A practical method of estimating medium-heterogeneity corrected dose without a Monte Carlo calculation in ocular brachytherapy using 125I COMS plaques

PURPOSE

To provide a practical method of estimating medium-heterogeneity corrected dose without a Monte Carlo (MC) calculation in ocular brachytherapy using 125I Collaborative Ocular Melanoma Study (COMS) plaques.

METHODS AND MATERIALS

Using egs_brachy, MC simulations (1) under task group-43 assumptions with fully loaded seed configurations in water (HOMO) and (2) with effects of plaque backing, insert and inter-seed interactions (HETERO) were performed for seven 125I COMS plaques (10 mm-22 mm in diameter), and homogeneous dose (DHOMO) and heterogeneous dose (DHETERO) for central-axis and off-axis points were determined. For DHOMO, 85 Gy was normalized to a depth of 5 mm. Central-axis heterogeneity correction factors (HCFs) from a depth of 0 mm (inner sclera) to 22 mm (opposite retina) were derived from a ratio of DHETERO to DHOMO. Off-axis HCFs for optic disc/macula and lens as a function of distance from optic disc/macula (DT/MT) for various basal dimensions of tumor (BD/BM) were derived from DHETERO/DHOMO.

RESULTS

Central-axis HCF varied with a dose reduction of 10.3-19.8% by heterogeneity. Off-axis HCF for optic disc/macula varied significantly depending on DT/MT and BD/BM with a dose reduction of 11.3-38.3%. Off-axis HCF for lens had a dependence on MT and BM with its variation of 11.0-19.0%. A clinical example of using HCFs to estimate DHETERO was presented.

CONCLUSIONS

The practical method of using depth-dependent central-axis HCF and DT/MT- and BD/BM-dependent off-axis HCF provided in this study will facilitate a heterogeneous dose estimate for 125I COMS plaques without MC calculations.

Current and Emerging Radiotherapy Options for Uveal Melanoma

What treatment options are there for patients having uveal melanoma? A randomized, prospective, multi-institutional clinical trial (COMS) showed no difference in survival between brachytherapy and enucleation for medium-sized lesions. With the obvious benefit of retaining the eye, brachytherapy has flourished and many different approaches have been developed such as low-dose-rate sources using alternate low-energy photon-emitting radionuclides, different plaque designs and seed-loading techniques, high-dose-rate brachytherapy sources and applicators, and low- and high-dose-rate beta-emitting sources and applicators. There also have been developments of other radiation modalities like external-beam radiotherapy using linear accelerators with high-energy photons, particle accelerators for protons, and gamma stereotactic radiosurgery. This article examines the dosimetric properties, targeting capabilities, and outcomes of these approaches. The several modalities examined herein have differing attributes and it may be that no single approach would be considered optimal for all patients and all lesion characteristics.

Generation and comparison of 3D dosimetric reference datasets for COMS eye plaque brachytherapy using model-based dose calculations

PURPOSE

A joint Working Group of the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) was created to aid in the transition from the AAPM TG-43 dose calculation formalism, the current standard, to model-based dose calculations. This work establishes the first test cases for low-energy photon-emitting brachytherapy using model-based dose calculation algorithms (MBDCAs).

ACQUISITION AND VALIDATION METHODS

Five test cases are developed: (1) a single model 6711 125 I brachytherapy seed in water, 13 seeds (2) individually and (3) in combination in water, (4) the full Collaborative Ocular Melanoma Study (COMS) 16 mm eye plaque in water, and (5) the full plaque in a realistic eye phantom. Calculations are done with four Monte Carlo (MC) codes and a research version of a commercial treatment planning system (TPS). For all test cases, local agreement of MC codes was within ∼2.5% and global agreement was ∼2% (4% for test case 5). MC agreement was within expected uncertainties. Local agreement of TPS with MC was within 5% for test case 1 and ∼20% for test cases 4 and 5, and global agreement was within 0.4% for test case 1 and 10% for test cases 4 and 5.

DATA FORMAT AND USAGE NOTES

Dose distributions for each set of MC and TPS calculations are available online (https://doi.org/10.52519/00005) along with input files and all other information necessary to repeat the calculations.

POTENTIAL APPLICATIONS

These data can be used to support commissioning of MBDCAs for low-energy brachytherapy as recommended by TGs 186 and 221 and AAPM Report 372. This work additionally lays out a sample framework for the development of test cases that can be extended to other applications beyond eye plaque brachytherapy.

Monte Carlo investigation of dose distribution of uniformly and non-uniformly loaded standard and notched eye plaques

