A dosimetric comparison of Yb169 and Ir192 for HDR brachytherapy of the breast, accounting for the effect of finite patient dimensions and tissue inhomogeneities

Feasibility of the analytical dose calculation method for Au-198 brachytherapy

PURPOSE

A dose calculation algorithm Computed Tomography (CT)-based analytical dose calculation method (CTanly), which can correct for subject inhomogeneity and size-dependent scatter doses, was applied to the 198Au seed. In this study, we evaluated the effectiveness of the CTanly method by comparing the gold standard Monte Carlo (MC) method and the conventional TG43 method on two virtual phantoms and patient CT images simulating oral cancer.

METHODS

As virtual phantoms, a water phantom and a heterogeneous phantom with soft tissue inserted cubic fat, lung, and bone were used. A 2-mm-thick lead plate was also inserted into the heterogeneous phantom as a dose attenuator. Virtual 198Au seeds and a 2-mm-thick lead plate were placed on the patient CT images. Dose distributions obtained via the TG43 and CTanly methods were compared with those of the MC by gamma analysis with 2%/2-mm thresholds. The computation durations were also compared.

RESULTS

In the water phantom, dose distributions comparable to those obtained via the MC method were obtained regardless of the algorithm. For the inhomogeneity phantom and patient case, the CTanly method showed an improvement in the gamma passing rate and dose distributions similar to those of the MC method were obtained. The computation time, which was days with the MC method, was reduced to minutes with the CTanly method.

CONCLUSIONS

The CTanly method is effective for 198Au seed dose calculations and takes a shorter time to obtain the dose distributions than the MC method.

Dosimetric Impact of Air Pockets in the Vaginal Cuff Brachytherapy Using Model-based Dose Calculation Algorithm

Background

Endometrial cancer is the most common disease of the female reproductive system. Vaginal cuff brachytherapy (VCB) has intrinsic advantages compared to external beam therapy when treated with radiation. A single-channel cylinder is a standard applicator in VCB. The present study aims to estimate a change in the dose to vaginal mucosa due to air pockets between the cylinder and vaginal mucosa by calculating with the Acuros BV algorithm and comparing it to the Task Group 43 (TG-43) algorithm.

Materials and Methods

Patients who presented with air packets were included retrospectively. For each patient, three plans were created: the first plan used TG-43, the second plan used dose recalculation with Acuros BV, and the third plan was generated by re-optimization by Acuros BV. On the same axial computed tomography image, the point doses at the cylinder's surface and the displaced mucosa were recorded and the ratios were then estimated.

Results

The average volume of air pockets was 0.08 cc (range of 0.01-0.3 cc), and 84% of air pockets displaced the vaginal mucosa by ≥0.2 cm. The average ratios of dose were 0.77 ± 0.09 (1 standard deviation [SD]) and 0.78 ± 0.09 (1 SD) for TG-43 and Acuros BV algorithms, respectively. Due to the air pocket, mucosa received a reduced dose by an average of 22.72% and an average of 23.29% for TG-43 and Acuros BV, respectively. The maximum displacement of mucosa and the ratio of doses were negatively correlated for both. In the Optimized Acuros BV plan, total dwell time increased by 1.8% but no considerable change in the dose ratios.

Conclusion

The calculated dose of mucous membrane forced out of the cylinder surface by air pockets by the Acuros BV algorithm was nonsignificantly different from TG-43. Therefore, even in the presence of air pockets, the TG-43 algorithm for calculating the VCB dose is appropriate.

