Browsing by Subject "Preclinical"

As complex radiotherapy techniques become more readily-practiced, comprehensive 3D dosimetry is a growing necessity for advanced quality assurance. However, clinical implementation has been impeded by a wide variety of factors, including the expense of dedicated optical dosimeter readout tools, high operational costs, and the overall difficulty of use. To address these issues, a novel dry-tank optical CT scanner was designed for PRESAGE 3D dosimeter readout, relying on 3D printed components and omitting costly parts from preceding optical scanners. This work details the design, prototyping, and basic commissioning of the Duke Integrated-lens Optical Scanner (DIOS).

The convex scanning geometry was designed in ScanSim, an in-house Monte Carlo optical ray-tracing simulation. ScanSim parameters were used to build a 3D rendering of a convex ‘solid tank’ for optical-CT, which is capable of collimating a point light source into telecentric geometry without significant quantities of refractive-index matched fluid. The model was 3D printed, processed, and converted into a negative mold via rubber casting to produce a transparent polyurethane scanning tank. The DIOS was assembled with the solid tank, a 3W red LED light source, a computer-controlled rotation stage, and a 12-bit CCD camera. Initial optical phantom studies show negligible spatial inaccuracies in 2D projection images and 3D tomographic reconstructions. A PRESAGE 3D dose measurement for a 4-field box treatment plan from Eclipse shows 95% of voxels passing gamma analysis at 3%/3mm criteria. Gamma analysis between tomographic images of the same dosimeter in the DIOS and DLOS systems show 93.1% agreement at 5%/1mm criteria. From this initial study, the DIOS has demonstrated promise as an economically-viable optical-CT scanner. However, further improvements will be necessary to fully develop this system into an accurate and reliable tool for advanced QA.

Pre-clinical animal studies are used as a conventional means of translational research, as a midpoint between in-vitro cell studies and clinical implementation. However, modern small animal radiotherapy platforms are primitive in comparison with conventional linear accelerators. This work also investigates a series of 3D printed tools to expand the treatment capabilities of the X-RAD 225Cx orthovoltage irradiator, and applies them to a feasibility study of hippocampal avoidance in rodent whole-brain radiotherapy.

As an alternative material to lead, a novel 3D-printable tungsten-composite ABS plastic, GMASS, was tested to create precisely-shaped blocks. Film studies show virtually all primary radiation at 225 kVp can be attenuated by GMASS blocks of 0.5cm thickness. A state-of-the-art software, BlockGen, was used to create custom hippocampus-shaped blocks from medical image data, for any possible axial treatment field arrangement. A custom 3D printed bite block was developed to immobilize and position a supine rat for optimal hippocampal conformity. An immobilized rat CT with digitally-inserted blocks was imported into the SmART-Plan Monte-Carlo simulation software to determine the optimal beam arrangement. Protocols with 4 and 7 equally-spaced fields were considered as viable treatment options, featuring improved hippocampal conformity and whole-brain coverage when compared to prior lateral-opposed protocols. Custom rodent-morphic PRESAGE dosimeters were developed to accurately reflect these treatment scenarios, and a 3D dosimetry study was performed to confirm the SmART-Plan simulations. Measured doses indicate significant hippocampal sparing and moderate whole-brain coverage.

Purpose: To develop and implement novel methods of extreme spatially fractionated radiation therapy (including GRID therapy) and subsequent evaluation in pre-clinical mice trials investigating the potential of novel radiation treatments with potential for promoting anti-cancer immunogenic response.

Methods: Spatially fractionated GRIDs were designed and precision-milled from 3mm thick lead sheets compatible with mounting on a 225 kVp small animal irradiator (X-Rad). Three pencil-beam GRIDs created arrays of 1mm diameter beams, and three “bar” GRIDs created 1x20mm rectangular fields. GRIDs projected 20x20mm fields at isocenter and beamlets were spaced at 1, 1.25, and 1.5mm, respectively. Output factors, peak-to-valley ratios, and dose distributions were determined with Gafchromic film. The bar GRID with 1mm beamlet spacing (50:50 open:closed ratio) was selected for the pre-clinical trial. Soft-tissue sarcoma (p53/MCA) was transplanted into C57BL/6 mice’s flanks. Four treatment arms were compared: unirradiated control (n=18), conventional radiation therapy (n=16), GRID therapy (n=17), and hemi-irradiation (n=17) where one-half of the beam was blocked. All irradiated mice received a single fraction of 15 Gy to irradiated regions. To date, this is the first study to compare GRID treatment against conventional RT at the same dose.

Results: Very high peak-to-valley ratios were achieved (bar GRIDs: 11.9±0.9, 13.6±0.4, 13.8±0.5; pencil-beam GRIDs: 18.7±0.6, 26.3±1.5, 31.0±3.3). Pencil-beam GRIDs spared twice the number of intra-tumor immune cells as bar GRIDs but left more of the tumor untreated (2-3% vs 14-17% area receiving 95% prescription, respectively). Penumbra was halved when GRIDs were 50% closer to treatment isocenter. The GRID selected for mouse trials was capable of sparing approximately 15% of intra-tumor CD8+ and CD4+ T cells. Preliminary results indicate mean times to tumor quintupling were: 12, 13, 14, and 20 days for unirradiated, GRID, hemi-irradiated, and conventional treatment groups, respectively. To date, all tumors have quintupled except for nine in the AP control group.