To investigate the effect of using non-uniform loading and notched plaques on dose distribution for eye plaques. Using EGSnrc Monte Carlo (MC) simulations, we investigate eye plaque dose distributions in water and in an anatomically representative eye phantom. Simulations were performed in accordance with TG43 formalism and compared against full MC simulations which account for inter-seed and inhomogeneity effects. For standard plaque configurations, uniformly and non-uniformly loaded plaque dose distributions in water showed virtually no difference between each other. For standard plaque, the MC calculated dose distribution in planes parallel to the plaque is narrower than the TG43 calculation due to attenuation at the periphery of the plaque by the modulay. MC calculated the dose behind the plaque is fully attenuated. Similar results were found for the notched plaque, with asymmetric attenuation along the plane of the notch. Cumulative dose volume histograms showed significant reductions in the calculated MC doses for both tumor and eye structures, compared to TG43 calculations. The effect was most pronounced for the notch plaque where the MC dose to the optic nerve was greatly attenuated by the modulay surrounding the optic nerve compared to the TG43. Thus, a reduction of optic nerve D95% from 14 to 0.2 Gy was observed, when comparing the TG43 calculation to the MC result. The tumor D95% reduced from 89.2 to 79.95 Gy for TG43 and MC calculations, respectively. TG43 calculations overestimate the absolute dose and the lateral dose distribution of both standard and notched eye plaques, leading to the dose overestimation for the target and organs at risk. The dose matching along the central axis for the non-uniformly loaded plaques to that of uniformly loaded ones was found to be sufficient for providing comparable coverage and can be clinically used in eye-cancer-busy centers.

Design and optimizing a novel ocular plaque brachytherapy with dual-core of 103Pd and 106Ru

In recent decades, eye plaques of brachytherapy have been extensively used as primary treatment as well as a complementary treatment for ocular cancer. The purpose of this study is the development of the eye plaque brachytherapy throughout a new design of eye plaque by combining the COMS plaque and the CCB BEBIG plaque loaded by IRA1-¹⁰³Pd and ¹⁰⁶Ru, respectively. A new dual-core plaque with a diameter of 20 mm was designed in the way that the BEBIG plaque with a diameter of 20 mm loaded by ¹⁰⁶Ru plate is attached to the COMS plaque with a diameter of 20 mm loaded by 24 of IRA1-¹⁰³Pd seeds. Dose calculations for the new plaque were performed by using the MCNP5 code. Dose calculations of dual-core plaque including ¹⁰³Pd seeds (gamma) and ¹⁰⁶Ru plate (beta) were separately done for the sake of MCNP constraints in gamma and beta particle transfer simultaneously. The new dual-core plaque delivers a much higher dose rate to the tumor compared with every single plaque, while the dose rate reached to healthy tissues is slightly higher than each plaque separately. Of course, this is acceptable because the treatment time reduces and subsequently the error in radiation therapy reduces.

Dose characteristics of Au-198 eye brachytherapy applicator: A Monte Carlo study

PURPOSE

The use of ocular plaques is a promising treatment option for eye melanoma brachytherapy. Although several studies have been done on various ocular plaques, little is known about the dose characterization of ¹⁹⁸Au plaque.

MATERIALS AND METHOD

The full mathematical model of the eye phantom, tumor, ¹⁰⁶Ru/¹⁰⁶Rh CCA, and ¹⁹⁸Au plaque were simulated using the Monte Carlo MCNPX code. The dose distribution was measured in the plaque's central axis direction, and a dose profile was also measured at a distance of 2.5 mm from the plaque surface.

RESULTS

The findings showed that ¹⁹⁸Au plaque has superior dosimetric characteristics than CCA plaque for tumors with a thickness of greater than 3.5 mm, while CCA plaque is better for tumors with a thickness of less than 3.5 mm. The dose to the sclera and choroid is higher in the case of CCA plaque, while the dose to the organs at risk (lens and optic nerve) is greater in the case of ¹⁹⁸Au applicator. In the case of ¹⁹⁸Au plaque, however, the dose to sensitive organs was within their permissible dose range.

CONCLUSION

In the treatment of medium and large tumors, ¹⁹⁸Au plaque is more successful than CCA plaque. It can produce a much more homogeneous lateral dose profile in the target. In the treatment of dome-shaped tumors, ¹⁹⁸Au plaque may be more successful than CCA plaque. As a result, the tumor's shape influences the plaque type selection.

How to design, fabricate, and validate a customized COMS-style eye plaque: Illustrated with a narrow-slotted plaque example

PURPOSE

A customized Collaborative Ocular Melanoma Study (COMS)-style eye plaque may provide superior dosimetric coverage compared with standard models for certain intraocular tumor locations and shapes. This work provides a recipe for developing and validating such customized plaques.

METHODS AND MATERIALS

The concept-into-clinical treatment process for a customized COMS-style eye plaque begins with a CAD model design that meets the specifications of the radiation oncologist and surgeon based on magnetic resonance, ultrasound, and clinical measurements, as well as a TG-43 hybrid heterogeneity-corrected dose prediction to model the dose distribution. Next, a 3D printed plastic prototype is created and reviewed. After design approval, a Modulay plaque is commercially fabricated. Quality assurance (QA) is subsequently performed to verify the physical measurements of the Modulay and Silastic and also includes dosimetric measurement of the calibration, depth dose, and dose profiles. Sterilization instructions are provided by the commercial fabricator. This customization procedure and QA methodology is demonstrated with a narrow-slotted plaque that was recently constructed for the treatment of a circumpapillary (e.g., surrounding the optic disk) ocular tumor.

RESULTS

The production of a customized COMS-style eye plaque is a multistep process. Dosimetric modeling is recommended to ensure that the design will meet the patient's needs, and QA is essential to confirm that the plaque has the proper dimensions and dose distribution. The customized narrow-slotted plaque presented herein was successfully implemented in the clinic, and provided superior dose coverage of juxtapapillary and circumpapillary tumors compared with standard or notched COMS-style plaques. Plaque development required approximately 30 h of physicist time and a fabrication cost of $1500.

