Interseed Attenuation Research Articles

Automatic planning for head and neck seed implant brachytherapy based on deep convolutional neural network dose engine.

Seed implant brachytherapy (SIBT) is an effective treatment modality for head and neck (H&N) cancers; however, current clinical planning requires manual setting of needle paths and utilizes inaccurate dose calculation algorithms. This study aims to develop an accurate and efficient deep convolutional neural network dose engine (DCNN-DE) and an automatic SIBT planning method for H&N SIBT. A cohort of 25 H&N patients who received SIBT was utilized to develop and validate the methods. The DCNN-DE was developed based on 3D-unet model. It takes single seed dose distribution from a modified TG-43 method, the CT image and a novel inter-seed shadow map (ISSM) as inputs, and predicts the dose map of accuracy close to the one from Monte Carlo simulations (MCS). The ISSM was proposed to better handle inter-seed attenuation. The accuracy and efficacy of the DCNN-DE were validated by comparing with other methods taking MCS dose as reference. For SIBT planning, a novel strategy inspired by clinical practice was proposed to automatically generate parallel or non-parallel potential needle paths that avoid puncturing bone and critical organs. A heuristic-based optimization method was developed to optimize the seed positions to meet clinical prescription requirements. The proposed planning method was validated by re-planning the 25 cases and comparing with clinical plans. The absolute percentage error in the TG-43 calculation for CTV V100 and D90 was reduced from 5.4% and 13.2% to 0.4% and 1.1% with DCNN-DE, an accuracy improvement of 93% and 92%, respectively. The proposed planning method could automatically obtain a plan in 2.5±1.5 min. The generated plans were judged clinically acceptable with dose distribution comparable with those of the clinical plans. The proposed method can generate clinically acceptable plans quickly with high accuracy in dose evaluation, and thus has a high potential for clinical use in SIBT.

DICOM CT-based Monte Carlo dose calculations for prostate permanent implant using 125I seed sources

Dose distributions are calculated for a clinical prostate implant consisting of multiple 125I sources using Monte Carlo-based egs_brachy code. A voxelised heterogeneous CT phantom, generated from DICOM CT images is used. Dose distributions vary significantly with scoring and transport media. About 3% interseed attenuation and 12% dose differences between prostate tissue and water are observed at D90. Monte Carlo-calculated dose values are smaller as compared to the TG-43 formalism-based TPS-generated dose values at a given location. Hence, patient-specific model-based dose calculation algorithm is preferred over TG-43 formalism-based TPS.

Haralick texture feature analysis for characterization of specific energy and absorbed dose distributions across cellular to patient length scales

Objective. To investigate an approach for quantitative characterization of the spatial distribution of dosimetric data by introducing Haralick texture feature analysis in this context. Approach. Monte Carlo simulations are used to generate 3D arrays of dosimetric data for 2 scenarios: (1) cell-scale microdosimetry: specific energy (energy imparted per unit mass) in cell-scale targets irradiated by photon spectra (125I, 192Ir, 6 MV); (2) tumour-scale dosimetry: absorbed dose in voxels for idealized models of 125I permanent implant prostate brachytherapy, considering ‘TG186’ (realistic tissues including 0% to 5% intraprostatic calcifications; interseed attenuation) and ‘TG43’ (water model, no interseed attenuation) conditions. Five prominent Haralick features (homogeneity, contrast, correlation, local homogeneity, entropy) are computed and trends are interpreted using fundamental radiation physics. Main results. In the cell-scale scenario, the Haralick measures quantify differences in 3D specific energy distributions due to source spectra. For example, contrast and entropy are highest for 125I reflecting the large variations in specific energy in adjacent voxels (photoelectric interactions; relatively short range of electrons), while 6 MV has the highest homogeneity with smaller variations in specific energy between voxels (Compton scattering dominates; longer range of electrons). For the tumour-scale scenario, the Haralick measures quantify differences due to TG186/TG43 simulation conditions and the presence of calcifications. For example, as calcifications increase from 0% to 5%, contrast increases while correlation decreases, reflecting the large differences in absorbed dose in adjacent voxels (higher absorbed dose in voxels with calcification due to photoelectric interactions). Significance. Haralick texture analysis provides a quantitative method for the characterization of 3D dosimetric distributions across cellular to tumour length scales, with promising future applications including analyses of multiscale tissue models, patient-specific data, and comparison of treatment approaches.

