Generation and comparison of 3D dosimetric reference datasets for COMS eye plaque brachytherapy using model-based dose calculations.

PURPOSE: A joint Working Group of the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) was created to aid in the transition from the AAPM TG-43 dose calculation formalism, the current standard, to model-based dose calculations. This work establishes the first test cases for low-energy photon-emitting brachytherapy using model-based dose calculation algorithms (MBDCAs). ACQUISITION AND VALIDATION METHODS: Five test cases are developed: (1) a single model 6711 125 I brachytherapy seed in water, 13 seeds (2) individually and (3) in combination in water, (4) the full Collaborative Ocular Melanoma Study (COMS) 16 mm eye plaque in water, and (5) the full plaque in a realistic eye phantom. Calculations are done with four Monte Carlo (MC) codes and a research version of a commercial treatment planning system (TPS). For all test cases, local agreement of MC codes was within ∼2.5% and global agreement was ∼2% (4% for test case 5). MC agreement was within expected uncertainties. Local agreement of TPS with MC was within 5% for test case 1 and ∼20% for test cases 4 and 5, and global agreement was within 0.4% for test case 1 and 10% for test cases 4 and 5. DATA FORMAT AND USAGE NOTES: Dose distributions for each set of MC and TPS calculations are available online (https://doi.org/10.52519/00005) along with input files and all other information necessary to repeat the calculations. POTENTIAL APPLICATIONS: These data can be used to support commissioning of MBDCAs for low-energy brachytherapy as recommended by TGs 186 and 221 and AAPM Report 372. This work additionally lays out a sample framework for the development of test cases that can be extended to other applications beyond eye plaque brachytherapy.

AAPM WGDCAB Report 372: A joint AAPM, ESTRO, ABG, and ABS report on commissioning of model-based dose calculation algorithms in brachytherapy.

The introduction of model-based dose calculation algorithms (MBDCAs) in brachytherapy provides an opportunity for a more accurate dose calculation and opens the possibility for novel, innovative treatment modalities. The joint AAPM, ESTRO, and ABG Task Group 186 (TG-186) report provided guidance to early adopters. However, the commissioning aspect of these algorithms was described only in general terms with no quantitative goals. This report, from the Working Group on Model-Based Dose Calculation Algorithms in Brachytherapy, introduced a field-tested approach to MBDCA commissioning. It is based on a set of well-characterized test cases for which reference Monte Carlo (MC) and vendor-specific MBDCA dose distributions are available in a Digital Imaging and Communications in Medicine-Radiotherapy (DICOM-RT) format to the clinical users. The key elements of the TG-186 commissioning workflow are now described in detail, and quantitative goals are provided. This approach leverages the well-known Brachytherapy Source Registry jointly managed by the AAPM and the Imaging and Radiation Oncology Core (IROC) Houston Quality Assurance Center (with associated links at ESTRO) to provide open access to test cases as well as step-by-step user guides. While the current report is limited to the two most widely commercially available MBDCAs and only for 192 Ir-based afterloading brachytherapy at this time, this report establishes a general framework that can easily be extended to other brachytherapy MBDCAs and brachytherapy sources. The AAPM, ESTRO, ABG, and ABS recommend that clinical medical physicists implement the workflow presented in this report to validate both the basic and the advanced dose calculation features of their commercial MBDCAs. Recommendations are also given to vendors to integrate advanced analysis tools into their brachytherapy treatment planning system to facilitate extensive dose comparisons. The use of the test cases for research and educational purposes is further encouraged.

Delivered dose changes in COMS plaque-based ocular brachytherapy arising from vitrectomy with silicone oil replacement.