CONCLUSION

Customized eye plaques may be used to treat intraocular tumors that cannot be adequately managed with standard models. The procedure by which a customized COMS-style plaque may be designed, fabricated, and validated was presented along with a clinical example.

Update of the CLRP eye plaque brachytherapy database for photon-emitting sources

PURPOSE

To update and extend the Carleton Laboratory for Radiotherapy Physics (CLRP) Eye Plaque (EP) dosimetry database for low-energy photon-emitting brachytherapy sources using egs_brachy, an open-source EGSnrc application. The previous database, CLRP_EPv1, contained datasets for the Collaborative Ocular Melanoma Study (COMS) plaques (10-22 mm diameter) with ¹⁰³ Pd or ¹²⁵ I seeds (BrachyDose-computed, 2008). The new database, CLRP_EPv2, consists of newly calculated three-dimensional (3D) dose distributions for 17 plaques [eight COMS, five Eckert & Ziegler BEBIG, and four others representative of models used worldwide] for ¹⁰³ Pd, ¹²⁵ I, and ¹³¹ Cs seeds.

ACQUISITION AND VALIDATION METHODS

Plaque models are developed with egs_brachy, based on published/manufacturer dimensions and material data. The BEBIG plaques (modeled for the first time) are identical in dimensions to COMS plaques but differ in elemental composition and/or density. Previously benchmarked seed models are used. Eye plaques and seeds are simulated at the center of full-scatter water phantoms, scoring in (0.05 cm)³ voxels spanning the eye for scenarios: (a) "HOMO": simulated TG43 conditions; (b) "HETERO": eye plaques and seeds fully modeled; (c) "HETsi" (BEBIG only): one seed is active at a time with other seed geometries present but not emitting photons (inactive); summation over all i seeds in a plaque then yields "HETsum" (includes interseed effects). For validation, doses are compared to those from CLRP_EPv1 and published data.

DATA FORMAT AND ACCESS

Data are available at https://physics.carleton.ca/clrp/eye_plaque_v2, http://doi.org/10.22215/clrp/EPv2. The data consist of 3D dose distributions (text-based EGSnrc "3ddose" file format) and graphical presentations of the comparisons to previously published data.

POTENTIAL APPLICATIONS

The CLRP_EPv2 database provides accurate reference 3D dose distributions to advance ocular brachytherapy dose evaluations. The fully-benchmarked eye plaque models will be freely distributed with egs_brachy, supporting adoption of model-based dose evaluations as recommended by TG-129, TG-186, and TG-221.

EyeDose: An open-source tool for using published Monte Carlo results to estimate the radiation dose delivered to the tumor and critical ocular structures for 125I Collaborative Ocular Melanoma Study eye plaques

PURPOSE

Radiation side effects and visual outcome for uveal melanoma patients managed with plaque radiotherapy are dependent on the radiation dose administered to the tumor and nearby healthy tissues. We have developed an open-source software tool, EyeDose, to simplify and standardize tumor and critical structure dose reporting for Collaborative Ocular Melanoma Study eye plaques.

METHODS AND MATERIALS

EyeDose is a MATLAB-based program that calculates point dose and volume dose metrics for standard models of the tumor and critical ocular structures. It uses published three-dimensional dose distributions for eye plaques, calculated with Monte Carlo methods, which are oriented with respect to the eye using the tumor's position on a fundus diagram. A standard model for the ocular structures was created using published measurements and patient CT scans. EyeDose reports radiation statistics for the fovea, optic disc, lens, lacrimal gland, retina, and tumor. The dosimetric margin for implant placement uncertainty is also calculated.

RESULTS

EyeDose calculations were validated against previously published Monte Carlo results for eight different tumor positions, including the dose to the fovea, optic disc, lacrimal gland, lens, and along the central axis. EyeDose accepts a spreadsheet input for rapidly processing large retrospective patient data sets, with an average run time of <40 s per patient. EyeDose is published as an open-source tool for easy adaptation at different institutions.

CONCLUSIONS

EyeDose calculates radiation statistics for Collaborative Ocular Melanoma Study eye plaque patients with Monte Carlo accuracy and without a treatment planning system. EyeDose streamlines data collection for large retrospective studies and can also be used prospectively to assess plaque applicability.

Model-Based Dose Calculation Algorithms for Brachytherapy Dosimetry

The purpose of this study was to review the limitations of dose calculation formalisms for photon-emitting brachytherapy sources based on the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) report and to provide recommendations to transition to model-based dose calculation algorithms. Additionally, an overview of these algorithms and approaches is presented. The influence of tissue and seed/applicator heterogeneities on brachytherapy dose distributions for breast, gynecologic, head and neck, rectum, and prostate cancers as well as eye plaques and electronic brachytherapy treatments were investigated by comparing dose calculations based on the TG-43 formalism and model-based dose calculation algorithms.

