low‐dose‐rate (LDR) brachytherapy of ocular melanoma using eye plaque treatment with 125I and 103Pd. In this work, the impact of the missing tissue on the TG‐43–recommended dosimetric parameters of different brachytherapy sources was investigated. The impact of missing tissue on the TG‐43–recommended dosimetric parameters of 137Cs, 192Ir, and 103Pd brachytherapy sources was investigated using the MCNP5 Monte Carlo code. These evaluations were performed by placing the sources at different locations inside a 30×30×30 cm3 cubical water phantom and comparing the results with the values of the source located at the center of the phantom, which is in a full scattering condition. The differences between the thickness of the overlying tissues for different source positions and the thickness of the overlying tissue in full scattering condition is referred to as missing tissue. The results of these investigations indicate that values of the radial dose function and 2D anisotropy function vary as a function of the thickness of missing tissue, only in the direction of the missing tissue. These changes for radial dose function were up to 5%, 11%, and 8% for 137Cs, 192Ir, and 103Pd, respectively. No significant changes are observed for the values of the dose rate constants. In this project, we have demonstrated that the TG‐43 dosimetric parameters may only change in the directions of the missing tissue. These results are more practical than the published data by different investigators in which a symmetric effect of the missing tissue on the dosimetric parameters of brachytherapy source are being considered, regardless of the implant geometry in real clinical cases.PACS number: 87.53.JW

Quantifying the dosimetric influences of radiation coverage and brachytherapy implant placement uncertainty on eye plaque size selection

Quantify the dosimetric adequacy of the 2003 American Brachytherapy Society report tumor margin recommendations for Collaborative Ocular Melanoma Study (COMS) eye plaque size selection for radiation coverage and clinical plaque placement uncertainties. Plaque heterogeneity-corrected dose distributions were generated for the range of available COMS plaque diameters (φplaque) and radionuclides. These dose distributions were used to determine the radiation dose distribution diameter (φ℞) at the eye surface for each plaque as a function of central axis prescription depth (d℞) to assess adequacy of a 2-3-mm margin for various gross tumor volume (GTV) basal diameters (φGTV). Four sets of ellipsoidal tumors (φGTV=5, 8, 11, and 14mm) with a range of apical heights (dGTV=2-8mm) were contoured in a reference CT environment. Plaque placement uncertainties were quantified as circumferential displacements (Δ) at the outer scleral surface. Tumor dose-volume histograms were generated and compared for all Δ with D90 and D95 used to evaluate tumor margin adequacy. For equivalent φplaque and prescription depths, φ℞ values were typically 0.4-0.8mm less for (103)Pd than for (125)I or (131)Cs. Δ≤3mm resulted in D90 and D95 values as low as 68% and 64% of the prescription dose, respectively. (103)Pd plaque dose distributions were more sensitive than (125)I or (131)Cs to placement uncertainties. The American Brachytherapy Society-recommended tumor margin may be inadequate for prescription dose coverage given COMS plaque radiation characteristics and placement uncertainties. Better coverage is achieved assuming a GTV-to-planning target volume total basal expansion of 3mm or greater and/or prescribing beyond the tumor apex.