PURPOSE: The purpose of the study was to determine dosimetric effects of performing concurrent I-125 Collaborative Ocular Melanoma Study plaque brachytherapy and vitrectomy with replacement using silicone oil, previously shown to be a means of shielding uninvolved parts of the eye. METHODS AND MATERIALS: Monte Carlo simulations using MCNP6 were performed to compare the dosimetry with all eye materials assigned as water, and for the vitreous (excluding the tumor), composed of polydimethylsiloxane oil for three generic, one large tumor, and two patient geometry scenarios. Dose was scored at the tumor apex, along the sclera, and within a 3D grid encompassing the eye. The assessed patient cases included vitrectomies to treat intraocular pathologies; not to enhance attenuation/shielding. RESULTS: The doses along the sclera and for the entire eye were decreased when the silicone oil replaced the vitreal fluid, with a maximum decrease at the opposite sclera of 63%. Yet, absolute changes in dose to critical structures were often small and likely not clinically significant. The dose at the tumor apex was decreased by 3.1-9.4%. Dose was also decreased at the edges of the tumor because of decreased backscatter at the tumor-oil interface. CONCLUSIONS: Concurrent silicone vitrectomy was found to reduce total radiation dose to the eye. Based on current radiation retinopathy predictive models, the evaluation of the absolute doses revealed only a subset of patients in which a clinically significant difference in outcomes is expected. Furthermore, the presence of the silicone oil decreased dose to the tumor edges, indicating that the tumor could be underdosed if the oil is unaccounted for.

Robotic-Assisted Needle Steering Around Anatomical Obstacles Using Notched Steerable Needles.

Robotic-assisted needle steering can enhance the accuracy of needle-based interventions. Application of current needle steering techniques are restricted by the limited deflection curvature of needles. Here, a novel steerable needle with improved curvature is developed and used with an online motion planner to steer the needle along curved paths inside tissue. The needle is developed by carving series of small notches on the shaft of a standard needle. The notches decrease the needle flexural stiffness, allowing the needle to follow tightly curved paths with small radius of curvature. In this paper, first, a finite element model of the notched needle deflection in tissue is presented. Next, the model is used to estimate the optimal location for the notches on needle's shaft for achieving a desired curvature. Finally, an ultrasound-guided motion planner for needle steering inside tissue is developed and used to demonstrate the capability of the notched needle in achieving high curvature and maneuvering around obstacles in tissue. We simulated a clinical scenario in brachytherapy, where the target is obstructed by the pubic bone and cannot be reached using regular needles. Experimental results show that the target can be reached using the notched needle with a mean accuracy of 1.2 mm. Thus, the proposed needle enables future research on needle steering toward deeper or more difficult-to-reach targets.

Advanced Collapsed cone Engine dose calculations in tissue media for COMS eye plaques loaded with I-125 seeds.

PURPOSE: To investigate the dose calculation accuracy of the Advanced Collapsed cone Engine (ACE) algorithm for ocular brachytherapy using a COMS plaque loaded with I-125 seeds for two heterogeneous patient tissue scenarios. METHODS: The Oncura model 6711 I-125 seed and 16 mm COMS plaque were added to a research version (v4.6) of the Oncentra® Brachy (OcB) treatment planning system (TPS) for dose calculations using ACE. Treatment plans were created for two heterogeneous cases: (a) a voxelized eye phantom comprising realistic eye materials and densities and (b) a patient CT dataset with variable densities throughout the dataset. ACE dose calculations were performed using a high accuracy mode, high-resolution calculation grid matching the imported CT datasets (0.5 × 0.5 × 0.5 mm3 ), and a user-defined CT calibration curve. The accuracy of ACE was evaluated by replicating the plan geometries and comparing to Monte Carlo (MC) calculated doses obtained using MCNP6. The effects of the heterogeneous patient tissues on the dose distributions were also evaluated by performing the ACE and MCNP6 calculations for the same scenarios but setting all tissues and air to water. RESULTS: Average local percent dose differences between ACE and MC within contoured structures and at points of interest for both scenarios ranged from 1.2% to 20.9%, and along the plaque central axis (CAX) from 0.7% to 7.8%. The largest differences occurred in the plaque penumbra (up to 17%), and at contoured structure interfaces (up to 20%). Other regions in the eye agreed more closely, within the uncertainties of ACE dose calculations (~5%). Compared to that, dose differences between water-based and fully heterogeneous tissue simulations were up to 27%. CONCLUSIONS: Overall, ACE dosimetry agreed well with MC in the tumor volume and along the plaque CAX for the two heterogeneous tissue scenarios, indicating that ACE could potentially be used for clinical ocular brachytherapy dosimetry. In general, ACE data matched the fully heterogeneous MC data more closely than water-based data, even in regions where the ACE accuracy was relatively low. However, depending on the plaque position, doses to critical structures near the plaque penumbra or at tissue interfaces were less accurate, indicating that improvements may be necessary. More extensive knowledge of eye tissue compositions is still required.

