TG-43 Calculations Research Articles

TU-A-137-01: High Energy Brachytherapy Dosimetry as It Applies to Accelerated Partial Breast Irradiation (APBI)

Report 229 is the newest report on high energy brachytherapy dosimetry. It investigates many known issues with accurately doing brachytherapy dose calculations and modeling of TG43 parameters for sources with photon energies >50keV. Two issues that are described in the report are the presence of tissue heterogeneities and scatter corrections when doing brachytherapy treatments. Both of these issues are well manifested when doing partial breast brachytherapy. The applicators for breast brachytherapy are either filled with high Z contrast material (Mammosite or Contura) or are filled with air (Savi) which can create either a dose decrease or dose increase in the PTV compared to a standard TG43 calculation. Often times when these devices are used, the applicators are in close proximity to the skin surface which results in a loss of CPE and can actually decrease the dose compared to what one would expect when doing a TG43 calculation. The applicators are also sometimes near the lung or ribs which also creates a dose discrepancy compared to treatment planning systems. The purpose of this talk will be to describe the physics behind these issues and provide an overview of published literature which investigates the nature of the dose discrepancies encountered with partial breast brachytherapy. It will also describe the magnitude of the dose discrepancy for a variety of clinical cases. Learning Objectives: 1. To understand the physics behind high energy brachytherapy as applied to APBI 2. To understand when TG43 calculations may not accurately describe the clinical situation 3. To understand the nature of these dose discrepancies and be able to provide magnitudes for clinical approximations 4. Some of the research presented was funded by an MRA with Varian Medical Systems.

SU-E-T-113: Quantitative Evaluation of Dosimetric Accuracy of a Multiple-Diode-Array Detector (MapCHECK2) for High-Dose-Rate Ir-192 Brachytherapy

Purpose: To evaluate dosimetric accuracy of a multiple-diode array detector (MapCHECK2 ) for high-dose-rate (HDR) brachytherapy Ir-192 source using two-dimensional dose distributions measured with MapCHECK2, EBT2 Gafchromic films, and TG-43 calculations.Materials and Methods: Plans with different patterns of dwell positions and times were delivered to MapCHECK2 and EBT2 films using a remote afterloader Ir-192 HDR brachytherapy Nucletron system. A catheter was placed between phantom slabs with Gafchromic films and the MapCHECK2 positioned 2cm above and 2cm below the source dwell positions, respectively. The dose distributions were also calculated with TG-43 using the phantom setup and I-192 source parameters to compare with dose measurements from MapCHECK2 and film. Results: Dose distributions measured with MapCHECK2 and calculated with TG-43 were within 5% agreement. Similarly, the dose distributions measured with MapCHECK and EBT2 film agreed within 5% in the range from 10–800 cGy. However, for doses larger than 800 cGy, EBT2 films saturated. In the low range, for doses below 10 cGy, the MapCHECK2 and film measurements were noisy and deviated up to 15% from the TG-43 calculations. The advantage of MapCHECK2 over Gafchromic film is linear response and stability over a wide range of dose levels and dose rates produced from Ir-192 sources. No over-response of MapCHECK2 dose from Ir-192 was observed. The 2D dose distributions from MapCHECK2 are obtained with small detectors and high spatial resolution at positions where the diode detectors are located. However, the separation of the detectors (7 mm) limits the overall spatial resolution of MapCHECK2 in comparison with films. Conclusion:MapCHECK2 provided accurate measurements of 2D-dose distributions from HDR Ir-192 source as validated by TG-43 calculations and EBT2 measurements. The response of MapCHECK2 was linear over a wide range of dose levels and dose-rates while the response of EBT2 Gafchromic film was non-linear and unstable.

SU-E-T-549: Validation of a Commercial Model-Based Collapsed Cone Dose Algorithm for High Dose-Rate Gynecological Brachytherapy Treatment Planning