Conclusions: Peak-to-valley ratios with kV grids were substantially superior to MV grids, which historically achieve ratios between 2.5 and 6.5. In data collected to date, GRID and hemi-irradiation did not significantly delay tumor growth as compared to an unirradiated control (P = 0.122 and P = 0.2437, respectively, P-values from logrank analysis). Differences between GRID and hemi-irradiation were not statistically significant (P = 0.5257). To date, the AP control group has performed significantly better than all other groups (P<0.001). These results do not corroborate the success of hemi-irradiation in Markovsky et al. 2019. GRID treatments may be more effective if a substantially higher dose and/or multiple fractions were employed.

Translational cancer research has been around for many years and has resulted

in many advancements in cancer treatment. Preclinical radiation therapy is an important

tool used in some studies to better understand the biological effects due to radiation.

Current preclinical radiation treatment techniques do not emulate the advanced

techniques used in cancer clinics, such as intensity modulated radiation therapy (IMRT).

In this work we explore the possibility of developing and implementing an IMRT

treatment capability for an orthovoltage micro irradiator used for small animal research.

In order to implement IMRT to the micro irradiator, every step of the radiation

therapy treatment process had to be evaluated, developed, and tested. The first step was

to develop and treatment planning software that can be used for small animal studies.

Using the open source Computational Environment for Radiotherapy Research (CERR)

and adapting it for use with an orthovoltage irradiator, monte carlo dose calculations

could be performed for small animal data sets. CERR does not have the ability to

optimize dose calculations, so a Matlab script was developed and written for inverse

optimization for treatment planning. Treatment plans were designed and optimized for

several small animal cases to evaluate the optimization algorithm. Following successful

simulation development, treatment delivery techniques needed to be developed. 3D

printing was used as a tool to create physical compensators that could be used as an

add-on device to the micro irradiator. With the capability of submillimeter printing

resolution, 3D printing has the capability to handle the high resolution required for very

small structures inside of small animals. Using the simulation data, another Matlab

script was developed to create both compensator and inverse compensator 3D models.

Many materials and techniques were evaluated to determine the best method for

compensator production. Materials were tested for attenuation properties, printing

capabilities, and ease of use until a satisfactory result was achieved.

Once the simulation and delivery techniques were developed to a satisfactory

level, an end to end test was designed to verify the IMRT capability. Using a 2.2 cm

diameter cylindrical Presage® dosimeter as the quality assurance (QA) device/patient, a

treatment plan was created based on the geometry of the Radiologic Physics Center

(RPC) Head and Neck phantom design. The dose tolerances used for the inverse

optimization were the same as the RPC Head and Neck protocol with a stricter tolerance

for the organ at risk (OAR). Compensators were produced for the plan and both 2D and

3D analysis was performed. Radiochromic film was used for 2D dose map analysis.

Gamma analysis was performed using 2D film data with varying criteria for distance to

agreement and dose difference. 3D analysis was done by delivering the treatment plan

to the Presage® dosimeter. Using optical-CT for dose readout of the dosimeter,

qualitative analysis was performed to show the 3D delivered dose data.

The end to end test showed strong evidence that IMRT could be implemented on

the small animal irradiator. The 9 field treatment plan was delivered in under 30

minutes with no mechanical or collisional issues. The 2D dose analysis showed 7 out of 9

treatment fields had a passing rate greater than 90% for a gamma analysis using 10%/0.5

mm tolerances. 3D dose analysis showed promising spatial resolution of the dose

modulation. As a feasibility and an initial testing study for a new treatment technique on

the small animal irradiator, these results showed the capability of the 3D printed

compensators to modulate dose with high spatial precision and moderately accurate

dose delivery.

Hyperpolarized (HP) gas MRI is emerging as a powerful, non-invasive method for imaging lung function. MRI of HP 129Xe and 3He was first introduced in small animals and was soon followed by its clinical implementation. 3He was preferred for imaging since it was more straight-forward to hyperpolarize in large volumes and had favorable magnetic resonance properties for high-resolution. However, the scarcity and high cost of this isotope has driven a transition to abundantly available 129Xe. This transition has stimulated a lot of clinical 129Xe MRI research. 129Xe ventilation, barrier-tissue uptake and red-blood-cell (RBC) transfer can now be depicted separately and three-dimensionally by exploiting xenon solubility and large chemical shifts in different pulmonary micro-environments. With this powerful capability, this technique has found clinical application across a broad range of lung diseases.