Borderline Lepromatous Leprosy in an Italian Man

We report a 77-year-old man from Italy with a painful, red right eye with decreased vision (Figure 1A) and multiple asymptomatic erythematous infiltrated nodules and edematous plaques on the face (Figure 1B), tongue (Figure 1C), and trunk and extremities (Figure 1D). Lesions appeared on his face 20 years ago and progressively enlarged and diffused on the trunk. Two of his relatives had been affected by lepromatous leprosy. A skin biopsy specimen showed a diffuse infiltrate of vacuolar macrophages engulfed with mycobacteria (Figure 1E) and the focal presence in the dermis of few multinucleated giant cells and a focal infiltrate of neutrophils. Figure 1. A 77-year-old man from Italy with borderline lepromatous leprosy. Shown are a red right eye (A) and multiple asymptomatic erythematous edematous infiltrated nodules and plaques on the face (B), tongue (C), and trunk and extremities (D). E, Histopathologic ... A polymerase chain reaction identified Mycobacterium leprae. Mycobacteria were confirmed by multiple skin smears stained with Ziehl-Neelsen stain. An ophthalmologic consultation diagnosed acute right uveitis. A neurologic consultation disclosed enlargement of both ulnar nerves and sensory and motor impairment in the fourth and fifth fingers. Electroneurography confirmed involvement of both ulnar nerves, and ultrasound examination demonstrated diffuse swelling of the ulnar nerve at the elbow. According to the Ridley-Jopling classification, borderline lepromatous leprosy was diagnosed and the patient received rifampicin (600 mg/month), minocycline (100 mg/day) instead of dapsone because of anemia, and clofazimine (300 mg/month and 50 mg/day). The man was treated with prednisone (30 mg/day for 6 months with progressive tapering), thalidomide (200 mg/day for 6 weeks with progressive tapering), and topical prednisolone drops (4 times/day for 4 weeks). Treatment cured concomitant type-1 and type-2 leprosy reactions. At one-year of follow-up, the patient showed no adverse reactions to treatment, remission of skin and mucosal lesions, and improvement of leprosy reactions. During 1990–2009, progressive reduction of 12 autochthonous leprosy cases (the most recent case in 2003) was observed in Italy with a simultaneous increase of imported leprosy cases among immigrants from leprosy-endemic areas (159 cases).1 Fear of segregation and marginalization caused the patient to avoid medical treatment for 20 years, resulting in an advanced stage of leprosy. This presentation was unusual because patients are usually given a diagnosis at an earlier stage. Single, anesthetic, hypopigmented, asymmetrically arranged macules and a few macules characterize tuberculoid and borderline tuberculoid leprosy, respectively. Multiple, symmetrically arranged macules, nodules, and plaques characterize borderline lepromatous and early lepromatous leprosy.

Evaluation of material heterogeneity dosimetric effects using radiochromic film for COMS eye plaques loaded with 125I seeds (model I25.S16)

(1) To measure absolute dose distributions in eye phantom for COMS eye plaques with (125)I seeds (model I25.S16) using radiochromic EBT film dosimetry. (2) To determine the dose correction function for calculations involving the TG-43 formalism to account for the presence of the COMS eye plaque using Monte Carlo (MC) method specific to this seed model. (3) To test the heterogeneous dose calculation accuracy of the new version of Plaque Simulator (v5.3.9) against the EBT film data for this seed model. Using EBT film, absolute doses were measured for (125)I seeds (model I25.S16) in COMS eye plaques (1) along the plaque's central axis for (a) uniformly loaded plaques (14-20 mm in diameter) and (b) a 20 mm plaque with single seed, and (2) in off-axis direction at depths of 5 and 12 mm for all four plaque sizes. The EBT film calibration was performed at (125)I photon energy. MC calculations using MCNP5 code for a single seed at the center of a 20 mm plaque in homogeneous water and polystyrene medium were performed. The heterogeneity dose correction function was determined from the MC calculations. These function values at various depths were entered into PS software (v5.3.9) to calculate the heterogeneous dose distributions for the uniformly loaded plaques (of all four sizes). The dose distributions with homogeneous water assumptions were also calculated using PS for comparison. The EBT film measured absolute dose rate values (film) were compared with those calculated using PS with homogeneous assumption (PS Homo) and heterogeneity correction (PS Hetero). The values of dose ratio (film∕PS Homo) and (film∕PS Hetero) were obtained. The central axis depth dose rate values for a single seed in 20 mm plaque measured using EBT film and calculated with MCNP5 code (both in ploystyrene phantom) were compared, and agreement within 9% was found. The dose ratio (film∕PS Homo) values were substantially lower than unity (mostly between 0.8 and 0.9) for all four plaque sizes, indicating dose reduction by COMS plaque compared with homogeneous assumption. The dose ratio (film∕PS Hetero) values were close to unity, indicating the PS Hetero calculations agree with those from the film study. Substantial heterogeneity effect on the (125)I dose distributions in an eye phantom for COMS plaques was verified using radiochromic EBT film dosimetry. The calculated doses for uniformly loaded plaques using PS with heterogeneity correction option enabled were corroborated by the EBT film measurement data. Radiochromic EBT film dosimetry is feasible in measuring absolute dose distributions in eye phantom for COMS eye plaques loaded with single or multiple (125)I seeds. Plaque Simulator is a viable tool for the calculation of dose distributions if one understands its limitations and uses the proper heterogeneity correction feature.