Purpose: To evaluate the accuracy of calculations from a model-based collapsed cone (MBCC) dose calculation algorithm developed for high dose-rate brachytherapy applications. Methods: The algorithm was developed for use in a commercial brachytherapy treatment planning system (Oncentra Brachy, Nucletron an Elekta Company). The MBCC algorithm was used to calculate dose in water for (1) a mHDR-v2 192Ir source, (2) a CT/MR-compatible ovoid applicator set, and (3) a shielded Fletcher-Williamson ovoid. For validation, MBCC algorithm calculations were compared to Monte Carlo (MC) simulations for equivalent geometries. The MC models were constructed using a multi-purpose code, and were previously benchmarked against film measurements. The resulting dose distributions from each calculation approach were compared using isodose overlays and gamma analysis using 2% dose difference and 2 mm distance to agreement metrics. Results: Comparison of the MBCC and MC calculations for the 192Ir source and the CT/MR compatible ovoid set demonstrated excellent agreement, with 100% of MBCC dose points showing agreement within the stated criteria. As expected, agreement between MBCC and MC was lower for the shielded Fletcher-Williamson tandem and ovoid applicator set due to the presence of high-density tungsten alloy shields. However, even in this case, the MBCC algorithm showed 95.5% and 97.4% of dose points within the stated tolerance of the MC calculation in the mid-plane and cross-plane of the ovoid, respectively. All MBCC dose points showed 100% agreement with MC calculations beyond 5 mm the ovoid shields. Conclusion: The MBCC dose algorithm demonstrates excellent agreement with MC calculations for various gynecological brachytherapy applicator components in water, and offers the potential to improve the accuracy of dose calculations compared with current, clinically-utilized TG-43 calculations. research was funded by a grant from Nucletron, an Elekta Company

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.

The dosimetric impact of heterogeneity corrections in high-dose-rate 192Ir brachytherapy for cervical cancer: Investigation of both conventional Point-A and volume-optimized plans

PurposeTo evaluate the dosimetric impact of heterogeneity corrections on both conventional and volume-optimized high-dose-rate (HDR) 192Ir brachytherapy tandem-and-ovoid treatment plans. Methods and MaterialsBoth conventional and volume-optimized treatment plans were retrospectively created using eight unique CT data sets. In the volume-optimized plans, the clinical target volume (CTV) and organs-at-risk (rectum, bladder, and sigmoid) were contoured on the CT data sets by a single physician. For each plan, dose calculations representing homogeneous water medium were performed using the Task Group (TG-43) formalism and dose calculations with heterogeneity corrections were performed using a commercially available treatment planning system. ResultsFor the conventional plans, the change in dose between TG-43 and heterogeneity-corrected calculations was assessed for the following points: Point-A (left and right) and International Commission on Radiation Units and Measurements (ICRU) 38 defined rectum and bladder points. It was found that the dose to the ICRU bladder decreased the most (−2.2±0.9%), whereas ICRU rectum (−1.7±0.8%), Point-A right (−1.1±0.4%), and Point-A left (−1.0±0.3%) also showed decreases with heterogeneity-corrected calculations. For the volume-optimized plans, the change in dose between TG-43 and heterogeneity-corrected calculations was assessed for the following dose–volume histogram parameters: D90 of the CTV and D2cc of the rectum, bladder, and sigmoid. It was found that D90 of the CTV decreased by −1.9±0.7% and D2cc decreased by −2.6±1.4%, −1.0±0.4%, and −2.0±0.6% for the rectum, bladder and sigmoid, respectively, with heterogeneity-corrected calculations. ConclusionsHeterogeneity corrections on high-dose rate plans were found to have only a small dosimetric impact over TG-43-based dose calculations for both conventional Point-A and volume-optimized plans.

Dosimetry comparison between TG-43 and Monte Carlo calculations using the Freiburg flap for skin high-dose-rate brachytherapy

PurposeThe purpose of this work was to evaluate whether the delivered dose to the skin surface and at the prescription depth when using a Freiburg flap applicator is in agreement with the one predicted by the treatment planning system (TPS) using the TG-43 dose-calculation formalism. Methods and MaterialsMonte Carlo (MC) simulations and radiochromic film measurements have been performed to obtain dose distributions with the source located at the center of one of the spheres and between two spheres. Primary and scatter dose contributions were evaluated to understand the role played by the scatter component. A standard treatment plan was generated using MC- and TG-43-based TPS applying the superposition principle. ResultsThe MC model has been validated by performing additional simulations in the same conditions but transforming air and Freiburg flap materials into water to match TG-43 parameters. Both dose distributions differ less than 1%. Scatter defect compared with TG-43 data is up to 15% when the source is located at the center of the sphere and up to 25% when the source is between two spheres. Maximum deviations between TPS- and MC-based distributions are of 5%. ConclusionsThe deviations in the TG-43-based dose distributions for a standard treatment plan with respect to the MC dose distribution calculated taking into account the composition and shape of the applicator and the surrounding air are lower than 5%. Therefore, this study supports the validity of the TPS used in clinical practice.

SU-E-T-391: Is 1D Anisotropy Function Recommended by TG-43U1 Protocol a Good Candidate for Clinical Brachytherapy Dosimetry?