Comparing the inter-seed effect for some iodine-125 brachytherapy sources through a Monte Carlo simulation approach

Background and ObjectiveOne of the principal deficits of TG-43(U1) dosimetry protocol is ignoring the attenuation effect of each seed during the multiseed brachytherapy implants. To take into account this shadowing effect, a parameter known as the inter-seed effect (ISE) has been introduced. Current study aims to evaluate and compare the ISE for some I-125 brachytherapy seeds through a Monte Carlo (MC) simulation approach. MethodsFive different models of I-125 seeds including 6711, ProstaSeed, STM 1251, 3500, and IAI-125A were simulated by MCNPX MC Code. Validity of simulated seed models was confirmed through comparing the corresponding dosimetric parameters such as radial dose function and anisotropy function with those reported by relevant literature. Then, the ISE for each seed at different distances was determined in three seeds implant configuration. Additionally, the relevant attenuation factors (AF) were also determined for each brachytherapy source. ResultsThe obtained results demonstrated that minimum inter-seed attenuation belongs to the ProstaSeed. In return, the maximum inter-seed attenuation was found for 3500 brachytherapy seed. The mean attenuation factor for 6711, ProstaSeed, STM 1251, 3500, and IAI-125A seed models was equal to 0.764, 0.829, 0.785, 0.594, and 0.772, respectively. ConclusionsIt can be concluded that the considered I-125 brachytherapy seeds show different inter-seed effects and mean AF values for the same multiseed implant arrangement. This finding can be attributed to discrepancies in internal seed design and configuration.

Developing A Method for Inter-Seed Effect Correction in 125I Interstitial Brachytherapy Using Artificial Neural Network

Introduction: Treatment planning systems use TG-43 dose calculation protocol for brachytherapy sources. Dose calculations based on TG-43 formalism do not correct the perturbations due to the presence of tissue inhomogeneity, applicators, and inter-seed effects. Inter-seed attenuation has an important effect on dosimetry in permanent implant brachytherapy. The aim of this study is to evaluate the inter-seed attenuation effect for I-125 permanent implants. Then, a software was developed to find the real dose distribution for different combinations of sources. Materials and methods: In the first step, a hypothetical generic source model was designed based on the configurations of different commercial source types. MCNP5 Monte Carlo code was utilized to simulate the single active generic source at the center of the phantom, and an inactive placed at various positions inside the phantom. An algorithm was introduced using artificial neural network models that can estimate the dose distribution in presence of inactive sources. Results: The Monte Carlo calculation results showed that the dose distribution is clearly affected by the inter-seed attenuation effect. Comparison of the artificial neural network results with the Monte Carlo simulation results show that the artificial neural networks can predict the inter-seed attenuation with acceptable accuracy, and with not much greater than 3% deviation.Discussion and conclusions: Inter-seed effect is dependent on the distance between the seeds. Decreasing distances would cause more effect. According to the results, it seems that the artificial neural network can be used as a tool for correction of inter-seed attenuation effect in treatment planning systems.

Monte Carlo Insight into Inter-Seed Attenuation (ISA) in Salvage Brachytherapy of Post Permanent Prostate Implant (PPI)

Monte Carlo Insight into Inter-Seed Attenuation (ISA) in Salvage Brachytherapy of Post Permanent Prostate Implant (PPI)

Real Time Modelling of The Dose Distribution Adapted to The Present Treatment Requirements (TG186)

The current approach for brachytherapy usually relies on superposition of single sources in a homogeneous liquid water phantom. This technique is fast and practical in the clinical context but not adapted to the present treatment requirements (TG186[a]) The influence of tissue heterogeneities, inter-seed attenuation and finite patient dimensions, have to be considered. A full Monte-Carlo (MC) code allows computing the Dose Distribution (DD), with a high precision, but is usually too time consuming to be routinely used in clinical context. This work proposes a new method for accurately calculating the DD within a time compatible with clinical requirements.