Accurate estimation of dose distributions inside an eye irradiated with 106Ru plaques

Irradiation of intraocular tumors requires dedicated techniques, such as brachytherapy with (106)Ru plaques. The currently available treatment planning system relies on the assumption that the eye is a homogeneous water sphere and on simplified radiation transport physics. However, accurate dose distributions and their assessment demand better models for both the eye and the physics. The Monte Carlo code PENELOPE, conveniently adapted to simulate the beta decay of (106)Ru over (106)Rh into (106)Pd, was used to simulate radiation transport based on a computerized tomography scan of a patient's eye. A detailed geometrical description of two plaques (models CCA and CCB) from the manufacturer BEBIG was embedded in the computerized tomography scan. The simulations were firstly validated by comparison with experimental results in a water phantom. Dose maps were computed for three plaque locations on the eyeball. From these maps, isodose curves and cumulative dose-volume histograms in the eye and for the structures at risk were assessed. For example, it was observed that a 4-mm anterior displacement with respect to a posterior placement of a CCA plaque for treating a posterior tumor would reduce from 40 to 0% the volume of the optic disc receiving more than 80Gy. Such a small difference in anatomical position leads to a change in the dose that is crucial for side effects, especially with respect to visual acuity. The radiation oncologist has to bring these large changes in absorbed dose in the structures at risk to the attention of the surgeon, especially when the plaque has to be positioned close to relevant tissues. The detailed geometry of an eye plaque in computerized and segmented tomography of a realistic patient phantom was simulated accurately. Dose-volume histograms for relevant anatomical structures of the eye and the orbit were obtained with unprecedented accuracy. This represents an important step toward an optimized brachytherapy treatment of ocular tumors.

Treating Medium-sized Choroidal Melanomas With Eye Plaque Brachytherapy (125-I)

Treating Medium-sized Choroidal Melanomas With Eye Plaque Brachytherapy (125-I)

Three-dimensional MRT/CT Treatment Planning for 106Ru Eye Plaques Commissioned Using Radiochromic Film in a Solid Water Phantom

Three-dimensional MRT/CT Treatment Planning for 106Ru Eye Plaques Commissioned Using Radiochromic Film in a Solid Water Phantom

RADIATION DOSE TO THE SURGEON DURING PLAQUE BRACHYTHERAPY

To evaluate the radiation dose to a surgeon's hands during I eye plaque procedures. Sixteen consecutive patients with uveal melanomas were scheduled for eye plaque brachytherapy. The same surgeon wore thermoluminescent dosimeters on the dominant index finger and thumb while placing and removing the eye plaque to measure radiation dose. Additional laboratory experiments were performed to measure unobstructed (by surgical gloves or other parts of the hand) radiation exposure from a plaque. Hand radiation doses during eye plaque brachytherapy are very low, but measurable, with plaques containing an average of 1.3 GBq of 125I. Using these data, a surgeon would need to perform more than 1,000 cases each year to approach or exceed the annual regulatory radiation dose limits for the extremities.