Purpose: The effect of the anisotropy on dose distributions for real permanent prostate implants with 125I seeds has been evaluated and compared with Monte Carlo (MC) calculations. Methods: soSeed (Bebig) was used. Dose distribution of the prostate implants was calculated using MCNP5 code and compared to TG43 calculation. A DVH comparison between 2D and 1D approximations was performed for prostate, urethra and rectum for 20 patients. Results: A DVHs comparison between the use of 1D & 2D anisotropy functions are presented for 20 prostates implants. For prostate, Dmean, D90 and D98 are underestimated by 1D approximation by about 3%, 1.5% and 1% respectively. It leads also to inconsistent results for Dmin and D100. 1D approximation overestimates the dose for about of 15% in planes situated at the two ends of implant containing no implanted seeds while it underestimates dose about 5% in the other planes. This yields a compensating effect in the resulting DVH's. These over dosage and under dosage must be attributed to an averaging effect between 2D and 1D anisotropy function. It can be noticed that the effect of 1D approximation on D98 and Dmin of prostate varies from patient to another. This is due to the many factors, for example, prostate volume, number of seeds and seeds positions relative to prostate…etc. D90 is always underestimated by 1D approximation where for 70% of patients D90 exceeds 185 Gy. For the urethra, 1D approximation underestimates the Dmax in which 60% of patients, their maximum dose for the urethra exceeds the limits. Finally, 1D approximation underestimates V100 about 50% les than 2D approximation Conclusions: The use of the 2D anisotropy function is preferred to the 1D anisotropy function as this approximation yields more consistent results for the 20 patients studied. The observed overestimation and the variability among the patients can influence dose—outcome.

A Review of Multi-Lumen Breast Brachytherapy for Savi With Acuros Software

Purpose: To compare new commercially available software called Acuros to conventional TG43 formalism dose calculations for patients undergoing accelerated partial breast irradiation with SAVI. Materials and Methods: Currently available software, based on TG43 formalism, calculates brachytherapy dose distributions assuming the surrounding media is equivalent to water. This approximation does not take into account tissue-air interfaces, which could erroneously increase the dose distributions near the air boundaries. Recent papers have analyzed tissue-air interfaces with MCNP calculations and in-vitro measurements. The results as stated in the papers confirm that tissue-air interfaces play a large role in the buildup of dose seen at these interfaces. In this research, patients who received treatments with SAVI interstitial devices were calculated on Varian Brachyvision utilizing both TG 43 and Acuros software. All patients were treated at Arizona Center for Cancer Care, Peoria Arizona. For both algorithms, clinically significant statistics from the RTOG 0413 APBI study were compared. Structures that are defined from RTOG 0413 protocol are as follows: balloon, breast tissue, and PTV_EVAL (which is defined as 1 cm expansion of the balloon but contained between 5mm from skin surface and the chest wall). Our study compared the results of the two algorithms for SAVI patients that were treated with varying volumes of PTV_EVAL and skin distances. Results: The data shows that in all cases of small skin distances!2 mm to large skin distances of 10 mm that the maximum skin doses were lower for all Acuros calculations when compared to the TG43 formalism. The results were the same regardless of the volume of the PTV_EVAL and the coume of air cavity that remains insdie the multi-lumen struts as seen by CT evaluation. Conclusions: The Acuros calculations showed a noticeable decrease in the maximum dose to the skin when compared to the TG43 formalism. This result is primarily due to the lack of full backscatter that would otherwise have been assumed when using the TG43 calculations. While the air cavities inside the device would have predicted a greater contribution at a distance the tissue-air interface reduced the effect.

Modified COMS Plaques for 125I and 103Pd Iris Melanoma Brachytherapy

Novel plaques are used to treat iris melanoma at the Mayo Clinic Rochester. The plaques are a modification of the Collaborative Ocular Melanoma Study (COMS) 22 mm plaque design with a gold alloy backing, outer lip, and silicone polymer insert. An inner lip surrounds a 10 mm diameter cutout region at the plaque center. Plaques span 360°, 270°, and 180° arcs. This article describes dosimetry for these plaques and others used in the treatment of anterior eye melanomas. The EGSnrc user-code BrachyDose is used to perform Monte Carlo simulations. Plaques and seeds are fully modeled. Three-dimensional dose distributions for different plaque models, TG-43 calculations, and (125)I (model 6711) and (103)Pd (model 200) seeds are compared via depth-dose curves, tabulation of doses at points of interest, and isodose contours. Doses at points of interest differ by up to 70% from TG-43 calculations. The inner lip reduces corneal doses. Matching plaque arc length to tumor extent reduces doses to eye regions outside the treatment area. Maintaining the same prescription dose, (103)Pd offers lower doses to critical structures than (125)I, with the exception of the sclera adjacent to the plaque. The Mayo Clinic plaques offer several advantages for anterior eye tumor treatments. Doses to regions outside the treatment area are significantly reduced. Doses differ considerably from TG-43 predictions, illustrating the importance of complete Monte Carlo simulations. Calculations take a few minutes on a single CPU, making BrachyDose sufficiently fast for routine clinical treatment planning.