Initial evaluation of Advanced Collapsed cone Engine dose calculations in water medium for I-125 seeds and COMS eye plaques.

PURPOSE: To investigate the dose calculation accuracy in water medium of the Advanced Collapsed cone Engine (ACE) for three sizes of COMS eye plaques loaded with low-energy I-125 seeds. METHODS: A model of the Oncura 6711 I-125 seed was created for use with ACE in Oncentra® Brachy (OcB) using primary-scatter separated (PSS) point dose kernel and Task Group (TG) 43 datasets. COMS eye plaque models of diameters 12, 16, and 20 mm were introduced into the OcB applicator library based on 3D CAD drawings of the plaques and Silastic inserts. To perform TG-186 level 1 commissioning, treatment plans were created in OcB for a single source in water and for each COMS plaque in water for two scenarios: with only one centrally loaded seed, or with all seed positions loaded. ACE dose calculations were performed in high accuracy mode with a 0.5 × 0.5 × 0.5 mm3 calculation grid. The resulting dose data were evaluated against Monte Carlo (MC) calculated doses obtained with MCNP6, using both local and global percent differences. RESULTS: ACE doses around the source for the single seed in water agreed with MC doses on average within < 5% inside a 6 × 6 × 6 cm3 region, and within < 1.5% inside a 2 × 2 × 2 cm3 region. The PSS data were generated at a higher resolution within 2 cm from the source, resulting in this improved agreement closer to the source due to fewer approximations in the ACE dose calculation. Average differences in both investigated plaque loading patterns in front of the plaques and on the plaque central axes were ≤ 2.5%, though larger differences (up to 12%) were found near the plaque lip. CONCLUSIONS: Overall, good agreement was found between ACE and MC dose calculations for a single I-125 seed and in front of the COMS plaques in water. More complex scenarios need to be investigated to determine how well ACE handles heterogeneous patient materials.

Experimental assessment of the Advanced Collapsed-cone Engine for scalp brachytherapy treatments.

PURPOSE: To experimentally assess the performance of the Advanced Collapsed-cone Engine (ACE) for 192Ir high-dose-rate brachytherapy treatment planning of nonmelanoma skin cancers of the scalp. METHODS AND MATERIALS: A layered slab phantom was designed to model the head (skin, skull, and brain) and surface treatment mold using tissue equivalent materials. Six variations of the phantom were created by varying skin thickness, skull thickness, and size of air gap between the mold and skin. Treatment planning was initially performed using the Task Group 43 (TG-43) formalism with CT images of each phantom variation. Doses were recalculated using standard and high accuracy modes of ACE. The plans were delivered to Gafchromic EBT3 film placed between different layers of the phantom. RESULTS: Doses calculated by TG-43 and ACE and those measured by film agreed with each other at most locations within the phantoms. For a given phantom variation, average TG-43- and ACE-calculated doses were similar, with a maximum difference of (3 ± 12)% (k = 2). Compared to the film measurements, TG-43 and ACE overestimated the film-measured dose by (13 ± 12)% (k = 2) for one phantom variation below the skull layer. CONCLUSIONS: TG-43- and ACE-calculated and film-measured doses were found to agree above the skull layer of the phantom, which is where the tumor would be located in a clinical case. ACE appears to underestimate the attenuation through bone relative to that measured by film; however, the dose to bone is below tolerance levels for this treatment.

Initial clinical assessment of "center-specific" automated treatment plans for low-dose-rate prostate brachytherapy.