Accuracy of Acuros TM BV as determined from GATE monte-carlo simulation

The American Association of Physicists in Medicine's Task Group No.43 has provided a standardised dose calculation methodology that is now the international benchmark for all brachytherapy dosimetry publications and treatment planning systems. However, limitations in this methodology has seen the development of Model-Based Dose Calculation Algorithms (MBDCA). In 2009, Varian (Varian Medical Systems, Palo Alto, CA, USA) released AcurosTM BrachyVision (ABV) which calculates dose by explicitly solving the Linear Boltzmann Transport Equation. In this study we have assessed the accuracy of ABV dose calculations within a range of materials relevant to high dose rate brachytherapy with an iridium-192 (192 Ir) source. Accuracy assessment has been achieved by implementing a modelled GamaMed Plus192 Ir source within a series of phantoms using the GEANT4 Application for Emission Tomography (GATE) to calculate dose for comparison with dose as determined by ABV. Comparisons between GATE and ABV were made using point-to-point profile comparisons and 1D gamma analysis. Source validation results yielded good agreement with published data. Spectrum and TG43U1 comparisons showed no major differences, with TG43U1 comparisons agreeing within ± 1%. Point-to-point comparisons showed large differences between GATE and ABV near the source and in low density materials. 1D gamma analysis pass criteria of 2%/1 mm and 2%/2 mm yielded passing rates ranging between 51.72-100% and 62.07-100% respectively. A critical analysis of this study's results suggest that ABV is unable to accurately calculate doses in low density materials. Furthermore, spatial accuracy of dose near the source is within 2 mm.

Evaluation of dosimetric functions for a new 169 Yb HDR Brachytherapy Source

169 Yb has been recently used as an HDR brachytherapy source for cancer treatment. In this paper, dosimetric parameters of a new design of 169 Yb HDR brachytherapy source were determined by Monte Carlo (MC) method and film dosimetry. In this new source, the radioactive core has been encapsulated twice for safety purposes. The calculations of dosimetric parameters carried out using MC simulation in water and air phantom. In order to exclude photon contamination's cutoff energy, δ was set at 10 keV. TG-43U1 data dosimetric, including Sk , Λ, g(r), F(r, θ) was computed using outputs from the simulation and their statistical uncertainties were calculated. Dose distribution around the new prototype source in PMMA phantom in the framework of AAPM TG-43 and TG-55 recommendations was measured by Radiochromic film (RCF) Gafchromic EBT3. Obtained air kerma strength, Sk , and the dose rate constant, Λ, from simulation has a value of 1.03U ± 0.03 and 1.21 cGyh-1 U-1  ± 0.03, respectively. The radial dose function was calculated at radial distances between 0.5 and 10 cm with a maximum value of 1.15 ± 0.03 at 5-6 cm distances. The anisotropy functions for radial distances of 0.5-7 cm and angle distances 0° to180° were calculated. The dosimetric data of the new HDR 169 Yb source were compared with another reference source of 169 Yb-HDR and were found that has acceptable compatibility. In addition, the anisotropy function of the MC simulation and film dosimetry method at a distance of 1 cm from this source was obtained and a good agreement was found between the anisotropy results.

Cardiac doses of accelerated partial breast irradiation with perioperative multicatheter interstitial brachytherapy

PURPOSE

To quantify mean heart dose (MHD) and doses to the left anterior descending artery (LAD) and left ventricle (LV) in a retrospective series of patients who underwent perioperative accelerated partial breast irradiation with multicatheter interstitial brachytherapy (MIB-APBI).

METHODS

Sixty-eight patients with low-risk left breast cancer were treated with MIB-APBI at our institution between 2012 and 2017. Interstitial tubes were inserted during the tumorectomy and sentinel node biopsy and APBI was started 6 days later. The prescribed dose was 34 Gy in 10 fractions (twice a day) to the clinical target volume (CTV). The heart, LAD, and LV were contoured and the distance between each structure and the CTV was measured. The MHD, mean and maximum LAD doses (LAD mean/max), and mean LV doses (LV mean) were calculated and corrected to biologically equivalent doses in 2‑Gy fractionation (EQD2). We also evaluated the impact of the distance between the cardiac structures and the CTV and of the volume receiving the prescribed dose (V100) and high-dose volume (V150) on heart dosimetry.

RESULTS

Mean EQD2 for MHD, LAD mean/max, and mean LV were 0.9 ± 0.4 Gy (range 0.3-2.2), 1.6 ± 1.1 Gy (range, 0.4-5.6), 2.6 ± 1.9 Gy (range, 0.7-9.2), and 1.3 ± 0.6 Gy (range, 0.5-3.4), respectively. MHD, LAD mean/max, and LV mean significantly correlated with the distance between the CTV and these structures, but all doses were below the recommended limits (German Society of Radiation Oncology; DEGRO). The MHD and LV mean were significantly dependent on V100.

CONCLUSION

Perioperative MIB-APBI resulted in low cardiac doses in our study. This finding provides further support for the value of this technique in well-selected patients with early-stage left breast cancer.