Fast 3D Modeling of Dose Distribution in Brachytherapy Adapted to Recommendations of International Organizations of Medical Practice

The current approach for brachytherapy usually relies on superposition of single sources in a homogeneous liquid water phantom. This technique is fast and practical in the clinical context but not adapted to the present treatment requirements [Beaulieu et al, TG186, Med Phys. 39, 6208 (2012)]. The influence of tissue heterogeneities, inter-seed attenuation and finite patient dimensions, have to be considered. A full Monte-Carlo (MC) code allows computing the Dose Distribution (DD), with a high precision, but is too time consuming to be routinely used in clinical context. This work proposes a new method for accurately calculating the DD within a time compatible with clinical requirements. Our code solves the three dimensional linear Boltzmann transport equation (LBTE). The model is based on a multi-group energy approach combined with a specific angular momentum closure, deduced from the principle of entropy minimization. The code has been recently applied to external radio-therapy and results suggest that the method is as accurate as Monte-Carlo code for the considered test cases [1]. The transport, attenuation and scattering are based on microscopic cross sections in the energy range from a few keV up to Mev’s. All particles, electrons, positrons, photons, primary or secondary are treated simultaneously and computations are performed in full three-dimensional geometry. Our deterministic method differs from existing commercial software [2] by the phase space discretization and the LBTE solving. Our method has been tested by using water phantoms with tissues and density/material inserts. The DD’s computed with our deterministic model have been compared with full three-dimensional converging MC simulations. We demonstrated that the model is capable of reproducing the DD’s with accuracy of a few percent compared to the reference MC calculations. The deterministic model is fast and hence it may be a viable candidate for a future Treatment Planning System. Limitations and possible improvements of the deterministic method are presented. Photon energies are relatively low in brachytherapy and the contribution of scattered photons to the total dose can be of the same order of magnitude as the primary dose, particularly in the presence of inhomogeneities. In addition, for these energies, the photoelectric effect, specifically in the inter seed attenuation, yields deviation from an in-water DD. The proposed method provides a fast, accurate and integrated dose computation without any corrections or adjustments. In addition, the discretization of the space dimension is well suited to problems in radiation therapy where voxel-based geometries of patients are conducted from tomographic images.

Monte Carlo in assessment of low dose rate prostate brachytherapy dose distributions

The purpose of this study was to investigate differences in dose distributions when Monte Carlo models included interseed attenuation of I125 seeds and tissue heterogeneity effects and when these interactions are ignored. The Monte Carlo calculations were performed using the PENELOPE/penEasy system code. The code is implemented in FORTRAN 77 and its structure is based on a set of subroutines that are invoked from a main program written by the user. The code simulates incoherent scattering, coherent scattering and Bremsstrahlung X-ray production at energies ranging from 1 keV to 1 GeV for photons and from 0.1 keV to 1 GeV for electrons. For simulations, a simplified array of 3 × 3 × 3 seeds was placed in a sphere of water (radius of 5 cm). The geometry and energy spectrum of individual seeds was based on the manufacturer’s description. The seeds were positioned in order to mimic a simplified clinical arrangement for various separations and tissue heterogeneities. All simulations were conducted using an off the shelf laptop computer of 1.8 GHz AMD A6 processor with a RAM memory of 6 GB. From the results observed it is clear that there are differences in dose distributions between simulations that account for interseed attenuation and tissue heterogeneity and those that do not. Future work would be to develop more accurate voxel based anatomical phantom models for simulations, with more particle histories and the inclusion of other Monte Carlo tallies would allow for a more robust quantitative analysis of the results and a thorough assessment the differences observed.

TU-AB-201-11: A Novel Theoretical Framework for MRI-Only Image Guided LDR Prostate and Breast Brachytherapy Implant Dosimetry

Purpose: To propose a novel framework for accurate model-based dose calculations using only MR images for LDR prostate and breast seed implant brachytherapy. Methods: Model-based dose calculation methodologies recommended by TG-186 require further knowledge about specific tissue composition, which is challenging with MRI. However, relying on MRI-only for implant dosimetry would reduce the soft tissue delineation uncertainty, costs, and uncertainties associated with multi-modality registration and fusion processes. We propose a novel framework to address this problem using quantitative MRI acquisitions and reconstruction techniques. The framework includes three steps: (1) Identify the locations of seeds(2) Identify the presence (or absence) of calcification(s)(3) Quantify the water and fat content in the underlying tissueSteps (1) and (2) consider the sources that limit patient dosimetry, particularly the inter-seed attenuation and the calcified regions; while step (3) targets the quantification of the tissue composition to consider the heterogeneities in the medium. Our preliminary work has shown that the seeds and the calcifications can be identified with MRI using both the magnitude and the phase images. By employing susceptibility-weighted imaging with specific post-processing techniques, the phase images can be further explored to distinguish the seeds from the calcifications. Absolute quantification of tissue, water, and fat content is feasible and was previously demonstrated in phantoms and in-vivo applications, particularly for brain diseases. The approach relies on the proportionality of the MR signal to the number of protons in an image volume. By employing appropriate correction algorithms for T1 - and T2*-related biases, B1 transmit and receive field inhomogeneities, absolute water/fat content can be determined. Results: By considering calcification and interseed attenuation, and through the knowledge of water and fat mass density, accurate patient-specific implant dosimetry can be achieved with MRI-only. Conclusion: The proposed framework showed that model-based dose calculation is feasible using MRI-only state-of-the-art techniques.