PURPOSE: To report results of an initial pilot study assessing iodine-125 prostate implant treatment plans created automatically by a new seed-placement method. METHODS AND MATERIALS: A novel mixed-integer linear programming method incorporating spatial constraints on seed locations in addition to standard dose-volume constraints was used to place seeds. The approach, described in detail elsewhere, was used to create treatment plans fully automatically on a retrospective basis for 20 patients having a wide range of prostate sizes and shapes. Corresponding manual plans used for patient treatment at a single institution were combined with the automated plans, and all 40 plans were anonymized, randomized, and independently evaluated by five clinicians using a common scoring tool. Numerical and clinical features of the plans were extracted for comparison purposes. RESULTS: A full 51% of the automated plans were deemed clinically acceptable without any modification by the five practitioners collectively versus 90% of the manual plans. Automated plan seed distributions were for the most part not substantially different from those for the manual plans. Two observed shortcomings of the automated plans were seed strands not intersecting the prostate and strands extending into the bladder. Both are amenable to remediation by adjusting existing spatial constraints. CONCLUSIONS: After spatial and dose-volume constraints are set, the mixed-integer linear programming method is capable of creating prostate implant treatment plans fully automatically, with clinical acceptability sufficient to warrant further investigation. These plans, intended to be reviewed and refined as necessary by an expert planner, have the potential to both save planner time and enhance treatment plan consistency.

A generic TG-186 shielded applicator for commissioning model-based dose calculation algorithms for high-dose-rate 192 Ir brachytherapy.

PURPOSE: A joint working group was created by the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) with the charge, among others, to develop a set of well-defined test case plans and perform calculations and comparisons with model-based dose calculation algorithms (MBDCAs). Its main goal is to facilitate a smooth transition from the AAPM Task Group No. 43 (TG-43) dose calculation formalism, widely being used in clinical practice for brachytherapy, to the one proposed by Task Group No. 186 (TG-186) for MBDCAs. To do so, in this work a hypothetical, generic high-dose rate (HDR) 192 Ir shielded applicator has been designed and benchmarked. METHODS: A generic HDR 192 Ir shielded applicator was designed based on three commercially available gynecological applicators as well as a virtual cubic water phantom that can be imported into any DICOM-RT compatible treatment planning system (TPS). The absorbed dose distribution around the applicator with the TG-186 192 Ir source located at one dwell position at its center was computed using two commercial TPSs incorporating MBDCAs (Oncentra® Brachy with Advanced Collapsed-cone Engine, ACE™, and BrachyVision ACUROS™) and state-of-the-art Monte Carlo (MC) codes, including ALGEBRA, BrachyDose, egs_brachy, Geant4, MCNP6, and Penelope2008. TPS-based volumetric dose distributions for the previously reported "source centered in water" and "source displaced" test cases, and the new "source centered in applicator" test case, were analyzed here using the MCNP6 dose distribution as a reference. Volumetric dose comparisons of TPS results against results for the other MC codes were also performed. Distributions of local and global dose difference ratios are reported. RESULTS: The local dose differences among MC codes are comparable to the statistical uncertainties of the reference datasets for the "source centered in water" and "source displaced" test cases and for the clinically relevant part of the unshielded volume in the "source centered in applicator" case. Larger local differences appear in the shielded volume or at large distances. Considering clinically relevant regions, global dose differences are smaller than the local ones. The most disadvantageous case for the MBDCAs is the one including the shielded applicator. In this case, ACUROS agrees with MC within [-4.2%, +4.2%] for the majority of voxels (95%) while presenting dose differences within [-0.12%, +0.12%] of the dose at a clinically relevant reference point. For ACE, 95% of the total volume presents differences with respect to MC in the range [-1.7%, +0.4%] of the dose at the reference point. CONCLUSIONS: The combination of the generic source and generic shielded applicator, together with the previously developed test cases and reference datasets (available in the Brachytherapy Source Registry), lay a solid foundation in supporting uniform commissioning procedures and direct comparisons among treatment planning systems for HDR 192 Ir brachytherapy.

Experimental verification of Advanced Collapsed-cone Engine for use with a multichannel vaginal cylinder applicator.