AAPM recommendations on medical physics practices for ocular plaque brachytherapy: Report of task group 221

The American Association of Physicists in Medicine (AAPM) formed Task Group 221 (TG-221) to discuss a generalized commissioning process, quality management considerations, and clinical physics practice standards for ocular plaque brachytherapy. The purpose of this report is also, in part, to aid the clinician to implement recommendations of the AAPM TG-129 report, which placed emphasis on dosimetric considerations for ocular brachytherapy applicators used in the Collaborative Ocular Melanoma Study (COMS). This report is intended to assist medical physicists in establishing a new ocular brachytherapy program and, for existing programs, in reviewing and updating clinical practices. The report scope includes photon- and beta-emitting sources and source:applicator combinations. Dosimetric studies for photon and beta sources are reviewed to summarize the salient issues and provide references for additional study. The components of an ocular plaque brachytherapy quality management program are discussed, including radiation safety considerations, source calibration methodology, applicator commissioning, imaging quality assurance tests for treatment planning, treatment planning strategies, and treatment planning system commissioning. Finally, specific guidelines for commissioning an ocular plaque brachytherapy program, clinical physics practice standards in ocular plaque brachytherapy, and other areas reflecting the need for specialized treatment planning systems, measurement phantoms, and detectors (among other topics) to support the clinical practice of ocular brachytherapy are presented. Expected future advances and developments for ocular brachytherapy are discussed.

Initial evaluation of Advanced Collapsed cone Engine dose calculations in water medium for I-125 seeds and COMS eye plaques

PURPOSE

To investigate the dose calculation accuracy in water medium of the Advanced Collapsed cone Engine (ACE) for three sizes of COMS eye plaques loaded with low-energy I-125 seeds.

METHODS

A model of the Oncura 6711 I-125 seed was created for use with ACE in Oncentra^® Brachy (OcB) using primary-scatter separated (PSS) point dose kernel and Task Group (TG) 43 datasets. COMS eye plaque models of diameters 12, 16, and 20 mm were introduced into the OcB applicator library based on 3D CAD drawings of the plaques and Silastic inserts. To perform TG-186 level 1 commissioning, treatment plans were created in OcB for a single source in water and for each COMS plaque in water for two scenarios: with only one centrally loaded seed, or with all seed positions loaded. ACE dose calculations were performed in high accuracy mode with a 0.5 × 0.5 × 0.5 mm³ calculation grid. The resulting dose data were evaluated against Monte Carlo (MC) calculated doses obtained with MCNP6, using both local and global percent differences.

RESULTS

ACE doses around the source for the single seed in water agreed with MC doses on average within < 5% inside a 6 × 6 × 6 cm³ region, and within < 1.5% inside a 2 × 2 × 2 cm³ region. The PSS data were generated at a higher resolution within 2 cm from the source, resulting in this improved agreement closer to the source due to fewer approximations in the ACE dose calculation. Average differences in both investigated plaque loading patterns in front of the plaques and on the plaque central axes were ≤ 2.5%, though larger differences (up to 12%) were found near the plaque lip.

CONCLUSIONS

Overall, good agreement was found between ACE and MC dose calculations for a single I-125 seed and in front of the COMS plaques in water. More complex scenarios need to be investigated to determine how well ACE handles heterogeneous patient materials.

Dosimetric and radiobiologic comparison of 103Pd COMS plaque brachytherapy and Gamma Knife radiosurgery for choroidal melanoma

PURPOSE

Plaque brachytherapy (BT) and Gamma Knife radiosurgery (GKRS) are highly conformal treatment options for choroidal melanoma. This study objectively compares physical dose and biologically effective dose (BED) distributions for these two modalities.

METHODS AND MATERIALS

Tumor and organ-at-risk (OAR) dose distributions from a CT-defined reference right eye were compared between ¹⁰³Pd COMS (Collaborative Ocular Melanoma Study Group) plaques delivering 70 Gy (plaque heterogeneity corrected) over 120 h to the tumor apex and GKRS plans delivering 22 Gy to the 40% isodose line for a representative sample of clinically relevant choroidal melanoma locations and sizes. Tumor and OAR biologically effective dose-volume histograms were generated using consensus radiobiologic parameters and modality-specific BED equations.

RESULTS

Published institutional prescriptive practices generally lead to larger tumor and OAR physical doses from COMS BT vs. GKRS. Radiobiologic dose conversions, however, revealed variable BEDs. Medium and large tumors receive >1.3 times higher BEDs with COMS BT vs. GKRS. OAR BEDs have even greater dependence on tumor size, location, and treatment modality. For example, COMS BT maximum BEDs to the optic nerve are lower than from GKRS for large anterior and all posterior tumors but are higher for anterior small and medium tumors.

CONCLUSIONS

BT and GKRS for choroidal melanoma have different physical dose and BED distributions with potentially unique clinical consequences. Using published institutional prescriptive practices, neither modality is uniformly favored, although COMS BT delivers higher physical doses and BEDs to tumors. These results suggest that lowering the physical prescription dose for COMS BT to more closely match the BED of GKRS might maintain equivalent tumor control with less potential morbidity.

Investigating the dosimetric impact of seed location uncertainties in Collaborative Ocular Melanoma Study-based eye plaques

PURPOSE

To quantify the dosimetric effects of random and systematic seed position uncertainties in Collaborative Ocular Melanoma Study-based eye plaques.