Dosimetry Modeling for Focal Low-Dose-Rate Prostate Brachytherapy

Focal brachytherapy targeted to an individual lesion(s) within the prostate may reduce side effects experienced with whole-gland brachytherapy. The outcomes of a consensus meeting on focal prostate brachytherapy were used to investigate optimal dosimetry of focal low-dose-rate (LDR) prostate brachytherapy targeted using multiparametric magnetic resonance imaging (mp-MRI) and transperineal template prostate mapping (TPM) biopsy, including the effects of random and systematic seed displacements and interseed attenuation (ISA). Nine patients were selected according to clinical characteristics and concordance of TPM and mp-MRI. Retrospectively, 3 treatment plans were analyzed for each case: whole-gland (WG), hemi-gland (hemi), and ultra-focal (UF) plans, with 145-Gy prescription dose and identical dose constraints for each plan. Plan robustness to seed displacement and ISA were assessed using Monte Carlo simulations. WG plans used a mean 28 needles and 81 seeds, hemi plans used 17 needles and 56 seeds, and UF plans used 12 needles and 25 seeds. Mean D90 (minimum dose received by 90% of the target) and V100 (percentage of the target that receives 100% dose) values were 181.3 Gy and 99.8% for the prostate in WG plans, 195.7 Gy and 97.8% for the hemi-prostate in hemi plans, and 218.3 Gy and 99.8% for the focal target in UF plans. Mean urethra D10 was 205.9 Gy, 191.4 Gy, and 92.4 Gy in WG, hemi, and UF plans, respectively. Mean rectum D2 cm(3) was 107.5 Gy, 77.0 Gy, and 42.7 Gy in WG, hemi, and UF plans, respectively. Focal plans were more sensitive to seed displacement errors: random shifts with a standard deviation of 4 mm reduced mean target D90 by 14.0%, 20.5%, and 32.0% for WG, hemi, and UF plans, respectively. ISA has a similar impact on dose-volume histogram parameters for all plan types. Treatment planning for focal LDR brachytherapy is feasible. Dose constraints are easily met with a notable reduction to organs at risk. Treating smaller targets makes seed positioning more critical.

A Novel Algorithm Accounting for Inter-Seed Attenuation Effect in Brachytherapy Treatment Planning Systems by Monte Carlo and Artificial Neural Networks

A Novel Algorithm Accounting for Inter-Seed Attenuation Effect in Brachytherapy Treatment Planning Systems by Monte Carlo and Artificial Neural Networks

SU-E-T-527: A Fast Novel Analytical Model Accounting for Interseed Attenuation Effect in Low Energy Brachytherapy Sources

Purpose: Currently, treatment planning systems make two assumptions that can affect dose calculation; Homogenous media and Radio‐transparency of surrounding seeds. In this project a fast novel analytical model to correct the inter‐seed attenuation (ISA) in multiseed implants for TG‐43U1 LDR brachytherapy have developed. Model sensitivity of the results to examined for multiple seeds of I‐125 and Ir‐192 in different distanc and angles in water phantom. Methods: In this model, dose perturbation (P‐value) and inter‐seed attenuation (ISA‐value) in a multi‐source brachytherapy implant is based on pre Monte Carlo simulated 3D kernels of perturbation and ISA data for single active and single dummy configuration, arranged at different distances and angles relative to each other. The final model results have been examined by a compared for the calculated Perturbation and ISA‐values with full Monte Carlo water simulations (FMCWS). Finally by multiply the model values in TG‐43U1 dose rate for any interest point, the ISA deficiency will be modified. Results: The results show that the perturbation model values at interest point or voxel are equal to multiply perturbation factor of single active‐inactive sources which are placed in straight line from active source to interest point of implant. The accuracy of the model for parallel multi‐source implants was 3.5% and 5% for Ir‐192 and I‐125, respectively. Sensitivity of the results for non‐parallel multi‐source implants was up to 5%. Conclusion: Base on this novel fast approach, one could expedite the processes of dose ISA corrections with the minimum increase of the planning time for standard and loos brachytherapy implants as compared to the FMCWS in TG‐43U1 brachytherapy treatment planning systems. This model is applicable for energy of I‐125 to Ir‐192 sources.