As clinical implementation progresses, it has become increasingly important to test these methods in well-controlled animal models. Such preclinical studies enable the testing of experimental drugs, tracking of disease progression by longitudinal imaging, validation against histology, and provide a platform to rapidly develop and validate novel methods of image acquisition and analysis. However, among the ~20 centers worldwide that have HP gas MRI capability, only 5 have demonstrated the capability to conduct preclinical studies. Preclinical 129Xe MRI is challenging owing to extensive requirements of animal handling, reliable delivery of polarized gas, and the challenges of high-resolution multi-breath imaging. While some applications for HP gas MRI in small animals have emerged, these have mostly been with 3He and the handful of work on 129Xe has been limited to 2D imaging. As was the case in the clinic, there is now an equally urgent need to drive the transition from 3He to 129Xe in the preclinical setting, demonstrate sufficient image quality, and rapidly discover new applications.

The objective of this work is to establish a robust and comprehensive 129Xe MRI infrastructure to investigate rodent models of lung disease, and to lay the foundation for the reverse-translation and dissemination of this capability. To this, the work in this thesis describes several milestones toward establishing routine, high-resolution 3D 129Xe MRI of gas-exchange on a modern preclinical imaging platform.

First, we established routine 3D 129Xe MRI in mice on a GE 2 Tesla magnet. Through rigorous optimization of multi-breath image acquisition strategies with constant-volume ventilation, we demonstrated high-resolution imaging of 129Xe gas- and dissolved-phases in mice with 156-µm and 312-µm isotropic resolution. In addition to imaging, we also comprehensively characterized 129Xe spectroscopic lineshapes in mice. The in vivo resonances of 129Xe are sensitive to micron-scale changes in lung physiology, but have been analyzed and reported inconsistently and inaccurately in the literature. Using innovative spectroscopic acquisition methods and robust fitting techniques, we introduced methodology to identify an accurate 129Xe reference frequency in vivo, and characterized the many dissolved-phase resonances that are arise as 129Xe is transported to distal tissues in the thoracic cavity.

Until this point, animal studies using 129Xe MRI required sacrificing the animal upon completion of imaging, owing to the requirement of tracheostomy to ventilate the rodent with HP gas. Also, our experiments could only be conducted on a single 2 Tesla magnet, because the ventilator and physiological monitoring system was hard-wired to this scanner. In order to address these limitations, we built a new ventilator with integrated physiological monitoring with a focus on portability, minimizing cost, and compatibility for longitudinal imaging. The portable and integrated ventilator made possible our first dissemination of preclinical 129Xe MRI—to the University of Oxford, UK.

Our robust 129Xe MRI and spectroscopy protocol was deployed to investigate two key mouse models of lung disease at 2 Tesla: lung cancer and invasive pulmonary aspergillosis (IPA). In lung cancer, longitudinal 129Xe MRI revealed tumors on 1H MRI and histology, and severe ventilation and gas-exchange defects. 129Xe spectroscopy additionally revealed a robust signature of cancer-associated cachexia. 129Xe MRI in IPA also revealed significant and complex ventilation and gas-exchange defects, which was bolstered by spectroscopic features.

Having a portable ventilator enabled experiments to be carried out at other magnets at our center. Since modern preclinical magnets now operate at high field strengths, we established preclinical 129Xe MRI on a Bruker 7 Tesla magnet at our center, to facilitate its broader dissemination. This is the most widely available preclinical imaging platform with an installed base estimated to exceed 500 units. This transition involved a comprehensive characterization and optimization of the noise floor of the system to maximize SNR, and developing several new image acquisition strategies to rapidly image short-lived 129Xe signal at 7 Tesla (dissolved-phase T2* ~0.5 ms). On this platform, we developed a robust 129Xe MRI protocol for quantitative gas-exchange mapping in rats, identical to that used by our clinical program to facilitate translation/reverse translation.

Finally, we used the new 7 Tesla platform to investigate the monocrotaline (MCT) rat model of pulmonary arterial hypertension (PAH). This model was chosen for 2 reasons: first, it provided a unique opportunity to deploy 129Xe gas-exchange MRI in a model that is translationally relevant to current clinical investigations; second, there is also a dire need for non-invasive assays to elucidate the pathogenesis of this disease in the lung, and to enable early detection. In this study, we comprehensively characterized the imaging and spectroscopic markers of this disease and validated results with histology. 129Xe MRI revealed significantly reduced signal in RBCs, as well as interesting abnormalities in the barrier-uptake and gas-phase signal that were consistent with the pathobiology of this disease model. This is the first study to have demonstrated the potential of 129Xe to be a valuable tool for assessing rodent models of pulmonary vascular disease.

This body of work has thus established a robust preclinical 129Xe MRI framework that can be routinely used for imaging across field strengths, vender platforms, rodent species, be translated/reverse-translated to/from our clinical program, and be disseminated to other centers. We have also demonstrated the potential of this imaging platform to identify different disease signatures in several clinically-relevant rodent models. We anticipate that this work will provide a fundamentally new capability to accelerate progress in lung imaging research.