METHODS AND MATERIALS

An eye plaque dose calculation routine was created using Task Group 43 formalism. A variety of clinical configurations were simulated, including two seed models: (125)I and (103)Pd, three eye plaque sizes, and eight plaque/eye orientations. Dose was calculated at four ocular anatomic sites and three central axis plaque depths. Random seed positional uncertainty was modeled by adding Gaussian random displacements, in one of three seed-motion degrees of freedom, to each seed's nominal coordinate. Distributions of dosimetric outcomes were obtained and fitted after 10(6) randomizations. Similar analysis was performed for deterministic, systematic shifts of the plaque along the eye surface and radially from the globe center.

RESULTS

Random seed placement uncertainties of 0.2-mm root mean square (RMS) (amplitude) produce dose changes that are typically <4% for each degree of freedom (95% confidence interval). Systematic seed placement uncertainties are generally greater than random uncertainty 95% confidence intervals (factor of 0.72-2.15), with the relative magnitudes depending on plaque size and location of interest. Eye plaque dosimetry is most sensitive to seed movement toward the center of the eye. Dosimetric uncertainty also increases with increasing dose gradients, which are typically greatest near the inner sclera, with smaller plaques, and with lower energy radionuclides (e.g., (103)Pd).

CONCLUSIONS

Dosimetric uncertainties due to the random seed positional displacements anticipated in the clinic are expected to be <4% for each degree of freedom in most circumstances.

Monte Carlo dosimetry for 103Pd, 125I, and 131Cs ocular brachytherapy with various plaque models using an eye phantom

PURPOSE

To investigate dosimetry for ocular brachytherapy for a range of eye plaque models containing(103)Pd, (125)I, or (131)Cs seeds with model-based dose calculations.

METHODS

Five representative plaque models are developed based on a literature review and are compared to the standardized COMS plaque, including plaques consisting of a stainless steel backing and acrylic insert, and gold alloy backings with: short collimating lips and acrylic insert, no lips and silicone polymer insert, no lips and a thin acrylic layer, and individual collimating slots for each seed within the backing and no insert. Monte Carlo simulations are performed using the EGSnrc user-code BrachyDose for single and multiple seed configurations for the plaques in water and within an eye model (including nonwater media). Simulations under TG-43 assumptions are also performed, i.e., with the same seed configurations in water, neglecting interseed and plaque effects. Maximum and average doses to ocular structures as well as isodose contours are compared for simulations of each radionuclide within the plaque models.

RESULTS

The presence of the plaque affects the dose distribution substantially along the plaque axis for both single seed and multiseed simulations of each plaque design in water. Of all the plaque models, the COMS plaque generally has the largest effect on the dose distribution in water along the plaque axis. Differences between doses for single and multiple seed configurations vary between plaque models and radionuclides. Collimation is most substantial for the plaque with individual collimating slots. For plaques in the full eye model, average dose in the tumor region differs from those for the TG-43 simulations by up to 10% for(125)I and (131)Cs, and up to 17% for (103)Pd, and in the lens region by up to 29% for (125)I, 34% for (103)Pd, and 28% for (131)Cs. For the same prescription dose to the tumor apex, the lowest doses to critical ocular structures are generally delivered with plaques containing (103)Pd seeds.

CONCLUSIONS

The combined effects of ocular and plaque media on dose are significant and vary with plaque model and radionuclide, suggesting the importance of model-based dose calculations employing accurate ocular and plaque media and geometries for eye plaque brachytherapy.

Model-based dose calculations for COMS eye plaque brachytherapy using an anatomically realistic eye phantom

PURPOSE

To investigate the effects of the composition and geometry of ocular media and tissues surrounding the eye on dose distributions for COMS eye plaque brachytherapy with(125)I, (103)Pd, or (131)Cs seeds, and to investigate doses to ocular structures.

METHODS

An anatomically and compositionally realistic voxelized eye model with a medial tumor is developed based on a literature review. Mass energy absorption and attenuation coefficients for ocular media are calculated. Radiation transport and dose deposition are simulated using the EGSnrc Monte Carlo user-code BrachyDose for a fully loaded COMS eye plaque within a water phantom and our full eye model for the three radionuclides. A TG-43 simulation with the same seed configuration in a water phantom neglecting the plaque and interseed effects is also performed. The impact on dose distributions of varying tumor position, as well as tumor and surrounding tissue media is investigated. Each simulation and radionuclide is compared using isodose contours, dose volume histograms for the lens and tumor, maximum, minimum, and average doses to structures of interest, and doses to voxels of interest within the eye.

RESULTS

Mass energy absorption and attenuation coefficients of the ocular media differ from those of water by as much as 12% within the 20-30 keV photon energy range. For all radionuclides studied, average doses to the tumor and lens regions in the full eye model differ from those for the plaque in water by 8%-10% and 13%-14%, respectively; the average doses to the tumor and lens regions differ between the full eye model and the TG-43 simulation by 2%-17% and 29%-34%, respectively. Replacing the surrounding tissues in the eye model with water increases the maximum and average doses to the lens by 2% and 3%, respectively. Substituting the tumor medium in the eye model for water, soft tissue, or an alternate melanoma composition affects tumor dose compared to the default eye model simulation by up to 16%. In the full eye model simulations, the average dose to the lens is larger by 7%-9% than the dose to the center of the lens, and the maximum dose to the optic nerve is 17%-22% higher than the dose to the optic disk for all radionuclides. In general, when normalized to the same prescription dose at the tumor apex, doses delivered to all structures of interest in the full eye model are lowest for(103)Pd and highest for (131)Cs, except for the tumor where the average dose is highest for (103)Pd and lowest for (131)Cs.