Model-based dose calculation algorithms have recently been incorporated into brachytherapy treatment planning systems, and their introduction requires critical evaluation before clinical implementation. Here, we present an experimental evaluation of Oncentra® Brachy Advanced Collapsed-cone Engine (ACE) for a multichannel vaginal cylinder (MCVC) applicator using radiochromic film. A uniform dose of 500 cGy was specified to the surface of the MCVC using the TG-43 dose formalism under two conditions: (a) with only the central channel loaded or (b) only the peripheral channels loaded. Film measurements were made at the applicator surface and compared to the doses calculated using TG-43, standard accuracy ACE (sACE), and high accuracy ACE (hACE). When the central channel of the applicator was used, the film measurements showed a dose increase of (11 ± 8)% (k = 2) above the two outer grooves on the applicator surface. This increase in dose was confirmed with the hACE calculations, but was not confirmed with the sACE calculations at the applicator surface. When the peripheral channels were used, a periodic azimuthal variation in measured dose was observed around the applicator. The sACE and hACE calculations confirmed this variation and agreed within 1% of each other at the applicator surface. Additionally for the film measurements with the central channel used, a baseline dose variation of (10 ± 4)% (k = 2) of the mean dose was observed azimuthally around the applicator surface, which can be explained by offset source positioning in the central channel.

A brief look at model-based dose calculation principles, practicalities, and promise.

Model-based dose calculation algorithms (MBDCAs) have recently emerged as potential successors to the highly practical, but sometimes inaccurate TG-43 formalism for brachytherapy treatment planning. So named for their capacity to more accurately calculate dose deposition in a patient using information from medical images, these approaches to solve the linear Boltzmann radiation transport equation include point kernel superposition, the discrete ordinates method, and Monte Carlo simulation. In this overview, we describe three MBDCAs that are commercially available at the present time, and identify guidance from professional societies and the broader peer-reviewed literature intended to facilitate their safe and appropriate use. We also highlight several important considerations to keep in mind when introducing an MBDCA into clinical practice, and look briefly at early applications reported in the literature and selected from our own ongoing work. The enhanced dose calculation accuracy offered by a MBDCA comes at the additional cost of modelling the geometry and material composition of the patient in treatment position (as determined from imaging), and the treatment applicator (as characterized by the vendor). The adequacy of these inputs and of the radiation source model, which needs to be assessed for each treatment site, treatment technique, and radiation source type, determines the accuracy of the resultant dose calculations. Although new challenges associated with their familiarization, commissioning, clinical implementation, and quality assurance exist, MBDCAs clearly afford an opportunity to improve brachytherapy practice, particularly for low-energy sources.

Semi-Automated Needle Steering in Biological Tissue Using an Ultrasound-Based Deflection Predictor.

The performance of needle-based interventions depends on the accuracy of needle tip positioning. Here, a novel needle steering strategy is proposed that enhances accuracy of needle steering. In our approach the surgeon is in charge of needle insertion to ensure the safety of operation, while the needle tip bevel location is robotically controlled to minimize the targeting error. The system has two main components: (1) a real-time predictor for estimating future needle deflection as it is steered inside soft tissue, and (2) an online motion planner that calculates control decisions and steers the needle toward the target by iterative optimization of the needle deflection predictions. The predictor uses the ultrasound-based curvature information to estimate the needle deflection. Given the specification of anatomical obstacles and a target from preoperative images, the motion planner uses the deflection predictions to estimate control actions, i.e., the depth(s) at which the needle should be rotated to reach the target. Ex-vivo needle insertions are performed with and without obstacle to validate our approach. The results demonstrate the needle steering strategy guides the needle to the targets with a maximum error of 1.22 mm.

Delivered dose uncertainty analysis at the tumor apex for ocular brachytherapy.