Monte Carlo dose calculations for high-dose-rate brachytherapy using GPU-accelerated processing

PURPOSE

Current clinical brachytherapy dose calculations are typically based on the Association of American Physicists in Medicine Task Group report 43 (TG-43) guidelines, which approximate patient geometry as an infinitely large water phantom. This ignores patient and applicator geometries and heterogeneities, causing dosimetric errors. Although Monte Carlo (MC) dose calculation is commonly recognized as the most accurate method, its associated long computational time is a major bottleneck for routine clinical applications. This article presents our recent developments of a fast MC dose calculation package for high-dose-rate (HDR) brachytherapy, gBMC, built on a graphics processing unit (GPU) platform.

METHODS AND MATERIALS

gBMC-simulated photon transport in voxelized geometry with physics in (192)Ir HDR brachytherapy energy range considered. A phase-space file was used as a source model. GPU-based parallel computation was used to simultaneously transport multiple photons, one on a GPU thread. We validated gBMC by comparing the dose calculation results in water with that computed TG-43. We also studied heterogeneous phantom cases and a patient case and compared gBMC results with Acuros BV results.

RESULTS

Radial dose function in water calculated by gBMC showed <0.6% relative difference from that of the TG-43 data. Difference in anisotropy function was <1%. In two heterogeneous slab phantoms and one shielded cylinder applicator case, average dose discrepancy between gBMC and Acuros BV was <0.87%. For a tandem and ovoid patient case, good agreement between gBMC and Acruos BV results was observed in both isodose lines and dose-volume histograms. In terms of the efficiency, it took ∼47.5 seconds for gBMC to reach 0.15% statistical uncertainty within the 5% isodose line for the patient case.

CONCLUSIONS

The accuracy and efficiency of a new GPU-based MC dose calculation package, gBMC, for HDR brachytherapy make it attractive for clinical applications.

Technical Note: Monte Carlo calculations of the AAPM TG-43 brachytherapy dosimetry parameters for a new titanium-encapsulated Yb-169 source

Due to a number of distinct advantages resulting from the relatively low energy gamma ray spectrum of Yb‐169, various designs of Yb‐169 sources have been developed over the years for brachytherapy applications. Lately, Yb‐169 has also been suggested as an effective and practical radioisotope option for a novel radiation treatment approach often known as gold nanoparticle‐aided radiation therapy (GNRT). In a recently published study, the current investigators used the Monte Carlo N‐Particle Version 5 (MCNP5) code to design a novel titanium‐encapsulated Yb‐169 source optimized for GNRT applications. In this study, the original MC source model was modified to accurately match the specifications of the manufactured Yb‐169 source. The modified MC model was then used to obtain a complete set of the AAPM TG‐43 parameters for the new titanium‐encapsulated Yb‐169 source. The MC‐calculated dose rate constant for this titanium‐encapsulated Yb‐169 source was 1.19 ± 0.03 cGy·h−1·U−1, indicating no significant change from the values reported for stainless steel‐encapsulated Yb‐169 sources. The source anisotropy and radial dose function for the new source were also found similar to those reported for the stainless steel‐encapsulated Yb‐169 sources. The current results suggest that the use of titanium, instead of stainless steel, to encapsulate the Yb‐169 core would not lead to any major change in the dosimetric characteristics of the Yb‐169 source. The results also show that the titanium encapsulation of the Yb‐169 source could be accomplished while meeting the design goals as described in the current investigators’ published MC optimization study for GNRT applications.

Production cross sections of 169Yb and Tm isotopes in deuteron-induced reactions on 169Tm

The excitation functions of deuteron-induced reactions on ¹⁶⁹Tm were measured using the stacked-foil method and high resolution gamma-ray spectrometry. The production cross sections of a medical radionuclide ¹⁶⁹Yb were investigated. The result was compared with the previous experiments and found to be in good agreement. In addition to ¹⁶⁹Yb, the production cross sections of Tm isotopes, ¹⁷⁰Tm, ¹⁶⁸Tm and ¹⁶⁷Tm, were measured. These results were compared with the TALYS calculations taken from the TENDL-2015 online data library.