Dosimetry of 125I and 103Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

Monte Carlo simulation of COMS ophthalmic applicators loaded with Bebig I25.S16 seeds and comparison with planning system predictions

PurposeTo simulate the Bebig model I125.S16 source and obtain AAPM Task Group Report 43 brachytherapy dosimetry parameters for comparison to consensus and previously published values. The seed model will then be incorporated into a Monte Carlo model of COMS eye plaques and simulation results will be used for seed-carrier set modeling in a commercial planning system. MethodsPENELOPE was used to simulate the seed and the applicators for different sizes and loading levels. The corresponding TG-43U1 dosimetric parameters of the seed were calculated. Bebig Plaque Simulator was used. ResultsThe air kerma strength, the dose rate constant and the radial dose and 2D anisotropy functions found showed a good agreement with those published by other authors. Dose distributions were determined for the 12 and 20 mm COMS plaques loaded with a single seed and for the 12 mm plaque fully loaded. The plaque effect on the eye dose and the interseed absorption were evaluated. If the plaque is loaded with a single seed, the dose in the central axis reduces about 10% at 5–6 mm depth with respect to the case in which the plaque is not present. This reduction does not depend on the plaque size. When the plaque is fully loaded, an additional reduction in the dose with respect to the dose in water is observed mainly due to the effect of the Silastic carrier. The mean dose reduction in the central axis of the 12 mm plaque due to the interseed absorption was 0.5%. A new physics file for the planning system was created with the results obtained from the simulations. Results obtained using this adapted model for the 12 mm plaque fully loaded agreed with the corresponding simulation. Dose rate at the prescription point differs 4.7% when the adapted model is used instead of the default model. ConclusionsSimulation results for COMS plaques are consistent with those published for other seeds. The planning system studied appears as a good tool for dose calculation in ophthalmic brachytherapy treatments. The new physics model, built up from Monte Carlo results, has been commissioned by comparing calculations made with the planning system to those obtained from Monte Carlo simulations.

TU-E-BRA-07: Post-Operative Eye Plaque Imaging Using Tomotherapy MVCT.

Intra-operative ultrasound is used to verify the positioning of episcleral eye plaques used to treat ocular melanoma. Ultrasound can be ambiguous because of image artifacts, and plaques may shift position after surgery. Ultrasound verification is particularly challenging for anterior tumors. Post-operative imaging could be used to trigger interventions that would prevent local treatment failure. We investigated if, and under what conditions, the Tomotherapy megavoltage computed tomography (MVCT) system could be used to perform post-implantation verification of eye plaque positioning. Plaques were placed on a preserved cow's eye, and imaged with the megavoltage CT of a Tomotherapy linear accelerator (Accuray, Sunnyvale, CA). The images were visually and quantitatively assessed to determine if they were of sufficient quality to verify tumor coverage and plaque tilt with respect to the sclera. We used the visibility of the lens as a proxy for visibility of a tumor. To test the utility of hypothetical higher beam current Tomotherapy images, we averaged sequential images of the same setup. The plaque, the lens of the eye, and the globe are visible in the images. The CNR of the lens with respect to the vitreous was 5.6 for a single image. For 10 images averaged, the CNR was 9.2. Estimated dose from a single image was 1.3 cGy (body CTDIvol); even 10 times this dose would be an acceptable image-guidance dose for radiotherapy patients. One limitation of the imaging procedure is the long scan time (up to 240 seconds), during which time any significant patient motion would lead to image artifacts. Human trials on eye plaque patients are planned. Tomotherapy MVCT imaging could be used to verify tumor coverage and plaque tilt after episcleral plaque implantation. Tumors should be visible in standard Tomotherapy images but higher beam current images would be preferred if available.

SU-E-T-429: Image-Guided Eye Plaque Brachytherapy Optimization: Implications for Patients at 2-Year Follow-up.