PURPOSE: To estimate the total dosimetric uncertainty at the tumor apex for ocular brachytherapy treatments delivered using 16 mm Collaborative Ocular Melanoma Study (COMS) and Super9 plaques loaded with (125)I seeds in order to determine the size of the apex margin that would be required to ensure adequate dosimetric coverage of the tumor. METHODS: The total dosimetric uncertainty was assessed for three reference tumor heights: 3, 5, and 10 mm, using the Guide to the expression of Uncertainty in Measurement/National Institute of Standards and Technology approach. Uncertainties pertaining to seed construction, source strength, plaque assembly, treatment planning calculations, tumor height measurement, plaque placement, and plaque tilt for a simple dome-shaped tumor were investigated and quantified to estimate the total dosimetric uncertainty at the tumor apex. Uncertainties in seed construction were determined using EBT3 Gafchromic film measurements around single seeds, plaque assembly uncertainties were determined using high resolution microCT scanning of loaded plaques to measure seed positions in the plaques, and all other uncertainties were determined from the previously published studies and recommended values. All dose calculations were performed using plaque simulator v5.7.6 ophthalmic treatment planning system with the inclusion of plaque heterogeneity corrections. RESULTS: The total dosimetric uncertainties at 3, 5, and 10 mm tumor heights for the 16 mm COMS plaque were 17.3%, 16.1%, and 14.2%, respectively, and for the Super9 plaque were 18.2%, 14.4%, and 13.1%, respectively (all values with coverage factor k = 2). The apex margins at 3, 5, and 10 mm tumor heights required to adequately account for these uncertainties were 1.3, 1.3, and 1.4 mm, respectively, for the 16 mm COMS plaque, and 1.8, 1.4, and 1.2 mm, respectively, for the Super9 plaque. These uncertainties and associated margins are dependent on the dose gradient at the given prescription depth, thus resulting in the changing uncertainties and margins with depth. CONCLUSIONS: The margins determined in this work can be used as a guide for determining an appropriate apex margin for a given treatment, which can be chosen based on the tumor height. The required margin may need to be increased for more complex scenarios (mushroom shaped tumors, tumors close to the optic nerve, oblique muscle related tilt, etc.) than the simple dome-shaped tumor examined and should be chosen on a case-by-case basis. The sources of uncertainty contributing most significantly to the total dosimetric uncertainty are seed placement within the plaques, treatment planning calculations, tumor height measurement, and plaque tilt. This work presents an uncertainty-based, rational approach to estimating an appropriate apex margin.

A generic high-dose rate (192)Ir brachytherapy source for evaluation of model-based dose calculations beyond the TG-43 formalism.

PURPOSE: In order to facilitate a smooth transition for brachytherapy dose calculations from the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) formalism to model-based dose calculation algorithms (MBDCAs), treatment planning systems (TPSs) using a MBDCA require a set of well-defined test case plans characterized by Monte Carlo (MC) methods. This also permits direct dose comparison to TG-43 reference data. Such test case plans should be made available for use in the software commissioning process performed by clinical end users. To this end, a hypothetical, generic high-dose rate (HDR) (192)Ir source and a virtual water phantom were designed, which can be imported into a TPS. METHODS: A hypothetical, generic HDR (192)Ir source was designed based on commercially available sources as well as a virtual, cubic water phantom that can be imported into any TPS in DICOM format. The dose distribution of the generic (192)Ir source when placed at the center of the cubic phantom, and away from the center under altered scatter conditions, was evaluated using two commercial MBDCAs [Oncentra(®) Brachy with advanced collapsed-cone engine (ACE) and BrachyVision ACUROS™ ]. Dose comparisons were performed using state-of-the-art MC codes for radiation transport, including ALGEBRA, BrachyDose, GEANT4, MCNP5, MCNP6, and PENELOPE2008. The methodologies adhered to recommendations in the AAPM TG-229 report on high-energy brachytherapy source dosimetry. TG-43 dosimetry parameters, an along-away dose-rate table, and primary and scatter separated (PSS) data were obtained. The virtual water phantom of (201)(3) voxels (1 mm sides) was used to evaluate the calculated dose distributions. Two test case plans involving a single position of the generic HDR (192)Ir source in this phantom were prepared: (i) source centered in the phantom and (ii) source displaced 7 cm laterally from the center. Datasets were independently produced by different investigators. MC results were then compared against dose calculated using TG-43 and MBDCA methods. RESULTS: TG-43 and PSS datasets were generated for the generic source, the PSS data for use with the ace algorithm. The dose-rate constant values obtained from seven MC simulations, performed independently using different codes, were in excellent agreement, yielding an average of 1.1109 ± 0.0004 cGy/(h U) (k = 1, Type A uncertainty). MC calculated dose-rate distributions for the two plans were also found to be in excellent agreement, with differences within type A uncertainties. Differences between commercial MBDCA and MC results were test, position, and calculation parameter dependent. On average, however, these differences were within 1% for ACUROS and 2% for ace at clinically relevant distances. CONCLUSIONS: A hypothetical, generic HDR (192)Ir source was designed and implemented in two commercially available TPSs employing different MBDCAs. Reference dose distributions for this source were benchmarked and used for the evaluation of MBDCA calculations employing a virtual, cubic water phantom in the form of a CT DICOM image series. The implementation of a generic source of identical design in all TPSs using MBDCAs is an important step toward supporting univocal commissioning procedures and direct comparisons between TPSs.