A Method to Check the Design and Specification of Seeds in Low-Dose-Rate Brachytherapy

Purpose: Using 125I or 103Pd in LDR brachytherapy for prostate implants is a worldwide well-established method. An important factor for the success of the treatment is the exact knowledge of the dose distribution. Each seed type has its own dosimetric characteristics. These can be expressed for example by the TG-43 calculation method that allow for an accurate dose calculation. Due to the different design of the seed models they have an own dosimetric characterization. When using a modern seed treatment planning system the user must rely on that the design of the seeds and the TG-43 data set that is based on this design.

Monte Carlo study of LDR seed dosimetry with an application in a clinical brachytherapy breast implant

A Monte Carlo (MC) study was carried out to evaluate the effects of the interseed attenuation and the tissue composition for two models of 125I low dose rate (LDR) brachytherapy seeds (Medi-Physics 6711, IBt InterSource) in a permanent breast implant. The effect of the tissue composition was investigated because the breast localization presents heterogeneities such as glandular and adipose tissue surrounded by air, lungs, and ribs. The absolute MC dose calculations were benchmarked by comparison to the absolute dose obtained from experimental results. Before modeling a clinical case of an implant in heterogeneous breast, the effects of the tissue composition and the interseed attenuation were studied in homogeneous phantoms. To investigate the tissue composition effect, the dose along the transverse axis of the two seed models were calculated and compared in different materials. For each seed model, three seeds sharing the same transverse axis were simulated to evaluate the interseed effect in water as a function of the distance from the seed. A clinical study of a permanent breast 125I implant for a single patient was carried out using four dose calculation techniques: (1) A TG-43 based calculation, (2) a full MC simulation with realistic tissues and seed models, (3) a MC simulation in water and modeled seeds, and (4) a MC simulation without modeling the seed geometry but with realistic tissues. In the latter, a phase space file corresponding to the particles emitted from the external surface of the seed is used at each seed location. The results were compared by calculating the relevant clinical metrics V85, V100, and V200 for this kind of treatment in the target. D90 and D50 were also determined to evaluate the differences in dose and compare the results to the studies published for permanent prostate seed implants in literature. The experimental results are in agreement with the MC absolute doses (within 5% for EBT Gafchromic film and within 7% for TLD-100). Important differences between the dose along the transverse axis of the seed in water and in adipose tissue are obtained (10% at 3.5 cm). The comparisons between the full MC and the TG-43 calculations show that there are no significant differences for V85 and V100. For V200, 8.4% difference is found coming mainly from the tissue composition effect. Larger differences (about 10.5% for the model 6711 seed and about 13% for the InterSource125) are determined for D90 and D50. These differences depend on the composition of the breast tissue modeled in the simulation. A variation in percentage by mass of the mammary gland and adipose tissue can cause important differences in the clinical dose metrics V200, D90, and D50. Even if the authors can conclude that clinically, the differences in V85, V100, and V200 are acceptable in comparison to the large variation in dose in the treated volume, this work demonstrates that the development of a MC treatment planning system for LDR brachytherapy will improve the dose determination in the treated region and consequently the dose-outcome relationship, especially for the skin toxicity.

Sci-Sat AM(2): Brachy-08: Monte Carlo calculations of 192Ir high dose rate brachytherapy treatment plans using CT and cone beam CT images.

The feasibility of using cone beam computed tomography (CBCT) images for Monte Carlo (MC) brachytherapy dose calculations has been investigated. To evaluate the effects of tissue heterogeneities and finite patient dimensions for 192Ir high dose rate treatment, CT-based MC calculations for breast and head and neck cases were first performed using the PTRAN_CT photon transport code. PTRAN_CT is an accelerated MC code specifically designed for patient-specific dose calculations. Muscles and adipose tissues, which are nearly indistinguishable in CBCT images, are found to cause minimal dose perturbations at 192Ir energies compared to water. The proximity of the tumor to the skin, however, will have an observable impact on the dose up to a few percent. Therefore, for CBCT calculations, a reasonable assignment of material and density values to the patient voxel geometry, with a good delineation of the skin and bony structures, will suffice for MC dose calculations. A CBCT-based calculation for an actual treatment plan with the tumor close to the cheek was performed. The results were compared to TG43 calculations to quantify the dose differences in the target and critical structures. Since the dose delivered to the tumor is mostly primary dose, deviations are found mostly in the organs at risk where scatter contribution becomes more significant. This study shows that for HDR brachytherapy applications, CBCT-based MC calculations is a feasible option despite inferior image quality and larger uncertainties in the Hounsfield Units compared to CT images. Research supported by Nucletron BV.