CONCLUSIONS

The eye is not radiologically water-equivalent, as doses from simulations of the plaque in the full eye model differ considerably from doses for the plaque in a water phantom and from simulated TG-43 calculated doses. This demonstrates the importance of model-based dose calculations for eye plaque brachytherapy, for which accurate elemental compositions of ocular media are necessary.

Dosimetry for 131Cs and 125I seeds in solid water phantom using radiochromic EBT film

PURPOSE

To measure the 2D dose distributions with submillimeter resolution for (131)Cs (model CS-1 Rev2) and (125)I (model 6711) seeds in a Solid Water phantom using radiochromic EBT film for radial distances from 0.06cm to 5cm. To determine the TG-43 dosimetry parameters in water by applying Solid Water to liquid water correction factors generated from Monte Carlo simulations.

METHODS

Each film piece was positioned horizontally above and in close contact with a (131)Cs or (125)I seed oriented horizontally in a machined groove at the center of a Solid Water phantom, one film at a time. A total of 74 and 50 films were exposed to the (131)Cs and (125)I seeds, respectively. Different film sizes were utilized to gather data in different distance ranges. The exposure time varied according to the seed air-kerma strength and film size in order to deliver doses in the range covered by the film calibration curve. Small films were exposed for shorter times to assess the near field, while larger films were exposed for longer times in order to assess the far field. For calibration, films were exposed to either 40kV (M40) or 50kV (M50) x-rays in air at 100.0cm SSD with doses ranging from 0.2Gy to 40Gy. All experimental, calibration and background films were scanned at a 0.02cmpixel resolution using a CCD camera-based microdensitometer with a green light source. Data acquisition and scanner uniformity correction were achieved with Microd3 software. Data analysis was performed using ImageJ, FV, IDL and Excel software packages. 2D dose distributions were based on the calibration curve established for 50kV x-rays. The Solid Water to liquid water medium correction was calculated using the MCNP5 Monte Carlo code. Subsequently, the TG-43 dosimetry parameters in liquid water medium were determined.

RESULTS

Values for the dose-rate constants using EBT film were 1.069±0.036 and 0.923±0.031cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The corresponding values determined using the Monte Carlo method were 1.053±0.014 and 0.924±0.016cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The radial dose functions obtained with EBT film measurements and Monte Carlo simulations were plotted for radial distances up to 5cm, and agreed within the uncertainty of the two methods. The 2D anisotropy functions obtained with both methods also agreed within their uncertainties.

CONCLUSION

EBT film dosimetry in a Solid Water phantom is a viable method for measuring (131)Cs (model CS-1 Rev2) and (125)I (model 6711) brachytherapy seed dose distributions with submillimeter resolution. With the Solid Water to liquid water correction factors generated from Monte Carlo simulations, the measured TG-43 dosimetry parameters in liquid water for these two seed models were found to be in good agreement with those in the literature.

Monte Carlo dosimetry of the eye plaque design used at the St. Erik Eye Hospital for (125)I brachytherapy

PURPOSE

At St. Erik Eye Hospital in Stockholm, Sweden, ocular tumors of apical height above 6 mm are treated with brachytherapy, using iodine-125 seeds attached to a gold alloy plaque while the treatment planning is performed assuming homogeneous water surroundings. The aim of this work was to investigate the dose-modifying effects of the plaque and the seed fixating silicone rubber glue.

METHODS AND MATERIALS

The impact of the gold plaque and silicone rubber glue was studied with the Monte Carlo N-particle transport code, version 5.

RESULTS

For the 2 cm most proximal to the plaque surface along the plaque's central axis, the eyeball received 104.6-93.0% of the dose in all-water conditions.

CONCLUSIONS

The 0.3 mm thick layer of silicone rubber glue, used for seed fixation, attenuates photons little enough to allow characteristic X-rays from the gold alloy plaque to reach the eyeball. Close to the plaque, the dose rates were higher with the plaque and glue present, than in homogeneous water conditions. This is in contrast to what has been reported for more commonly used eye plaques, demonstrating the importance of investigating the dosimetry of individual treatment systems.

Dosimetric influence of seed spacers and end-weld thickness for permanent prostate brachytherapy

PURPOSE

The aim of this study was to analyze the dosimetric influence of conventional spacers and a cobalt chloride complex contrast (C4) agent, a novel marker for MRI that can also serve as a seed spacer, adjacent to (103)Pd, (125)I, and (131)Cs sources for permanent prostate brachytherapy.

METHODS AND MATERIALS

Monte Carlo methods for radiation transport were used to estimate the dosimetric influence of brachytherapy end-weld thicknesses and spacers near the three sources. Single-source assessments and volumetric conditions simulating prior patient treatments were computed. Volume-dose distributions were imported to a treatment planning system for dose-volume histogram analyses.