Radiochromic film calibration for low-energy seed brachytherapy dose measurement.

PURPOSE: Radiochromic film dosimetry is typically performed for high energy photons and moderate doses characterizing external beam radiotherapy (XRT). The purpose of this study was to investigate the accuracy of previously established film calibration procedures used in XRT when applied to low-energy, seed-based brachytherapy at higher doses, and to determine necessary modifications to achieve similar accuracy in absolute dose measurements. METHODS: Gafchromic EBT3 film was used to measure radiation doses upwards of 35 Gy from 75 kVp, 200 kVp, 6 MV, and (∼28 keV) I-125 photon sources. For the latter irradiations a custom phantom was built to hold a single I-125 seed. Film pieces were scanned with an Epson 10000XL flatbed scanner and the resulting 48-bit RGB TIFF images were analyzed using both FilmQA Pro software andMATLAB. Calibration curves relating dose and optical density via a rational functional form for all three color channels at each irradiation energy were determined with and without the inclusion of uncertainties in the measured optical densities and dose values. The accuracy of calibration curve variations obtained using piecewise fitting, a reduced film measurement area for I-125 irradiation, and a reduced number of dose levels was also investigated. The energy dependence of the film lot used was also analyzed by calculating normalized optical density values. RESULTS: Slight differences were found in the resulting calibration curves for the various fitting methods used. The accuracy of the calibration curves was found to improve at low doses and worsen at high doses when including uncertainties in optical densities and doses, which may better represent the variability that could be seen in film optical density measurements. When exposing the films to doses > 8 Gy, two-segment piecewise fitting was found to be necessary to achieve similar accuracies in absolute dose measurements as when using smaller dose ranges. When reducing the film measurement area for the I-125 irradiations, the accuracy of the calibration curve was degraded due to the presence of localized film heterogeneities. No degradation in the calibration curves was found when reducing the number of calibration points down to only 4, but with piecewise fitting, 6 calibration points as well as a blank film are required. Variations due to photon energy in film optical density of up to 3% were found above doses of 2 Gy. CONCLUSIONS: A modified procedure for performing EBT3 film calibration was established for use with low-energy brachytherapy seeds and high dose exposures. The energy dependence between 6 MV and I-125 photons is significant such that film calibrations should be done with an appropriately low-energy source when performing low-energy brachytherapy dose measurements. Two-segment piecewise fitting with the inclusion of errors in measured optical density as well as dose was found to result in the most accurate calibration curves. Above doses of 1 Gy, absolute dose measurements can be made with an accuracy of 1.6% for 6 MV beams and 5.7% for I-125 seed exposures if using the I-125 source for calibration, or 2.3% if using the 75 kVp photon beam for calibration.

Fast dose kernel interpolation using Fourier transform with application to permanent prostate brachytherapy dosimetry.

PURPOSE: Boyer and Mok proposed a fast calculation method employing the Fourier transform (FT), for which calculation time is independent of the number of seeds but seed placement is restricted to calculation grid points. Here an interpolation method is described enabling unrestricted seed placement while preserving the computational efficiency of the original method. METHODS: The Iodine-125 seed dose kernel was sampled and selected values were modified to optimize interpolation accuracy for clinically relevant doses. For each seed, the kernel was shifted to the nearest grid point via convolution with a unit impulse, implemented in the Fourier domain. The remaining fractional shift was performed using a piecewise third-order Lagrange filter. RESULTS: Implementation of the interpolation method greatly improved FT-based dose calculation accuracy. The dose distribution was accurate to within 2% beyond 3 mm from each seed. Isodose contours were indistinguishable from explicit TG-43 calculation. Dose-volume metric errors were negligible. Computation time for the FT interpolation method was essentially the same as Boyer's method. CONCLUSIONS: A FT interpolation method for permanent prostate brachytherapy TG-43 dose calculation was developed which expands upon Boyer's original method and enables unrestricted seed placement. The proposed method substantially improves the clinically relevant dose accuracy with negligible additional computation cost, preserving the efficiency of the original method.