An analytical model to determine interseed attenuation effect in low‐dose‐rate brachytherapy

Brachytherapy treatment planning systems (BTPS) are employing the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG‐43)‐recommended dosimetric parameters of sources, which are measured in water. The majority of brachytherapy implant volumes are not homogeneous media. Particularly, an implant with multiple seeds significantly changes homogeneity of the implant volume. Heterogeneities, such as attenuation by adjacent seeds or interseed attenuation (ISA), are neglected to this day in all BTPS. The goal of this project is to determine a novel analytical method to evaluate the impact of the dose perturbations (P‐value) and/or interseed attenuation effect (ISA‐value). This method will be validated for low‐ and high‐energy brachytherapy seeds such as 125I and 192Ir using Monte Carlo (MC) simulation techniques. In this analytical model, determination of dose perturbation and interseed attenuation in a multisource brachytherapy implant is based on MC‐simulated 3D kernels of P‐values and ISA data for single active and single dummy configurations, arranged at different distances and orientations relative to each other. The accuracy of the final model in multisource implant configurations has been examined by a comparison of the calculated P‐values and ISA‐values with full Monte Carlo water simulations (FMCWS). This model enabled us to determine the total perturbation and ISA values for any multisource implant, and the results are in excellent agreement with the FMCWS data. The advantage of this model to FMCWS for daily clinical application is the speed of the calculations and ease of the implementation. The new perturbation and ISA formulism have shown a better accuracy for 192Ir than 125I due to Compton scattering and its independence of the atomic number of the chemical composition of the phantom materials. The maximum difference between the ISA model and FMCWS for all cases was less than 5%. This new model can provide inputs for brachytherapy planning software to consider the ISA effect in dose calculations based on TG‐43U1 algorithm. This approach is applicable for energy range of 125I to 192Ir sources.PACS number: 87.53.Jw

Monte Carlo investigation of I‐125 interseed attenuation for standard and thinner seeds in prostate brachytherapy with phantom validation using a MOSFET

In permanent seed implant prostate brachytherapy the actual dose delivered to the patient may be less than that calculated by TG-43U1 due to interseed attenuation (ISA) and differences between prostate tissue composition and water. In this study the magnitude of the ISA effect is assessed in a phantom and in clinical prostate postimplant cases. Results are compared for seed models 6711 and 9011 with 0.8 and 0.5 mm diameters, respectively. A polymethyl methacrylate (PMMA) phantom was designed to perform ISA measurements in a simple eight-seed arrangement and at the center of an implant of 36 seeds. Monte Carlo (MC) simulation and experimental measurements using a MOSFET dosimeter were used to measure dose rate and the ISA effect. MC simulations of 15 CT-based postimplant prostate treatment plans were performed to compare the clinical impact of ISA on dose to prostate, urethra, rectum, and the volume enclosed by the 100% isodose, for 6711 and 9011 seed models. In the phantom, ISA reduced the dose rate at the MOSFET position by 8.6%-18.3% (6711) and 7.8%-16.7% (9011) depending on the measurement configuration. MOSFET measured dose rates agreed with MC simulation predictions within the MOSFET measurement uncertainty, which ranged from 5.5% to 7.2% depending on the measurement configuration (k = 1, for the mean of four measurements). For 15 clinical implants, the mean ISA effect for 6711 was to reduce prostate D90 by 4.2 Gy (3%), prostate V100 by 0.5 cc (1.4%), urethra D10 by 11.3 Gy (4.4%), rectal D2cc by 5.5 Gy (4.6%), and the 100% isodose volume by 2.3 cc. For the 9011 seed the mean ISA effect reduced prostate D90 by 2.2 Gy (1.6%), prostate V100 by 0.3 cc (0.7%), urethra D10 by 8.0 Gy (3.2%), rectal D2cc by 3.1 Gy (2.7%), and the 100% isodose volume by 1.2 cc. Differences between the MC simulation and TG-43U1 consensus data for the 6711 seed model had a similar impact, reducing mean prostate D90 by 6 Gy (4.2%) and V100 by 0.6 cc (1.8%). ISA causes the delivered dose in prostate seed implant brachytherapy to be lower than the dose calculated by TG-43U1. MC simulation of phantom seed arrangements show that dose at a point can be reduced by up to 18% and this has been validated using a MOSFET dosimeter. Clinical simulations show that ISA reduces DVH parameter values, but the reduction is less for thinner seeds.