RESULTS

Single-source assessment revealed that brachytherapy spacers primarily attenuated the dose distribution along the source long axis. The magnitude of the attenuation at 1 cm on the long axis ranged from -10% to -5% for conventional spacers and approximately -2% for C4 spacers, with the largest attenuation for (103)Pd. Spacer perturbation of dose distributions was less than manufacturing tolerances for brachytherapy sources as gleaned by an analysis of end-weld thicknesses. Volumetric Monte Carlo assessment demonstrated that TG-43 techniques overestimated calculated doses by approximately 2%. Specific dose-volume histogram metrics for prostate implants were not perturbed by inclusion of conventional or C4 spacers in clinical models.

CONCLUSIONS

Dosimetric perturbations of single-seed dose distributions by brachytherapy spacers exceeded 10% along the source long axes adjacent to the spacers. However, no dosimetric impact on volumetric parameters was noted for brachytherapy spacers adjacent to (103)Pd, (125)I, or (131)Cs sources in the context of permanent prostate brachytherapy implants.

Dosimetric characteristics of three new design 125I brachytherapy sources

For roughly 25 years, 125I sources have been used in the treatment of various malignant diseases such as prostate cancer. Three new brachytherapy sources, IR01-125I, IR02-125I and IR03-125I, have been developed and are designed for permanent implant application. The Monte Carlo radiation transport code version MCNP 5 was used to calculate the dosimetry parameters around the sources in accordance with the updated report of the American Association of Physicists in Medicine (AAPM), Task Group No. 43. For each source, the dose rate constant Λ, the radial dose function gL(r), and the anisotropy function F(r, θ), were obtained. The results indicated a dose rate constant of 0.932 ± 0.01, 0.934 ± 0.01 and 0.939 ± 0.01 Gy h−1 U−1 for the IR01-125I, IR02-125I and IR03-125I sources respectively. With the goal of determining an optimal design for a 125I source, each seed's parameters were compared with other seeds. In this study, the optimal source IR03-125I provides the most isotropic dose distribution in water. Finally, the results for optimal source were compared with published results for those of other commercial sources.

Keeping an eye on the ring: COMS plaque loading optimization for improved dose conformity and homogeneity

PURPOSE

To improve tumor dose conformity and homogeneity for COMS plaque brachytherapy by investigating the dosimetric effects of varying component source ring radionuclides and source strengths.

MATERIAL AND METHODS

The MCNP5 Monte Carlo (MC) radiation transport code was used to simulate plaque heterogeneity-corrected dose distributions for individually-activated source rings of 14, 16 and 18 mm diameter COMS plaques, populated with (103)Pd, (125)I and (131)Cs sources. Ellipsoidal tumors were contoured for each plaque size and MATLAB programming was developed to generate tumor dose distributions for all possible ring weighting and radionuclide permutations for a given plaque size and source strength resolution, assuming a 75 Gy apical prescription dose. These dose distributions were analyzed for conformity and homogeneity and compared to reference dose distributions from uniformly-loaded (125)I plaques. The most conformal and homogeneous dose distributions were reproduced within a reference eye environment to assess organ-at-risk (OAR) doses in the Pinnacle(3) treatment planning system (TPS). The gamma-index analysis method was used to quantitatively compare MC and TPS-generated dose distributions.

RESULTS

Concentrating > 97% of the total source strength in a single or pair of central (103)Pd seeds produced the most conformal dose distributions, with tumor basal doses a factor of 2-3 higher and OAR doses a factor of 2-3 lower than those of corresponding uniformly-loaded (125)I plaques. Concentrating 82-86% of the total source strength in peripherally-loaded (131)Cs seeds produced the most homogeneous dose distributions, with tumor basal doses 17-25% lower and OAR doses typically 20% higher than those of corresponding uniformly-loaded (125)I plaques. Gamma-index analysis found > 99% agreement between MC and TPS dose distributions.

CONCLUSIONS

A method was developed to select intra-plaque ring radionuclide compositions and source strengths to deliver more conformal and homogeneous tumor dose distributions than uniformly-loaded (125)I plaques. This method may support coordinated investigations of an appropriate clinical target for eye plaque brachytherapy.

Dosimetry of (125)I and (103)Pd 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.

Experimental measurements and Monte Carlo calculations for (103)Pd 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-(103)Pd 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 6.82U/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.

Thermoluminescent and Monte Carlo dosimetry of IR06-103Pd brachytherapy source

This work presents experimental dosimetry results for a new P103d brachytherapy seed, in accordance with the AAPM TG‐43U1 recommendation that all new low‐energy interstitial brachytherapy seeds should undergo one Monte Carlo (MC) and at least one experimental dosimetry characterization. Measurements were performed using TLD‐GR200A circular chip dosimeters using standard methods employing thermoluminescent dosimeters in a Perspex phantom. The Monte Carlo N‐particle (MCNP) code, version 5 was used to evaluate the dose‐rate distributions around this model P103d source in water and Perspex phantoms. The consensus value for dose‐rate constant of the IR06‐P103d source was found equal to 0.690 cG⋅h−1⋅U−1. The anisotropy function, F(r, θ), and the radial dose function, gL(r), of the seed were measured in Perspex phantom and calculated in both Perspex and liquid water phantom. The measured values were also found in good agreement with corresponding MC calculations.