Transrectal ultrasound based prostate volume determination: is the frustum algorithm more accurate than planimetry?

PURPOSE: To compare reconstructed volumes calculated via planimetry and frustum algorithms in the context of stepped transrectal ultrasound (US) imaging, and to estimate the reconstruction error for prostate volumes. METHODS: Prostate contours for 40 permanent implant patients were delineated on magnetic resonance (MR) and transrectal US images by a radiation oncologist. Simulated images of ellipsoid and truncated cone geometrical objects were constructed to determine volume calculation accuracy. Simulation results were used to deduce the algorithm-associated error made when calculating transrectal US prostate volumes. RESULTS: For imaging without deliberate slice positioning, planimetry reconstruction was mostly accurate while the frustum algorithm underestimated the volume. The discrepancy was mostly due to the end slice reconstruction. For slice positioning that reflected US image acquisition, planimetry overestimated by half the superior slice volume on average while frustum underestimated by half the inferior slice volume. The estimated algorithm errors for prostate contours were 4% and -3%, respectively. CONCLUSIONS: The planimetry and frustum algorithms offer different interpretations for reconstruction and yield systematic differences in calculated volumes. Both algorithms introduce bias into transrectal US prostate volume determinations that may have clinical implications, planimetry overestimating and frustum underestimating the volume.

Dose calculation for photon-emitting brachytherapy sources with average energy higher than 50 keV: report of the AAPM and ESTRO.

PURPOSE: Recommendations of the American Association of Physicists in Medicine (AAPM) and the European Society for Radiotherapy and Oncology (ESTRO) on dose calculations for high-energy (average energy higher than 50 keV) photon-emitting brachytherapy sources are presented, including the physical characteristics of specific (192)Ir, (137)Cs, and (60)Co source models. METHODS: This report has been prepared by the High Energy Brachytherapy Source Dosimetry (HEBD) Working Group. This report includes considerations in the application of the TG-43U1 formalism to high-energy photon-emitting sources with particular attention to phantom size effects, interpolation accuracy dependence on dose calculation grid size, and dosimetry parameter dependence on source active length. RESULTS: Consensus datasets for commercially available high-energy photon sources are provided, along with recommended methods for evaluating these datasets. Recommendations on dosimetry characterization methods, mainly using experimental procedures and Monte Carlo, are established and discussed. Also included are methodological recommendations on detector choice, detector energy response characterization and phantom materials, and measurement specification methodology. Uncertainty analyses are discussed and recommendations for high-energy sources without consensus datasets are given. CONCLUSIONS: Recommended consensus datasets for high-energy sources have been derived for sources that were commercially available as of January 2010. Data are presented according to the AAPM TG-43U1 formalism, with modified interpolation and extrapolation techniques of the AAPM TG-43U1S1 report for the 2D anisotropy function and radial dose function.

Impact of edema and seed movement on the dosimetry of prostate seed implants.

This article summarizes current knowledge concerning the characterization of prostatic edema and intra-prostatic seed movement as these relate to dosimetry of permanent prostate implants, and reports the initial application to clinical data of a new edema model used in calculating pre- and post-implant dose distributions. Published edema magnitude and half-life parameters span a broad range depending on implant technique and measurement uncertainty, hence clinically applicable values should be determined locally. Observed intra-prostatic seed movements appear to be associated with particular aspects of implant technique and could be minimized by technique modification. Using an extended AAPM TG-43 formalism incorporating the new edema model, relative dose error RE associated with neglecting edema was calculated for three I-125 seed implants (18.9 cc, 37.6 cc, 60.2 cc) performed at our center. Pre- and post-plan RE average values and ranges in a 50 × 50 × 50 mm(3) calculation volume were similar at ~2% and ~0-3.5%, respectively, for all three implants; however, the spatial distribution of RE varied for different seed configurations. Post-plan values of D90 and V100 for prostate were reduced by ~2% and ~1%, respectively. In cases where RE is not clinically negligible as a consequence of large edema magnitude and / or use of Pd-103 seeds, the dose calculation method demonstrated here can be applied to account for edema explicitly and there by improve the accuracy of clinical dose estimates.