Polymer gel dosimetry for measuring the dose near thin high-Z materials irradiated with high energy photon beams

PURPOSE

To investigate the feasibility of three-dimensional (3D) dose measurements near thin high-Z materials placed in a water-like medium by using a polymer gel dosimeter (PGD) when the medium was irradiated with high energy photon beams.

METHODS

PGD is potentially a useful tool for this application because it can record the dose around a small object made of a high-Z material in a continuous 3D medium. In this study, the authors manufactured a methacrylic acid-based normoxic PGD, nMAG. Two 0.5 mm thick lead foils (1 × 1 cm) were placed in foil supports with 0.7 cm separation in a 1000 ml polystyrene container filled with nMAG. The authors used two foil configurations, i.e., orthogonal and parallel. In the orthogonal configuration, two foils were placed in the direction orthogonal to the beam axis. The parallel configuration had two foils arranged in parallel to the beam axis. The phantom was irradiated with an 18 MV photon beam of 5 × 5 cm field size. It was imaged with a three-Tesla (3 T) magnetic resonance imaging (MRI) scanned using the Car-Purcell-Meiboom-Gill pulse sequence. The spin-spin relaxation time (R2) to-dose calibration data were obtained by using small vials filled with nMAG and exposing to known doses. The DOSXYZnrc Monte Carlo (MC) code was used to get the expected dose distributions. More than 35 × 10⁶ of histories were simulated so that the average error was less than 1%. An in-house matlab-based software was used to obtain the dose distributions from the measured R2 data as well as to compare the measurements and the MC predictions. The dose change due to the presence of the foils was studied by comparing the dose distributions with and without foils (or the reference).

RESULTS

For the orthogonal configuration, the measured dose along the beam axis showed an increase in the upstream side of the first foil, between the foils, and on the downstream side of the second foil. The range of increased dose area was 1.1 cm in the upstream of the first foil. However, in the downstream of the second foil, it was 0.2 cm, beyond which the dose fell below the reference dose by 10%. The dose profile between the foils showed a well-like shape with the minimum dose still larger than the reference dose by 1.8%. The minimum dose point was closer to the first foil than to the second foil. For the parallel configuration, the dose between foils was the largest at the center. The increased dose area opposite to the gap between foils extended outward to 1 cm. The spatial dose distributions of PGD and MC showed the same geometrical patterns except for the points inside the foils for both orthogonal and parallel foil arrangements.

CONCLUSIONS

The authors demonstrated that the nMAG PGD with MRI could be used to measure the 3D dosimetric structures at the mm-scale in the vicinity of the foil. The current study provided more accurate 3D spatial dose distribution than the previous studies. Furthermore, the measurements were validated by the MC simulation.

Impact of heterogeneity-corrected dose calculation using a grid-based Boltzmann solver on breast and cervix cancer brachytherapy

Purpose

To analyze the impact of heterogeneity-corrected dose calculation on dosimetric quality parameters in gynecological and breast brachytherapy using Acuros, a grid-based Boltzmann equation solver (GBBS), and to evaluate the shielding effects of different cervix brachytherapy applicators.

Material and methods

Calculations with TG-43 and Acuros were based on computed tomography (CT) retrospectively, for 10 cases of accelerated partial breast irradiation and 9 cervix cancer cases treated with tandem-ring applicators. Phantom CT-scans of different applicators (plastic and titanium) were acquired. For breast cases the V_20Gyαβ3 to lung, the D_0.1cm³, D_1cm³, D_2cm³ to rib, the D_0.1cm³, D_1cm³, D_10cm³ to skin, and D_max for all structures were reported. For cervix cases, the D_0.1cm³, D_2cm³ to bladder, rectum and sigmoid, and the D₅₀, D₉₀, D₉₈, V₁₀₀ for the CTV_HR were reported. For the phantom study, surrogates for target and organ at risk were created for a similar dose volume histogram (DVH) analysis. Absorbed dose and equivalent dose to 2 Gy fractionation (EQD2) were used for comparison.

Results

Calculations with TG-43 overestimated the dose for all dosimetric indices investigated. For breast, a decrease of ~8% was found for D_10cm³ to the skin and 5% for D_2cm³ to rib, resulting in a difference ~ –1.5 Gy EQD2 for overall treatment. Smaller effects were found for cervix cases with the plastic applicator, with up to –2% (–0.2 Gy EQD2) per fraction for organs at risk and –0.5% (–0.3 Gy EQD2) per fraction for CTV_HR. The shielding effect of the titanium applicator resulted in a decrease of 2% for D_2cm³ to the organ at risk versus 0.7% for plastic.