Episcleral eye plaque brachytherapy has been utilized in the treatment of intra-ocular malignancies, delivering large prescription doses to the apex of the tumor. Advances in dose calculation and image guidance, via calibrated fundus images, enable localization of the tumor and determination of dose to the macula, optic disc, and lens. A two-year post-implant study aims to correlate dosimetry with local tumor control and changes in visual acuity, as well as assess the need for plaque optimization with respect to critical structures. A retrospective, two-year follow-up study of 21 patients who have received episcleral eye plaque brachytherapy at our institution was used to correlate dosimetry with clinical outcomes and evaluate the need for eye plaque optimization. BEBIG Plaque Simulator wasused in treatment planning; fundus photographs were registered for tumor localization and the TG43-U1 formulism enabled dose calculation of I-125- loaded COMS plaques. Doses to the apex, macula, and optic disc were correlated to changes in apex height and visual acuity. Selected patients were replanned using optimization strategies to reduce dose to critical structures. A total of seven patients (33%) noted improved eyesight at two years. 11 (52%) patients lost at least two lines of vision at two years. Two patients saw increases in apical height (9%) within two years. Optimized eye plaque plans were able to reduce optic disc and macular doses (average 68Gy and 80Gy, respectively) by 36% and 25% on the average, while maintaining the prescribed dose. Image guidance and optimization are important tools that can aid in treatment of intra-ocular malignancies, as these techniques provide physicists with the ability to spare critical structures while delivering the prescription dose, thus increasing the possibility of local control and vision sparing.

SU‐E‐T‐328: Method for Verifying the Air Kerma Strength of Pre‐Assembled I‐125 Plaques for Ocular Melanoma

Purpose: To develop a method of easily verifying the activity or air kerma strength of pre‐assembled COMS and Eye Physics EP917 eye plaques. Methods: A Capintec CRC‐7 Dose Calibrator with its standard vial/syringe sample holder was used to measure the activity of pre‐assembled eye plaques using IsoAid Advantage I‐125 seeds. Plaque activity measurements were made by placing the plaque face up in the center of a 10 cm tall Styrofoam insert in the source holder. The source holder was rotated to four angles (0°, 90°, 180°, and 270°). The average of these four values was compared to the assay air kerma strength, decayed to the plaque measurement date, to establish a plaque calibration factor. The average and standard deviation of several plaque measurements were then calculated to determine the repeatability of the process. Activity measurements were also made to determine the contribution of each seed position to the total measured activity of the EP 917 plaque. Results: Activity measurements were taken on seven vendor‐preloaded Eye Physics EP917 eye plaques. For the EP917, the average plaque calibration factor was 0.344 with a standard deviation of 0.012. For the COMS plaques with a silastic carrier, the plaque calibration factors ranged from 0.237 for a COMS 16N to 0.281 for a COMS 20. The average plaque calibration factor was 0.256 and the standard deviation was 0.015 The individual seed measurements were made using three different EP‐917 plaques. The data from these measurements show that the contribution of a single seed to the total plaque activity is approximately 5.9% or 1/17th of the total activity. Conclusions: Plaque calibration factors based on this methodology may be used to verify the activity of preassembled plaques to within 5%.

SU-E-T-517: Characterization of Relative Doses and Source Strengths for Various Plaque Sizes and Tumor Dimensions in the Treatment of Uveal Melanoma.

To characterize the relative doses and source strengths for various eye plaque sizes and tumor dimensions in the treatment of uveal melanoma. Several tumors with basal diameters ranging from 6-16mm and apical height of 5-10mm were planned using the Pinnacle3 treatment planning system following the American Brachytherapy Society's recommendations. The data is based on 5-day implant to deliver 85Gy with a dose rate of 70.9cGy/hr. Choice of plaque size is based on a 2-3 mm margin on either side of the tumor. Doses to the fixed locations: sclera, center of the eye and opposite sclera were obtained. From our results we see larger source strengths needed for smaller plaques due to smaller number of sources used. The exception is with 14mm and 16mm plaques containing the same number of sources where the 16mm plaque has slightly higher source strengths. For both the inner and outer sclera, the relative dose increases with increasing height. For a 12mm plaque, the relative dose of the outer sclera increases from 4-10 times that of the Rx dose at the apex of the tumor. For a 20mm eye plaque, the relative dose of the outer sclera increases from 2-5 times that of the prescription dose at the apex of the tumor. As apical height increases, the relative dose at the center of the eye increases to about 75% of the Rx dose at 10cm depth for all plaque sizes. At the inner sclera surface opposite the center of the tumor base the relative dose is 20-22% of the Rx dose for all plaque sizes. The data provided will be a helpful tool in evaluating and predicting complications in the retina and sclera based on the tumor dimensions and selected plaque size.