Model-based dose calculations for125I lung brachytherapy

Purpose: Model-baseddose calculations (MBDCs) are performed using patient computed tomography (CT) data for patients treated with intraoperative125I lung brachytherapy at the Mayo Clinic Rochester. Various metallic artifact correction and tissue assignment schemes are considered and their effects on dose distributions are studied. Dose distributions are compared to those calculated under TG-43 assumptions. Methods: Dose distributions for six patients are calculated using phantoms derived from patient CT data and the EGSnrc user-code BrachyDose.125I (GE Healthcare/Oncura model 6711) seeds are fully modeled. Four metallic artifact correction schemes are applied to the CT data phantoms: (1) no correction, (2) a filtered back-projection on a modified virtual sinogram, (3) the reassignment of CT numbers above a threshold in the vicinity of the seeds, and (4) a combination of (2) and (3). Tissue assignment is based on voxel CT number and mass density is assigned using a CT number to mass density calibration. Three tissue assignment schemes with varying levels of detail (20, 11, and 5 tissues) are applied to metallic artifact corrected phantoms. Simulations are also performed under TG-43 assumptions, i.e., seeds in homogeneous water with no interseed attenuation. Results: Significant dose differences (up to 40% for D90) are observed between uncorrected and metallic artifact corrected phantoms. For phantoms created with metallic artifact correction schemes (3) and (4), dose volume metrics are generally in good agreement (less than 2% differences for all patients) although there are significant local dose differences. The application of the three tissue assignment schemes results in differences of up to 8% for D90; these differences vary between patients. Significant dose differences are seen between fully modeled and TG-43 calculations with TG-43 underestimating the dose (up to 36% in D90) for larger volumes containing higher proportions of healthy lung tissue. Conclusions: Metallic artifact correction is necessary for accurate application of MBDCs for lung brachytherapy; simpler threshold replacement methods may be sufficient for early adopters concerned with clinical dose metrics. Rigorous determination of voxel tissue parameters and tissue assignment is required for accurate dose calculations as different tissue assignment schemes can result in significantly different dose distributions. Significant differences are seen between MBDCs and TG-43 dose distributions with TG-43 underestimating dose in volumes containing healthy lung tissue.

ALGEBRA: ALgorithm for the heterogeneous dosimetry based on GEANT4 for BRAchytherapy

Task group 43 (TG43)-based dosimetry algorithms are efficient for brachytherapy dose calculation in water. However, human tissues have chemical compositions and densities different than water. Moreover, the mutual shielding effect of seeds on each other (interseed attenuation) is neglected in the TG43-based dosimetry platforms. The scientific community has expressed the need for an accurate dosimetry platform in brachytherapy. The purpose of this paper is to present ALGEBRA, a Monte Carlo platform for dosimetry in brachytherapy which is sufficiently fast and accurate for clinical and research purposes. ALGEBRA is based on the GEANT4 Monte Carlo code and is capable of handling the DICOM RT standard to recreate a virtual model of the treated site. Here, the performance of ALGEBRA is presented for the special case of LDR brachytherapy in permanent prostate and breast seed implants. However, the algorithm is also capable of handling other treatments such as HDR brachytherapy.

Consequences of dose heterogeneity on the biological efficiency of 103Pd permanent breast seed implants

