In image-guided surgery, deformation of soft tissues can cause substantial errors in targeting internal targets, since deformation can affect the translation of preoperative image-based surgical ...plans during surgery. Having a realistic tissue deformation simulator could enhance the accuracy of internal targets localization by giving an accurate estimation of the deformation applied to a preoperative model of the organ. A key challenge is to address the sim-to-real gap between the simulator and the actual intraoperative behavior of the tissue. The sim-to-real transfer challenge is addressed by formulating the problem as a probabilistic inference over a low-dimensional representation of deformed objects. The proposed method utilizes a generative variational autoencoder structure based on graph neural networks (GNN-VAE) to generate a probabilistic low-dimensional representation of the outputs of a physics-based simulator. To match simulation data to real data, the resultant low-dimensional distribution (i.e., prior distribution) is updated iteratively using an ensemble smoother with multiple data assimilation. The advantages of the proposed method are first, it only uses simulation data for training the GNN-VAE, and no retraining of GNN-VAE is required intraoperatively, second, it does not require estimating the mechanical properties of the tissue it is simulating, and third, is able to work with any physic-based simulator. The proposed framework was verified both in experimental and simulation studies and showed it can reduce the registration error in tissue deformation.
Robotics-assisted needle steering can enhance targeting accuracy in percutaneous interventions. This paper presents a novel dynamical model for robotically controlled needle steering. This is the ...first model that predicts both needle shape and tip position in soft tissue, and accepts needle insertion velocity, needle 180° axial rotation, and needle base force/torque as inputs. A hybrid formulation of needle steering dynamics in soft tissue is presented, which considers the needle as a two-body rigid/flexible coupled system composed of a moving, discrete, and rigid part attached to a vibrating compliant part that is subject to external excitation forces. The former is the carrier representing the surgeon's hand or the needle inserting robot, while the latter is a beam modeling the continuous deflection of the needle inside tissue. A novel time-delayed tissue model and a fracture mechanics-based model are developed to model the tissue reaction forces and cutting force at the needle tip, respectively. Experiments are performed on synthetic and ex vivo animal tissues to identify the model parameters and validate the needle steering model. The maximum error of the 2-D model in predicting the needle tip position in the insertion plane was 1.59 mm in the case of no axial rotation and 0.74 mm with axial rotation.
In low-dose-rate permanent-seed (LDR-PS) implant brachytherapy, it is crucial to predict the movement of internal target points (planned radioactive seed locations) under the effect of external ...forces. Accurate prediction of the target locations is critical for precise seed implantation, as inaccurate seed implantation diminishes the effectiveness of radiotherapy. Therefore, developing a model to simulate tissue dynamics is necessary. All physics-based tissue models have model-reality mismatches due to unmodeled dynamics, a problem which should be addressed. In this work, we propose the KF-ADMM method as a solution, which compensates for a portion of unmodeled dynamic terms existing in the alternating direction method of multipliers (ADMM)-based projective dynamics (PD) tissue simulator through Kalman filtering. This method provides accurate predictions of the location of inner tissue points with an error of around 0.8 mm. Experiments on a breast tissue phantom are performed to evaluate the efficacy of the proposed approach. According to the results, the accuracy of tissue deformation is enhanced by <inline-formula><tex-math notation="LaTeX">52 \%</tex-math></inline-formula> on average, and the convergence rate is accelerated compared to an ADMM-based PD tissue simulator.
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.
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 125I 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.
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.
In surgical operations, tissue manipulation can be automated to reduce the surgeon's workload. This work addresses the application of tissue manipulation in breast brachytherapy, which involves ...manipulating an internal target inside the breast. Unassisted breast brachytherapy causes excessive target movement that reduces seed implantation accuracy. To address this target movement in breast brachytherapy, first, the internal target point will be manipulated accurately and then the brachytherapy needle will be inserted into the immobilized tissue. In this paper, a model-based tissue manipulation method is introduced. To simulate nonlinear large tissue deformation for the first time, a minimum-energy-based deformable tissue solver is utilized. Based on the theory of positive bases, the optimal number of actuators is determined to guarantee controllability of the internal target. A model predictive controller (MPC) is designed to implement multi-point tissue manipulation. A breast phantom is used to test the accuracy of the deformation model and the effectiveness of the proposed control method. The results show that the tissue deformation simulation error is 1.6 mm and the internal target can be regulated with negligible steady-state errors using an MPC controller.
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 192Ir‐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.
Needle insertion is a common minimally invasive medical procedure used for therapy and diagnosis. Among the therapeutic procedures is prostate brachytherapy, during which needle insertion is applied ...to implant radioactive seeds within the prostate. During insertion, the needle tends to deflect from a desired straight path, thus causing misplacement of the seeds. While currently the needle is steered manually to correct for needle deflection, robotic assistance can be used toward this goal. A requirement for accurate robotic needle steering is needle deflection estimation or prediction obtained from needle deflection modeling. Various mechanics-based deflection models based on needle-tissue interactions have been introduced in the literature. Many models require the tissue Young's modulus as a parameter input that can be measured or quantified using methods of varying limitations with regard to complexity or access in the operating room. This paper proposes an intraoperative method for the identification of tissue Young's modulus using lateral actuation of the needle. The needle-tissue system's response in terms of needle deflection and thus tissue displacement is observed during lateral needle displacement. The tissue Young's modulus is then identified based on the energy stored in the needle-tissue system. Using this method, the actuated needle itself is the tool used to obtain the tissue Young's modulus, facilitating clinical implementation. Experimental studies are presented to confirm a high accuracy of the identified tissue Young's modulus when compared with an independent measurement. Moreover, the prediction accuracy of a deflection model that is calibrated with the proposed method is verified experimentally.