SU-E-T-13: Comparison of Dose Rates with and without Gold Backing of USC #9 Radioactive Eye Plaque Using MCNP5.

To show the effect of gold backing on dose rates for the USC #9 radioactive eye plaque. An I125 source (IsoAid model IAI-125A) and gold backing was modeled using MCNP5 Monte Carlo code. A single iodine seed was simulated with and without gold backing. Dose rates were calculated in two orthogonal planes. Dose calculation points were structured in two orthogonal planes that bisect the center of the source. A 2×2 cm matrix of spherical points of radius 0.2 mm was created in a water phantom of 10 cm radius. 0.2 billion particle histories were tracked. Dose differences with and without the gold backing were analyzed using Matlab. The gold backing produced a 3% increase in the dose rate near the source surface (<1mm) relative to that without the backing. This was presumably caused by fluorescent photons from the gold. At distances between 1 and 2 cm, the gold backing reduced the dose rate by up to 12%, which we attribute to a lack of scatter resulting from the attenuation from the gold. Dose differences were most pronounced in the radial direction near the source center but off axis. The dose decreased by 25%, 65% and 81% at 1, 2, and 3 mm off axis at a distance of 1 mm from the source surface. These effects were less pronounced in the perpendicular dimension near the source tip, where maximum dose decreases of 2% were noted. I 125 sources embedded directly into gold troughs display dose differences of 2 - 90%, relative to doses without the gold backing. This is relevant for certain types of plaques used in treatment of ocular melanoma. Large dose reductions can be observed and may have implications for scleral dose reduction.

WE‐A‐BRB‐05: Methods to Optimize COMS Plaque Loading for Improved Dose Homogeneity and Conformity

Purpose: To investigate the dosimetric optimization methods of varied intra‐plaque seed strength and radionuclide choice for COMS eye plaque brachytherapy. Methods: The COMS plaque Silastic insert arranges seeds about concentric circles (rings) of different radii. Thus, each plaque dose distribution may be considered as the superposition of its constituent rings. The MCNP5 radiation transport code was used to simulate a 16mm COMS plaque populated with 103Pd, 125I, and 131Cs sources. Each constituent source ring was then activated to determine its contribution to the total dose distribution while accounting for the dosimetric influences of inter‐seed attenuation and scatter. Tumor dose distributions were generated from &gt;106 permutations of ring weightings and radionuclides for a given apical prescription dose. These dose distributions were analyzed for target dose uniformity and conformity, and compared to that of a uniformly‐loaded 125I plaque. Results: Using 131Cs seeds in only the plaque‘s outer ring yielded the most homogeneous dose distribution (+16% vs. 125I). The proximal sclera dose was 42% lower, yet center of the eye and opposite retina doses were 16% and 30% higher, respectively. Combinations with 103Pd in the plaque inner ring weighted to deliver &gt;98% of the prescription dose yielded the most conformal dose distributions (&gt;24% vs. 12 5I). Though doses to the eye center and opposite retina were &gt;32% and &gt;55% lower, respectively, the proximal sclera dose was &gt;26% higher. Various combinations using only 103Pd with the outer ring contributing &gt;50% of the prescription dose produced dose distributions both more homogeneous and more conformal than 125I. Proximal sclera, eye center, and opposite retina doses were typically 8‐12%, 11‐14%, and 38–40% lower, respectively, than using 125I. Conclusions: Intra‐plaque radionuclide and seed strength variations produced more homogeneous and conformal dose distributions than uniformly‐loaded COMS plaques. This optimization approach is generalizable to all plaque sizes and tumor dimensions.