Brachytherapy is associated with highly heterogeneous spatial dose distributions. This heterogeneity is usually ignored when estimating the biological effective dose (BED). In addition, the heterogeneities of the medium including the tissue heterogeneity (TH) and the interseed attenuation (ISA) are also contributing to the heterogeneity of the dose distribution, but they are both ignored in Task Group 43 (TG43)-based protocols. This study investigates the effect of dose heterogeneity, TH and ISA on metrics that are commonly used to quantify biological efficiency in brachytherapy. The special case of 29 breast cancer patients treated with permanent 103Pd seed implant is considered here. BED is compared to equivalent uniform BED (EUBED) capable of considering the spatial heterogeneity of the dose distribution. The effects of TH and ISA on biological efficiency of treatments are taken into account by comparing TG43 with Monte Carlo (MC) dose calculations for each patient. The effect of clonogenic repopulation is also considered. The analysis is performed for different sets of (α/β, α) ratios of (2, 0.3), (4, 0.27) and (10, 0.3) [Gy, Gy−1] covering the whole range of reported α/β values in the literature. BED is sometimes larger and sometimes smaller than EUBED(TG43) indicating that the effect of the dose heterogeneity is not similar among patients. The effect of the dose heterogeneity can be characterized by using the D99 dose metric. For each set of the radiobiological parameters considered, a D99 threshold is found over which dose heterogeneity will cause an overestimation of the biological efficiencies while the inverse happens for smaller D99 values. EUBED(MC) is always larger than EUBED(TG43) indicating that by neglecting TH and ISA in TG43-based dosimetry algorithms, the biological efficiencies may be underestimated by about 10 Gy. Overall, by going from BED to the more accurate EUBED(MC) there is a gain of about 9.6 to 13 Gy on the biological efficiency. The efficiency gain is about 10.8 to 14 Gy when the repopulation is considered. Dose heterogeneity does not have a constant impact on the biological efficiencies and may under- or overestimate the efficacy in different patients. However, the combined effect of neglecting dose heterogeneity, TH and ISA results in underestimation of the biological efficiencies in permanent breast seed implants.

A dosimetry study of the Oncoseed 6711 using glass rod dosimeters and EGS5 Monte Carlo code in a geometry lacking radiation equilibrium scatter conditions

The aim of this study was to develop a dose calculation method which is applicable to the interseed attenuation and the geometry lacking the equilibrium radiation scatter conditions in brachytherapy. The dose obtained from measurement with a radiophotoluminescent glass rod dosimeter (GRD) was compared to the dose calculated with the Monte Carlo (MC) code "EGS5," using the 125I source structure detailed in by Kennedy et al. The GRDs were irradiated with 125I Oncoseed 6711 in a human head phantom. The phantom was a cylinder made of 2 mm thick PMMA with a diameter of 18 cm and length of 16 cm. Some of the GRD positions were so close to the phantom surface that the backscatter margin was less than 5 cm, insufficient for photons. The EGS5 simulations were found to reproduce the relative dose distributions as measured with the GRDs to within 25% uncertainty in the geometry lacking the equilibrium radiation scatter conditions. The absolute value of the GRD measurement agreed with the American Association of Physicist in Medicine Task Group No 43 Updated Protocol (AAPM-TG43U1) formalism to within 3% of the reference point (r = 1 cm, theta = 90 degrees), where the TG43U1 is especially reliable because of the abundant data accumulation in composing the formalism. The factor to normalize the measured or calculated dose to the TG43U1 estimate at the reference point was evaluated to be 0.97 for the GRD measurement and 1.8 for the MC calculation, which uses the integration of the apparent activity with the time as the amount of disintegration during the irradiation. Also, F(r,theta) and g(r) estimated by this calculation method were consistent with those proposed in the TG43U1. The results of this investigation support the validity of both the MC calculation method and GRD measurement in this study as well as the TG-43U1 formalism. Also, this calculation is applicable to interseed attenuation and the geometry lacking the equilibrium radiation scatter.

Postimplant Dosimetry Using a Monte Carlo Dose Calculation Engine: A New Clinical Standard

To use the Monte Carlo (MC) method as a dose calculation engine for postimplant dosimetry. To compare the results with clinically approved data for a sample of 28 patients. Two effects not taken into account by the clinical calculation, interseed attenuation and tissue composition, are being specifically investigated. An automated MC program was developed. The dose distributions were calculated for the target volume and organs at risk (OAR) for 28 patients. Additional MC techniques were developed to focus specifically on the interseed attenuation and tissue effects. For the clinical target volume (CTV) D(90) parameter, the mean difference between the clinical technique and the complete MC method is 10.7 Gy, with cases reaching up to 17 Gy. For all cases, the clinical technique overestimates the deposited dose in the CTV. This overestimation is mainly from a combination of two effects: the interseed attenuation (average, 6.8 Gy) and tissue composition (average, 4.1 Gy). The deposited dose in the OARs is also overestimated in the clinical calculation. The clinical technique systematically overestimates the deposited dose in the prostate and in the OARs. To reduce this systematic inaccuracy, the MC method should be considered in establishing a new standard for clinical postimplant dosimetry and dose-outcome studies in a near future.