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

Monte Carlo-based dosimetry of proposed bi-radionuclide (125I and 106Ru/106Rh) eye plaque: A feasibility study

BACKGROUND

Combining the sharp dose fall off feature of beta-emitting 106Ru/106Rh radionuclide with larger penetration depth feature of photon-emitting125I radionuclide in a bi-radionuclide plaque, prescribed dose to the tumor apex can be delivered while maintaining the tumor dose uniformity and sparing the organs at risk. The potential advantages of bi-radionuclide plaque could be of interest in context of ocular brachytherapy.

PURPOSE

The aim of the study is to evaluate the dosimetric advantages of a proposed bi-radionuclide plaque for two different designs, consisting of indigenous 125I seeds and 106Ru/106Rh plaque, using Monte Carlo technique. The study also explores the influence of other commercial 125I seed models and presence or absence of silastic/acrylic seed carrier on the calculated dose distributions. The study further included the calculation of depth dose distributions for the bi-radionuclide eye plaque for which experimental data are available.

METHODS

The proposed bi-radionuclide plaque consists of a 1.2-mm-thick silver (Ag) spherical shell with radius of curvature of 12.5 mm, 20 µm-thick-106Ru/106Rh encapsulated between 0.2 mm Ag disk, and a 0.1-mm-thick Ag window, and water-equivalent gel containing 12 symmetrically arranged 125I seeds. Two bi-radionuclide plaque models investigated in the present study are designated as Design I and Design II. In Design I, 125I seeds are placed on the top of the plaque, while in Design II 106Ru/106Rh source is positioned on the top of the plaque. In Monte Carlo calculations, the plaque is positioned in a spherical water phantom of 30 cm diameter.

RESULTS

The proposed bi-radionuclide eye plaque demonstrated superior dose distributions as compared to 125I or 106Ru plaque for tumor thicknesses ranges from 5 to 10 mm. Amongst the designs, dose at a given voxel for Design I is higher as compared to the corresponding voxel dose for Design II. This difference is attributed to the higher degree of attenuation of 125I photons in Ag as compared to beta particles. Influence of different 125I seed models on the normalized lateral dose profiles of Design I (in the absence of carrier) is negligible and within 5% on the central axis depth dose distribution as compared to the corresponding values of the plaque that has indigenous 125I seeds. In the presence of a silastic/acrylic seed carrier, the normalized central axis dose distributions of Design I are smaller by 3%-12% as compared to the corresponding values in the absence of a seed carrier. For the published bi-radionuclide plaque model, good agreement is observed between the Monte Carlo-calculated and published measured depth dose distributions for clinically relevant depths.

CONCLUSION

Regardless of the type of 125I seed model utilized and whether silastic/acrylic seed carrier is present or not, Design I bi-radionuclide plaque offers superior dose distributions in terms of tumor dose uniformity, rapid dose fall off and lesser dose to nearby critical organs at risk over the Design II plaque. This shows that Design I bi-radionuclide plaque could be a promising alternative to 125I plaque for treatment of tumor sizes in the range 5 to 10 mm.

Validation of the TOPAS Monte Carlo toolkit for LDR brachytherapy simulations

PURPOSE

This work has the purpose of validating the Monte Carlo toolkit TOol for PArticle Simulation (TOPAS) for low-dose-rate (LDR) brachytherapy uses.

METHODS AND MATERIALS

Simulations of 12 LDR sources and 2 COMS eye plaques (10 mm and 20 mm in diameter) and comparisons with published reference data from the Carleton Laboratory for Radiotherapy Physics (CLRP), the TG-43 consensus data and the TG-129 consensus data were performed. Sources from the IROC Houston Source Registry were modeled. The OncoSeed 6711 and the SelectSeed 130.002 were also modeled for historical reasons. For each source, the dose rate constant, the radial dose function and the anisotropy functions at 0.5, 1 and 5 cm were extracted. For the eye plaques (loaded with ¹²⁵I sources), dose distribution maps, dose profiles along the central axis and transverse axis were calculated.

RESULTS

Dose rate constants for 11 of the 12 sources are within 4% of the consensus data and within 2% of the CLRP data. The radial dose functions and anisotropy functions are mostly within 2% of the CLRP data. In average, 92% of all voxels are within 1% of the CLRP data for the eye plaques dose distributions. The dose profiles are within 0.5% (central axis) and 1% (transverse axis) of the reference data.

CONCLUSION

The TOPAS MC toolkit was validated for LDR brachytherapy applications. Single-seed and multi-seed results agree with the published reference data. TOPAS has several benefits such as a simplified approach to MC simulations and an accessible brachytherapy package including comprehensive learning resources.

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.