PACS number: 87.53.Jw

Dosimetric characteristic of a new 125I brachytherapy source

A new brachytherapy (125)I source has been investigated at Iranian Agricultural, Medical and Industrial Research School. Dosimetric characteristics [dose-rate constant Λ, radial dose function g(l)(r) and anisotropy function F(r,)] of IRA-(125)I were theoretically determined in terms of the updated AAPM task group 43 (TG-43U1) recommendations. Versions 5 and 4C of the Monte Carlo radiation transport code were used to calculate the dosimetry parameters around the source. The Monte Carlo calculated dose-rate constant of the (125)I source in water was found to be 92×10(-4) Gy h(-1) U(-1) with an approximate uncertainty of ±3 %. Brachytherapy seed model, 6711-(125)I, carrying (125)I radionuclides, was modelled and benchmarked against previously published values. Finally, the calculated results were compared with the published results of those of other source manufacturers.

Comparison of dose calculation methods for brachytherapy of intraocular tumors

PURPOSE

To investigate dosimetric differences among several clinical treatment planning systems (TPS) and Monte Carlo (MC) codes for brachytherapy of intraocular tumors using 125I or 103Pd plaques, and to evaluate the impact on the prescription dose of the adoption of MC codes and certain versions of a TPS (Plaque Simulator with optional modules).

METHODS

Three clinical brachytherapy TPS capable of intraocular brachytherapy treatment planning and two MC codes were compared. The TPS investigated were Pinnacle v8.0dp1, BrachyVision v8.1, and Plaque Simulator v5.3.9, all of which use the AAPM TG-43 formalism in water. The Plaque Simulator software can also handle some correction factors from MC simulations. The MC codes used are MCNP5 v1.40 and BrachyDose/EGSnrc. Using these TPS and MC codes, three types of calculations were performed: homogeneous medium with point sources (for the TPS only, using the 1D TG-43 dose calculation formalism); homogeneous medium with line sources (TPS with 2D TG-43 dose calculation formalism and MC codes); and plaque heterogeneity-corrected line sources (Plaque Simulator with modified 2D TG-43 dose calculation formalism and MC codes). Comparisons were made of doses calculated at points-of-interest on the plaque central-axis and at off-axis points of clinical interest within a standardized model of the right eye.

RESULTS

For the homogeneous water medium case, agreement was within approximately 2% for the point- and line-source models when comparing between TPS and between TPS and MC codes, respectively. For the heterogeneous medium case, dose differences (as calculated using the MC codes and Plaque Simulator) differ by up to 37% on the central-axis in comparison to the homogeneous water calculations. A prescription dose of 85 Gy at 5 mm depth based on calculations in a homogeneous medium delivers 76 Gy and 67 Gy for specific 125I and 103Pd sources, respectively, when accounting for COMS-plaque heterogeneities. For off-axis points-of-interest, dose differences approached factors of 7 and 12 at some positions for 125I and 103Pd, respectively. There was good agreement (approximately 3%) among MC codes and Plaque Simulator results when appropriate parameters calculated using MC codes were input into Plaque Simulator. Plaque Simulator and MC users are perhaps at risk of overdosing patients up to 20% if heterogeneity corrections are used and the prescribed dose is not modified appropriately.

CONCLUSIONS

Agreement within 2% was observed among conventional brachytherapy TPS and MC codes for intraocular brachytherapy dose calculations in a homogeneous water environment. In general, the magnitude of dose errors incurred by ignoring the effect of the plaque backing and Silastic insert (i.e., by using the TG-43 approach) increased with distance from the plaque's central-axis. Considering the presence of material heterogeneities in a typical eye plaque, the best method in this study for dose calculations is a verified MC simulation.

Monte Carlo calculation of dosimetry parameters for the IR08-103Pd brachytherapy source

PURPOSE

For the treatment of some cancerous tumors using brachytherapy methods and low-energy photon sources, such as 125I and 103Pd, the American Association of Physicists in Medicine Task Group No. 43U1 report recommends that the dosimetric parameters of a new brachytherapy source must be determined in two experimental and Monte Carlo theoretical methods before using each new source clinically. This study presents the results of Monte Carlo calculations of the dosimetric parameters for IR08-103Pd brachytherapy source design. IR08-103Pd seed has been manufactured at the Agricultural, Medical and Industrial Research School.

METHODS

Version 5 of the (MCNP) Monte Carlo radiation transport code was used to calculate the dosimetry parameters around the source. Three geometric models of the seed, based on different locations of beads inside the titanium capsule, were simulated. The seed contains five resin beads of 0.6 mm diameter having 103Pd uniformly absorbed in the bead volume, which were contained within a cylindrical titanium capsule having 0.8 mm outside diameter and 4.8 mm length.

RESULTS

The Monte Carlo calculated dose rate constant of the IR08-103Pd seed was found to be 0.695 +/- 0.021 cGyU(-1) h(-1). Also in this study, the geometry function G(r, theta), line and point-source radial dose functions gL(r) and gP(r), and the anisotropy function F(r, theta), have been calculated at distances from 0.25 to 7 cm. The results of these calculations have been compared with measured values for an actual IR08-103Pd seed.

CONCLUSIONS

There are no statistical significant dosimetric differences among the three seed orientations in this study (i.e., ideal, vertical, and diagonal). However, the observed differences between the calculated and measured values could be explained by the measurement uncertainty and the configuration of the resin beads within the capsule and capsule orientation.