Experimental measurements and Monte Carlo calculations for 103Pd dosimetry of the 12 mm COMS eye plaque

Monte Carlo simulations and TLD dosimetry have been performed to determine the dose distributions along the central axis of the 12 mm COMS eye plaques loaded with IRA1-103Pd seeds. Several simulations and measurements have been employed to investigate the effect of Silastic insert and air in front of the eye on dosimetry results along the central axis of the plaque and at some critical ocular structures. Measurements were performed using TLD-GR200A circular chip dosimeters in a PMMA eye phantom. The central axis TLD chips locations were arranged in one central column of eye phantom, in 3 mm intervals. The off-axis TLD chips locations were arranged in three off-axis columns around the central axis column. Version 5 of the MCNP code was also used to evaluate the dose distribution around the plaque. The presence of the Silastic insert results in dose reduction of 14% at 5 mm; also about 7% dose reduction appears at the interface point, due to the air presence and lack of the scattering condition. The overall dosimetric parameters for the COMS eye plaque loaded with new palladium seeds are similar to a commercial widely used seed such as Theragenics200. As the dose calculations under TG-43 assumptions do not consider the effect of the plaque backing and Silastic insert for accurate dosimetry, it's suggested to apply the effect of the eye plaque materials and air on dosimetry results along the central axis of the plaque and at some critical ocular structures.

Comparison of 16 mm OSU‐Nag and COMS eye plaques

OSU‐NAG eye plaques use fewer sources than COMS‐plaques of comparable size, and do not employ a Silastic seed carrier insert. Monte Carlo modeling was used to calculate 3D dose distributions for a 16 mm OSU‐NAG eye plaque and a 16 mm COMS eye plaque loaded with either Iodine‐125 or Cesium‐131 brachytherapy sources. The OSU‐NAG eye plaque was loaded with eight sources forming two squares, whereas the COMS eye plaque was loaded with thirteen sources approximating three isocentric circles. A spherical eyeball 24.6 mm in diameter and an ellipsoid‐like tumor 6 mm in height and 12 mm in the major and minor axes were used to evaluate the doses delivered. To establish a fair comparison, a water seed carrier was used instead of the Silastic seed carrier designed for the traditional COMS eye plaque. Calculations were performed on the dose distributions along the eye plaque axis and the DVHs of the tumor, as well as the 3D distribution. Our results indicated that, to achieve a prescription dose of 85 Gy at 6 mm from the inner sclera edge for a six‐day treatment, the OSU‐NAG eye plaque will need 6.16 U/source and 6.82 U/source for 125I and 131Cs, respectively. The COMS eye plaque will require 4.02 U/source and 4.43 U/source for the same source types. The dose profiles of the two types of eye plaques on their central axes are within 9% difference for all applicable distances. The OSU‐NAG plaque delivers about 10% and 12% more dose than the COMS for 125I and 131Cs sources, respectively, at the inner sclera edge, but 6% and 3% less dose at the opposite retina. The DVHs of the tumor for two types of plaques were within 6% difference. In conclusion, the dosimetric quality of the OSU‐NAG eye plaque used in eye plaque brachytherapy is comparable to the COMS eye plaque.PACS number: 87.56B, 87.55k, 87.55kh

BEDVH-A method for evaluating biologically effective dose volume histograms: Application to eye plaque brachytherapy implants