Conclusions

Lower doses were reported when calculating with Acuros compared to TG-43. Differences in dose parameters were larger in breast cases. A lower impact on clinical dose parameters was found for the cervix cases. Applicator material causes systematic shielding effects that can be taken into account.

Comparison of Dose Distributions With TG-43 and Collapsed Cone Convolution Algorithms Applied to Accelerated Partial Breast Irradiation Patient Plans

PURPOSE

To compare the treatment plans for accelerated partial breast irradiation calculated by the new commercially available collapsed cone convolution (CCC) and current standard TG-43-based algorithms for 50 patients treated at our institution with either a Strut-Adjusted Volume Implant (SAVI) or Contura device.

METHODS AND MATERIALS

We recalculated target coverage, volume of highly dosed normal tissue, and dose to organs at risk (ribs, skin, and lung) with each algorithm. For 1 case an artificial air pocket was added to simulate 10% nonconformance. We performed a Wilcoxon signed rank test to determine the median differences in the clinical indices V90, V95, V100, V150, V200, and highest-dosed 0.1 cm(3) and 1.0 cm(3) of rib, skin, and lung between the two algorithms.

RESULTS

The CCC algorithm calculated lower values on average for all dose-volume histogram parameters. Across the entire patient cohort, the median difference in the clinical indices calculated by the 2 algorithms was <10% for dose to organs at risk, <5% for target volume coverage (V90, V95, and V100), and <4 cm(3) for dose to normal breast tissue (V150 and V200). No discernable difference was seen in the nonconformance case.

CONCLUSIONS

We found that on average over our patient population CCC calculated (<10%) lower doses than TG-43. These results should inform clinicians as they prepare for the transition to heterogeneous dose calculation algorithms and determine whether clinical tolerance limits warrant modification.

Design of an Yb-169 source optimized for gold nanoparticle-aided radiation therapy

PURPOSE

To find an optimum design of a new high-dose rate ytterbium (Yb)-169 brachytherapy source that would maximize the dose enhancement during gold nanoparticle-aided radiation therapy (GNRT), while meeting practical constraints for manufacturing a clinically relevant brachytherapy source.

METHODS

Four different Yb-169 source designs were considered in this investigation. The first three source models had a single encapsulation made of one of the following materials: aluminum, titanium, and stainless steel. The last source model adopted a dual encapsulation design with an inner aluminum capsule surrounding the Yb-core and an outer titanium capsule. Monte Carlo (MC) simulations using the Monte Carlo N-Particle code version 5 (MCNP5) were conducted initially to investigate the spectral changes caused by these four source designs and the associated variations in macroscopic dose enhancement across the tumor loaded with gold nanoparticles (GNPs) at 0.7% by weight. Subsequent MC simulations were performed using the EGSnrc and norec codes to determine the secondary electron spectra and microscopic dose enhancement as a result of irradiating the GNP-loaded tumor with the mcnp-calculated source spectra.

RESULTS

Effects of the source filter design were apparent in the current MC results. The intensity-weighted average energy of the Yb-169 source varied from 108.9 to 122.9 keV, as the source encapsulation material changed from aluminum to stainless steel. Accordingly, the macroscopic dose enhancement calculated at 1 cm away from the source changed from 51.0% to 45.3%. The sources encapsulated by titanium and aluminum/titanium combination showed similar levels of dose enhancement, 49.3% at 1 cm, and average energies of 113.0 and 112.3 keV, respectively. While the secondary electron spectra due to the investigated source designs appeared to look similar in general, some differences were noted especially in the low energy region (<50 keV) of the spectra suggesting the dependence of the photoelectron yield on the atomic number of source filter material, consistent with the macroscopic dose enhancement results. A similar trend was also shown in the so-called microscopic dose enhancement factor, for example, resulting in the maximum values of 138 and 119 for the titanium- and the stainless steel-encapsulated Yb-169 sources, respectively.

