Effect of tissue composition on dose distribution in brachytherapy with various photon emitting sources

Effective atomic number and photon buildup factor of bismuth doped tissue for photon and particles beam interaction

Introduction: The doping of high Z nanoparticles into the tumor tissue increases the therapeutic efficiency of radiotherapy called nanoparticle enhanced radiotherapy (NERT). In the present study, we are identifying the effective types of radiation and effective doping concentration of bismuth radiosensitizer for NERT application by analyzing effective atomic number (Zeff) and photon buildup factor (PBF) of bismuth (Bi) doped soft tissue for the photon, electron, proton, alpha particle, and carbon ion interactions.

Material and methods: The direct method was used for the calculation of Zeff for photon and electron beams (10 keV-30 MeV). The phy-X/ZeXTRa software was utilized for the particle beams such as proton, alpha particle, and carbon ions (1-15 MeV). Bismuth doping concentrations of 5, 10, 15, 20, 25 and 30 mg/g were considered. The PBF was calculated over 15 keV-15 MeV energies using phy-X/PSD software.

Results: The low energy photon (<100 keV) interaction with a higher concentration of Bi dopped tissue gives the higher values of Zeff. The Zeff increased with the doping concentration of bismuth for all types of radiation. The Zeff was dependent on the type of radiation, the energy of radiation, and the concentration of Bi doping. The particle beams such as electron, proton, alpha particle, and carbon ion interaction gives the less values of Zeff has compared to photon beam interaction. On the other hand, the photon buildup factor values were decreased while increasing the Bi doping concentration.

Conclusions: According to Zeff and PBF, the low energy photon and higher concentration of radiosensitizer are the most effective for nanoparticle enhanced radiotherapy application. Based on the calculated values of Zeff, the particle beams such as electron, proton, alpha particle, and carbon ions were less effective for NERT application. The presented values of Zeff and PBF are useful for the radiation dosimetry in NERT.

A Monte Carlo investigation of the dose distribution for new I-125 Low Dose Rate brachytherapy source in water and in different media

Permanent and temporary implantation of I-125 brachytherapy sources has become an official method for the treatment of different cancers. In this technique, it is essential to determine dose distribution around the brachytherapy source to choose the optimal treatment plan. In this study, the dosimetric parameters for a new interstitial brachytherapy source I-125 (IrSeed-125) were calculated with GATE/GEANT4 Monte Carlo code. Dose rate constant, radial dose function and 2D anisotropy function were calculated inside a water phantom (based on the recommendations of TG-43U1 protocol), and inside several tissue phantoms around the IrSeed-125 capsule. Acquired results were compared with MCNP simulation and experimental data. The dose rate constant of IrSeed-125 in the water phantom was about 1.038 cGy·h−1U−1 that shows good consistency with the experimental data. The radial dose function at 0.5, 0.9, 1.8, 3 and 7 cm radial distances were obtained as 1.095, 1.019, 0.826, 0.605, and 0.188, respectively. The results of the IrSeed-125 is not only in good agreement with those calculated by other simulation with MCNP code but also are closer to the experimental results. Discrepancies in the estimation of dose around IrSeed-125 capsule in the muscle and fat tissue phantoms are greater than the breast and lung phantoms in comparison with the water phantom. Results show that GATE/GEANT4 Monte Carlo code produces accurate results for dosimetric parameters of the IrSeed-125 LDR brachytherapy source with choosing the appropriate physics list. There are some differences in the dose calculation in the tissue phantoms in comparison with water phantom, especially in long distances from the source center, which may cause errors in the estimation of dose around brachytherapy sources that are not taken account by the TG43-U1 formalism.

Monte Carlo dosimetry for a new 32P brachytherapy source using FLUKA code

Purpose

Dosimetric characterization of a new 32P brachytherapy source was studied and the validity of the FLUKA code to reproduce the dosimetric parameters in a water phantom was evaluated. In addition, dose rate distributions around the 32P source sheathed by a catheter and unsheathed source were investigated in different tissue phantoms.