A method is introduced to examine the influence of implant duration T, radionuclide, and radiobiological parameters on the biologically effective dose (BED) throughout the entire volume of regions of interest for episcleral brachytherapy using available radionuclides. This method is employed to evaluate a particular eye plaque brachytherapy implant in a radiobiological context. A reference eye geometry and 16 mm COMS eye plaque loaded with (103)Pd, (125)I, or (131)Cs sources were examined with dose distributions accounting for plaque heterogeneities. For a standardized 7 day implant, doses to 90% of the tumor volume ( (TUMOR)D(90)) and 10% of the organ at risk volumes ( (OAR)D(10)) were calculated. The BED equation from Dale and Jones and published α/β and μ parameters were incorporated with dose volume histograms (DVHs) for various T values such as T = 7 days (i.e., (TUMOR) (7)BED(10) and (OAR) (7)BED(10)). By calculating BED throughout the volumes, biologically effective dose volume histograms (BEDVHs) were developed for tumor and OARs. Influence of T, radionuclide choice, and radiobiological parameters on (TUMOR)BEDVH and (OAR)BEDVH were examined. The nominal dose was scaled for shorter implants to achieve biological equivalence. (TUMOR)D(90) values were 102, 112, and 110 Gy for (103)Pd, (125)I, and (131)Cs, respectively. Corresponding (TUMOR) (7)BED(10) values were 124, 140, and 138 Gy, respectively. As T decreased from 7 to 0.01 days, the isobiologically effective prescription dose decreased by a factor of three. As expected, (TUMOR) (7)BEDVH did not significantly change as a function of radionuclide half-life but varied by 10% due to radionuclide dose distribution. Variations in reported radiobiological parameters caused (TUMOR) (7)BED(10) to deviate by up to 46%. Over the range of (OAR)α/β values, (OAR) (7)BED(10) varied by up to 41%, 3.1%, and 1.4% for the lens, optic nerve, and lacrimal gland, respectively. BEDVH permits evaluation of the relative biological effectiveness for brachytherapy implants. For eye plaques, (TUMOR)BEDVH and (OAR)BEDVH were sensitive to implant duration, which may be manipulated to affect outcomes.

Intraoperative Ultrasonography-Guided Positioning of Iodine 125 Plaque Brachytherapy in the Treatment of Choroidal Melanoma

To report intraoperative ultrasonography-guided positioning of iodine 125 (I(125)) plaques for brachytherapy of choroidal melanoma as a quality improvement measure. Retrospective, single-center, consecutive case-cohort study. One hundred fifty consecutive patients with choroidal melanoma. Patients with choroidal melanoma who were treated with I(125) plaque brachytherapy from January 2007 through January 2011 with at least 6 months of clinical follow-up were included. Patient and tumor characteristics at diagnosis were tabulated. The need for plaque repositioning if intraoperative ultrasonography showed the plaque to be either not centered on the tumor or if there was less than 1.0 mm of plaque margin beyond the tumor border was recorded. The rate of local treatment failure and occurrence of distant metastasis were determined. The average interval from surgery to last follow-up was 21.5 months. Fifty-four (36%) of 150 patients required plaque repositioning. Of tumors located in the macula, equator, and periphery, 15 (36.6%), 26 (36.6%), and 13 (34.2%) required repositioning. There was no case of local treatment failure during a mean follow-up of 21.5 months (range, 6-48 months). Clinical evidence of choroidal melanoma metastasis developed in 9 patients. The cumulative 2-year Kaplan-Meier rate of local treatment failure in the cohort was statistically lower compared with the Collaborative Ocular Melanoma Study, which did not require ultrasonography-guided plaque positioning. Intraoperative ultrasonography identified the need to reposition I(125) plaques to achieve centration and plaque margin (>1.0 mm) beyond the tumor border in 36% of eyes. Neither tumor size nor tumor location correlated with the need to reposition the plaque. There was no case of local treatment failure during follow-up in this series. Correct plaque position is an essential component of quality outcomes in brachytherapy. Intraoperative ultrasonography reduces geographic errors in placement in eye plaque therapy and may help to reduce local treatment failure in choroidal melanoma.