CONCLUSIONS

The current results consistently show that the dose enhancement achievable from the Yb-169 source is closely related with the atomic number (Z) of source encapsulation material. While the observed range of improvement in the dose enhancement may be considered moderate after factoring all uncertainties in the MC results, the current study provides a reasonable support for the encapsulation of the Yb-core with lower-Z materials than stainless steel, for GNRT applications. Overall, the titanium capsule design can be favored over the aluminum or dual aluminum/titanium capsule designs, due to its superior structural integrity and improved safety during manufacturing and clinical use.

Evaluation of (101)Rh as a brachytherapy source

PURPOSE

Recently a number of hypothetical sources have been proposed and evaluated for use in brachytherapy. In the present study, a hypothetical (101)Rh source with mean photon energy of 121.5 keV and half-life of 3.3 years, has been evaluated as an alternative to the existing high-dose-rate (HDR) sources. Dosimetric characteristics of this source model have been determined following the recommendation of the Task Group 43 (TG-43) of the American Association of the Physicist in Medicine (AAPM), and the results are compared with the published data for (57)Co source and Flexisource (192)Ir sources with similar geometries.

MATERIAL AND METHODS

MCNPX Monte Carlo code was used for simulation of the (101)Rh hypothetical HDR source design. Geometric design of this hypothetical source was considered to be similar to that of Flexisource (192)Ir source. Task group No. 43 dosimetric parameters, including air kerma strength per mCi, dose rate constant, radial dose function, and two dimensional (2D) anisotropy functions were calculated for the (101)Rh source through simulations.

RESULTS

Air kerma strength per activity and dose rate constant for the hypothetical (101)Rh source were 1.09 ± 0.01 U/mCi and 1.18 ± 0.08 cGy/(h.U), respectively. At distances beyond 1.0 cm in phantom, radial dose function for the hypothetical (101)Rh source is higher than that of (192)Ir. It has also similar 2D anisotropy functions to the Flexisource (192)Ir source.

CONCLUSIONS

(101)Rh is proposed as an alternative to the existing HDR sources for use in brachytherapy. This source provides medium energy photons, relatively long half-life, higher dose rate constant and radial dose function, and similar 2D anisotropy function to the Flexisource (192)Ir HDR source design. The longer half-life of the source reduces the frequency of the source exchange for the clinical environment.

Studies on the development of ¹⁶⁹Yb-brachytherapy seeds: New generation brachytherapy sources for the management of cancer

This paper describes development of (169)Yb-seeds by encapsulating 0.6-0.65 mm (ϕ) sized (169)Yb2O3 microspheres in titanium capsules. Microspheres synthesized by a sol-gel route were characterized by XRD, SEM/EDS and ICP-AES. Optimization of neutron irradiation was accomplished and (169)Yb-seeds up to 74 MBq of (169)Yb could be produced from natural Yb2O3 microspheres, which have the potential for use in prostate brachytherapy. A protocol to prepare (169)Yb-brachytherapy sources (2.96-3.7 TBq of (169)Yb) with the use of enriched targets was also formulated.

Prospects for in vivo estimation of photon linear attenuation coefficients using postprocessing dual-energy CT imaging on a commercial scanner: comparison of analytic and polyenergetic statistical reconstruction algorithms

PURPOSE

Accurate patient-specific photon cross-section information is needed to support more accurate model-based dose calculation for low energy photon-emitting modalities in medicine such as brachytherapy and kilovoltage x-ray imaging procedures. A postprocessing dual-energy CT (pDECT) technique for noninvasive in vivo estimation of photon linear attenuation coefficients has been experimentally implemented on a commercial CT scanner and its accuracy assessed in idealized phantom geometries.

METHODS

Eight test materials of known composition and density were used to compare pDECT-estimated linear attenuation coefficients to NIST reference values over an energy range from 10 keV to 1 MeV. As statistical image reconstruction (SIR) has been shown to reconstruct images with less random and systematic error than conventional filtered backprojection (FBP), the pDECT technique was implemented with both an in-house polyenergetic SIR algorithm, alternating minimization (AM), as well as a conventional FBP reconstruction algorithm. Improvement from increased spectral separation was also investigated by filtering the high-energy beam with an additional 0.5 mm of tin. The law of propagated uncertainty was employed to assess the sensitivity of the pDECT process to errors in reconstructed images.