Material and methods

The new 32P source was modeled using FLUKA Monte Carlo code. According to the AAPM TG-60 recommendations, reference of absorbed dose rate, radial dose function, anisotropy function, and an away-along table for quality assurance purposes inside water phantom were calculated. Moreover, the results of the radial dose function and dose rate were obtained for the sheathed source and unsheathed sources at radial distances in different tissue phantoms: liver, fat tissue, 9-component soft tissue, and 4-component soft tissue.

Results

The calculated dosimetric parameters of the new 32P source by FLUKA code in water phantom agreed well with that of the GEANT4 calculation. The 2D away-along dose results were similar to the GEANT4 simulation for distances less than 0.25 cm, small differences were apparent at long distances from the source. Dose rate evaluation for the sheathed source shows that the presence of a catheter increases the dose values up to 2.11% in comparison with the unsheathed source in water phantom. Our results show that the radial dose function calculated in water, as generalized by AAPM TG-60, differed in tissue, especially at large distances from the source.

Conclusions

This work fully characterizes dosimetric parameters of the sheathed and unsheathed new 32P brachytherapy sources in water and different tissue phantoms by using FLUKA code. The results demonstrate that the dose distribution in water differed from the calculated ones in tissue phantoms due to the densities and atomic composition for tissues that are not taken account by the TG-60 formalism.

HDR brachytherapy in vivo source position verification using a 2D diode array: A Monte Carlo study

PURPOSE

This study aims to assess the accuracy of source position verification during high-dose rate (HDR) prostate brachytherapy using a novel, in-house developed two-dimensional (2D) diode array (the Magic Plate), embedded exactly below the patient within a carbon fiber couch. The effect of tissue inhomogeneities on source localization accuracy is examined.

METHOD

Monte Carlo (MC) simulations of 12 source positions from a HDR prostate brachytherapy treatment were performed using the Geant4 toolkit. An Ir-192 Flexisource (Isodose Control, Veenendaal, the Netherlands) was simulated inside a voxelized patient geometry, and the dose deposited in each detector of the Magic Plate evaluated. The dose deposited in each detector was then used to localize the source position using a proprietary reconstruction algorithm.

RESULTS

The accuracy of source position verification using the Magic Plate embedded in the patient couch was found to be affected by the tissue inhomogeneities within the patient, with an average difference of 2.1 ± 0.8 mm (k = 1) between the Magic Plate predicted and known source positions. Recalculation of the simulations with all voxels assigned a density of water improved this verification accuracy to within 1 mm.

CONCLUSION

Source position verification using the Magic Plate during a HDR prostate brachytherapy treatment was examined using MC simulations. In a homogenous geometry (water), the Magic Plate was able to localize the source to within 1 mm, however, the verification accuracy was negatively affected by inhomogeneities; this can be corrected for by using density information obtained from CT, making the proposed tool attractive for use as a real-time in vivo quality assurance (QA) device in HDR brachytherapy for prostate cancer.

Tissue composition effect on dose distribution in neutron brachytherapy/neutron capture therapy

AIM

The aim of this study is to assess the effect of the compositions of various soft tissues and tissue-equivalent materials on dose distribution in neutron brachytherapy/neutron capture therapy.

BACKGROUND

Neutron brachytherapy and neutron capture therapy are two common radiotherapy modalities.

MATERIALS AND METHODS

Dose distributions were calculated around a low dose rate (252)Cf source located in a spherical phantom with radius of 20.0 cm using the MCNPX code for seven soft tissues and three tissue-equivalent materials. Relative total dose rate, relative neutron dose rate, total dose rate, and neutron dose rate were calculated for each material. These values were determined at various radial distances ranging from 0.3 to 15.0 cm from the source.

RESULTS

Among the soft tissues and tissue-equivalent materials studied, adipose tissue and plexiglass demonstrated the greatest differences for total dose rate compared to 9-component soft tissue. The difference in dose rate with respect to 9-component soft tissue varied with compositions of the materials and the radial distance from the source. Furthermore, the total dose rate in water was different from that in 9-component soft tissue.

CONCLUSION

Taking the same composition for various soft tissues and tissue-equivalent media can lead to error in treatment planning in neutron brachytherapy/neutron capture therapy. Since the International Commission on Radiation Units and Measurements (ICRU) recommends that the total dosimetric uncertainty in dose delivery in radiotherapy should be within ±5%, the compositions of various soft tissues and tissue-equivalent materials should be considered in dose calculation and treatment planning in neutron brachytherapy/neutron capture therapy.

A simple analytical method for heterogeneity corrections in low dose rate prostate brachytherapy

In low energy brachytherapy, the presence of tissue heterogeneities contributes significantly to the discrepancies observed between treatment plan and delivered dose. In this work, we present a simplified analytical dose calculation algorithm for heterogeneous tissue. We compare it with Monte Carlo computations and assess its suitability for integration in clinical treatment planning systems. The algorithm, named as RayStretch, is based on the classic equivalent path length method and TG-43 reference data. Analytical and Monte Carlo dose calculations using Penelope2008 are compared for a benchmark case: a prostate patient with calcifications. The results show a remarkable agreement between simulation and algorithm, the latter having, in addition, a high calculation speed. The proposed analytical model is compatible with clinical real-time treatment planning systems based on TG-43 consensus datasets for improving dose calculation and treatment quality in heterogeneous tissue. Moreover, the algorithm is applicable for any type of heterogeneities.

Effect of tissue composition on dose distribution in electron beam radiotherapy

OBJECTIVE

The aim of this study is to evaluate the effect of tissue composition on dose distribution in electron beam radiotherapy.

METHODS

A Siemens Primus linear accelerator and a phantom were simulated using MCNPX Monte Carlo code. In a homogeneous cylindrical phantom, six types of soft tissue and three types of tissue-equivalent materials were investigated. The tissues included muscle (skeletal), adipose tissue, blood (whole), breast tissue, soft tissue (9-components) and soft tissue (4-component). The tissue-equivalent materials were water, A-150 tissue-equivalent plastic and perspex. Electron dose relative to dose in 9-component soft tissue at various depths on the beam's central axis was determined for 8, 12, and 14 MeV electron energies.

RESULTS

The results of relative electron dose in various materials relative to dose in 9-component soft tissue were reported for 8, 12 and 14 MeV electron beams as tabulated data. While differences were observed between dose distributions in various soft tissues and tissue-equivalent materials, which vary with the composition of material, electron energy and depth in phantom, they can be ignored due to the incorporated uncertainties in Monte Carlo calculations.

CONCLUSION

Based on the calculations performed, differences in dose distributions in various soft tissues and tissue-equivalent materials are not significant. However, due to the difference in composition of various materials, further research in this field with lower uncertainties is recommended.

A Monte Carlo study on dose distribution evaluation of Flexisource (192)Ir brachytherapy source

AIM

The aim of this study is to evaluate the dose distribution of the Flexisource (192)Ir source.

BACKGROUND

Dosimetric evaluation of brachytherapy sources is recommended by task group number 43 (TG. 43) of American Association of Physicists in Medicine (AAPM).

MATERIALS AND METHODS

MCNPX code was used to simulate Flexisource (192)Ir source. Dose rate constant and radial dose function were obtained for water and soft tissue phantoms and compared with previous data on this source. Furthermore, dose rate along the transverse axis was obtained by simulation of the Flexisource and a point source and the obtained data were compared with those from Flexiplan treatment planning system (TPS).

RESULTS

The values of dose rate constant obtained for water and soft tissue phantoms were equal to 1.108 and 1.106, respectively. The values of the radial dose function are listed in the form of tabulated data. The values of dose rate (cGy/s) obtained are shown in the form of tabulated data and figures. The maximum difference between TPS and Monte Carlo (MC) dose rate values was 11% in a water phantom at 6.0 cm from the source.

CONCLUSION

Based on dosimetric parameter comparisons with values previously published, the accuracy of our simulation of Flexisource (192)Ir was verified. The results of dose rate constant and radial dose function in water and soft tissue phantoms were the same for Flexisource and point sources. For Flexisource (192)Ir source, the results of TPS calculations in a water phantom were in agreement with the simulations within the calculation uncertainties. Furthermore, the results from the TPS calculation for Flexisource and MC calculation for a point source were practically equal within the calculation uncertainties.