RESULTS

Mean pDECT-estimated linear attenuation coefficients for the eight test materials agreed within 1% of NIST reference values for energies from 1 MeV down to 30 keV, with mean errors rising to between 3% and 6% at 10 keV, indicating that the method is unbiased when measurement and calibration phantom geometries are matched. Reconstruction with FBP and AM algorithms conferred similar mean pDECT accuracy. However, single-voxel pDECT estimates reconstructed on a 1 × 1 × 3 mm(3) grid are shown to be highly sensitive to reconstructed image uncertainty; in some cases pDECT attenuation coefficient estimates exhibited standard deviations on the order of 20% around the mean. Reconstruction with the statistical AM algorithm led to standard deviations roughly 40% to 60% less than FBP reconstruction. Additional tin filtration of the high energy beam exhibits similar pDECT estimation accuracy as the unfiltered beam, even when scanning with only 25% of the dose. Using the law of propagated uncertainty, low Z materials are found to be more sensitive to image reconstruction errors than high Z materials. Furthermore, it is estimated that reconstructed CT image uncertainty must be limited to less than 0.25% to achieve a target linear-attenuation coefficient estimation uncertainty of 3% at 28 keV.

CONCLUSIONS

That pDECT supports mean linear attenuation coefficient measurement accuracies of 1% of reference values for energies greater than 30 keV is encouraging. However, the sensitivity of the pDECT measurements to noise and systematic errors in reconstructed CT images warrants further investigation in more complex phantom geometries. The investigated statistical reconstruction algorithm, AM, reduced random measurement uncertainty relative to FBP owing to improved noise performance. These early results also support efforts to increase DE spectral separation, which can further reduce the pDECT sensitivity to measurement uncertainty.

Current state of the art brachytherapy treatment planning dosimetry algorithms

Following literature contributions delineating the deficiencies introduced by the approximations of conventional brachytherapy dosimetry, different model-based dosimetry algorithms have been incorporated into commercial systems for (192)Ir brachytherapy treatment planning. The calculation settings of these algorithms are pre-configured according to criteria established by their developers for optimizing computation speed vs accuracy. Their clinical use is hence straightforward. A basic understanding of these algorithms and their limitations is essential, however, for commissioning; detecting differences from conventional algorithms; explaining their origin; assessing their impact; and maintaining global uniformity of clinical practice.

Review of clinical brachytherapy uncertainties: analysis guidelines of GEC-ESTRO and the AAPM

Background and purpose

A substantial reduction of uncertainties in clinical brachytherapy should result in improved outcome in terms of increased local control and reduced side effects. Types of uncertainties have to be identified, grouped, and quantified.

Methods

A detailed literature review was performed to identify uncertainty components and their relative importance to the combined overall uncertainty.

Results

Very few components (e.g., source strength and afterloader timer) are independent of clinical disease site and location of administered dose. While the influence of medium on dose calculation can be substantial for low energy sources or non-deeply seated implants, the influence of medium is of minor importance for high-energy sources in the pelvic region. The level of uncertainties due to target, organ, applicator, and/or source movement in relation to the geometry assumed for treatment planning is highly dependent on fractionation and the level of image guided adaptive treatment. Most studies to date report the results in a manner that allows no direct reproduction and further comparison with other studies. Often, no distinction is made between variations, uncertainties, and errors or mistakes. The literature review facilitated the drafting of recommendations for uniform uncertainty reporting in clinical BT, which are also provided. The recommended comprehensive uncertainty investigations are key to obtain a general impression of uncertainties, and may help to identify elements of the brachytherapy treatment process that need improvement in terms of diminishing their dosimetric uncertainties. It is recommended to present data on the analyzed parameters (distance shifts, volume changes, source or applicator position, etc.), and also their influence on absorbed dose for clinically-relevant dose parameters (e.g., target parameters such as D₉₀ or OAR doses). Publications on brachytherapy should include a statement of total dose uncertainty for the entire treatment course, taking into account the fractionation schedule and level of image guidance for adaptation.

Conclusions

This report on brachytherapy clinical uncertainties represents a working project developed by the Brachytherapy Physics Quality Assurances System (BRAPHYQS) subcommittee to the Physics Committee within GEC-ESTRO. Further, this report has been reviewed and approved by the American Association of Physicists in Medicine.

Doses to internal organs for various breast radiation techniques--implications on the risk of secondary cancers and cardiomyopathy

Background

Breast cancers are more frequently diagnosed at an early stage and currently have improved long term outcomes. Late normal tissue complications induced by adjuvant radiotherapy like secondary cancers or cardiomyopathy must now be avoided at all cost. Several new breast radiotherapy techniques have been developed and this work aims at comparing the scatter doses of internal organs for those techniques.

Methods

A CT-scan of a typical early stage left breast cancer patient was used to describe a realistic anthropomorphic phantom in the MCNP Monte Carlo code. Dose tally detectors were placed in breasts, the heart, the ipsilateral lung, and the spleen. Five irradiation techniques were simulated: whole breast radiotherapy 50 Gy in 25 fractions using physical wedge or breast IMRT, 3D-CRT partial breast radiotherapy 38.5 Gy in 10 fractions, HDR brachytherapy delivering 34 Gy in 10 treatments, or Permanent Breast ¹⁰³Pd Seed Implant delivering 90 Gy.

Results

For external beam radiotherapy the wedge compensation technique yielded the largest doses to internal organs like the spleen or the heart, respectively 2,300 mSv and 2.7 Gy. Smaller scatter dose are induced using breast IMRT, respectively 810 mSv and 1.1 Gy, or 3D-CRT partial breast irradiation, respectively 130 mSv and 0.7 Gy. Dose to the lung is also smaller for IMRT and 3D-CRT compared to the wedge technique. For multicatheter HDR brachytherapy a large dose is delivered to the heart, 3.6 Gy, the spleen receives 1,171 mSv and the lung receives 2,471 mSv. These values are 44% higher in case of a balloon catheter. In contrast, breast seeds implant is associated with low dose to most internal organs.

Conclusions

The present data support the use of breast IMRT or virtual wedge technique instead of physical wedges for whole breast radiotherapy. Regarding partial breast irradiation techniques, low energy source brachytherapy and external beam 3D-CRT appear safer than ¹⁹²Ir HDR techniques.

On the use of HDR 60Co source with the MammoSite radiation therapy system

This work summarizes Monte Carlo results in order to evaluate the potential of using HDR 60Co sources in accelerated partial breast irradiation (APBI) with the MammoSite applicator. Simulations have been performed using the MCNP5 Monte Carlo Code, in simple geometries comprised of two concentric spheres; the internal consisting of selected concentrations, C, of a radiographic contrast solution in water (Omnipaque 300) to simulate the MammoSite balloon and the external consisting of water to simulate surrounding tissue. The magnitude of the perturbation of delivered dose due to the radiographic contrast medium used in the MammoSite applicator is calculated. At the very close vicinity of the balloon surface, a dose build-up region is observed, which leads to a dose overestimation by the treatment planning system (TPS) which depends on Omnipaque 300 solution concentration (and is in order of 2.3%, 3.0%, and 4.5%, respectively, at 1 mm away from the balloon - water interface, for C=10%, 15%, and 20%). However, dose overestimation by the TPS is minimal for points lying at the prescription distance (d=1 cm) or beyond, for all simulated concentrations and radii of MammoSite balloon. An analytical estimation of the integral dose outside the CTV in the simple geometries simulated shows that dose to the breast for MammoSite applications is expected to be comparable using HDR 60Co and 192Ir sources, and higher than that for 169Yb. The higher enegies of 60Co sources result to approximately twice radiation protection requirements as compared to 169Ir sources. However, they allow for more accurate dosimetry calculation with currently used treatment planning algorithms for 60Co sources, compared to 169Ir.

Monte Carlo characterization of the M-19 high dose rate Iridium-192 brachytherapy source

The MCNP5 Monte Carlo code was used to simulate the dosimetry of an M-19 iridium-192 high dose rate brachytherapy source in both air/vacuum and water environments with the in-air photon spectrum filtered to remove low-energy photons below delta=10 keV. Dosimetric data was organized into an away-along table and was used to derive the updated AAPM Task Group Report No. 43 (TG-43U1) parameters including S(K), D(r, theta), lamda, gL(r), F(r, theta), phi an(r), and phi an, and their respective statistical uncertainties.