Development and optimisation of grid inserts for a preclinical radiotherapy system and corresponding Monte Carlo beam simulations

Objective. To develop a physical grid collimator compatible with the X-RAD preclinical radiotherapy system and create a corresponding Monte Carlo (MC) model.Approach. This work presents a methodology for the fabrication of a grid collimator designed for utilisation on the X-RAD preclinical radiotherapy system. Additionally, a MC simulation of the grid is developed, which is compatible with the X-RAD treatment planning system. The grid was manufactured by casting a low melting point alloy, cerrobend, into a silicone mould. The silicone was moulded around a 3D-printed replica of the grid, enabling the production of diverging holes with precise radii and spacing. A MC simulation was conducted on an equivalent 3D grid model and validated using 11 layers of GAFChromic EBT-3 film interspersed in a 3D-printed water-equivalent phantom. A 3D dose distribution was constructed from the film layers, enabling a direct comparison with the MC Simulation.Main results. The film and the MC dose distribution demonstrated a gamma passing rate of 99% for a 1%, 0.5 mm criteria with a 10% threshold applied. The peak-to-valley dose ratio and output factor at the surface were determined to be 20.4 and 0.79, respectively.Significance. The pairing of the grid collimator with a MC simulation can significantly enhance the practicality of grid therapy on the X-RAD. This combination enables further exploration of the biological implications of grid therapy, supported by a knowledge of the complex dose distributions. Moreover, this methodology can be adapted for use in other systems and scenarios.

A GPU-accelerated Monte Carlo dose computation engine for small animal radiotherapy

BACKGROUND

Accurate dose computation is critical in precision small animal radiotherapy. The Monte Carlo simulation method is the gold standard for radiation dose computation but has not been widely implemented in practice due to its low computation efficiency.

PURPOSE

This study aims to develop a GPU-accelerated radiation dose engine (GARDEN) based on the Monte Carlo simulation method for fast and accurate dose computation.

METHODS

In the GARDEN simulation, Compton scattering, Rayleigh scattering, and photoelectric effect were considered. The Woodcock tracking algorithm and GPU-specific acceleration techniques were used to obtain a high computational efficiency. Benchmark studies against both Geant4 simulations and experimental measurements were performed for various phantoms and beams. Finally, a conformal arc treatment plan was designed for a lung tumor to further evaluate the accuracy and efficiency in small animal radiotherapy.

RESULT

The engine attained a speed-up of 1232 times in a homogeneous water phantom and 935 times in a water-bone-lung heterogeneous phantom when compared with Geant4. Both the depth-dose curves and cross-sectional dose profiles for various radiation field sizes showed a great match between measurements and the GARDEN calculations. For in vivo dose validation, the differences between calculations and measurements in the mouse thorax and abdomen were 2.50% ± 1.50% and 1.56% ± 1.40%, respectively. The computation time for an arc treatment plan delivered from 36 angles was 2 s at a <1% uncertainty level using an NVIDIA GeForce RTX 2060 SUPER GPU. When compared with Geant4, the 3D gamma comparison passing rate was 98.7% at 2%/0.3 mm criteria.

CONCLUSION

GARDEN can perform fast and accurate dose computations in heterogeneous tissue environments and is expected to play a vital role in image-guided precision small animal radiotherapy.

A sparse orthogonal collimator for small animal intensity-modulated radiation therapy. Part II: hardware development and commissioning

PURPOSE

A dose-modulation device for small animal radiotherapy is required to use clinically analogous treatment techniques, which will likely increase the translatability of preclinical research results. Because the clinically used multileaf collimator (MLC) is impractical for miniaturization, we have developed a simpler, better-suited sparse orthogonal collimator (SOC) for delivering small animal intensity-modulated radiation therapy (IMRT) using a rectangular aperture optimization (RAO) treatment planning system.

METHODS

The SOC system was modeled in computer-aided design software and fabricated with machined tungsten leaves and three-dimensional (3D) printed leaf housing. A graphical user interface was developed for controlling and calibrating the SOC leaves, which are driven by Arduino-controlled stepper motors. A Winston-Lutz test was performed to assess mechanical alignment, and abutting field and grid dose patterns were created to analyze intra- and intercalibration leaf positioning error. Leaf transmission and penumbra were measured over the full range of gantry angles and leaf positions, respectively. Three SOC test plans were delivered, and film measurements were compared to the intended dose distributions. The differences in maximum, mean, and minimum, as well as pixelwise absolute dose differences, were compared for each structure, and a gamma analysis was performed for the target structures using criteria of 4% dose difference and 0.3 mm distance to agreement.

RESULTS

The Winston-Lutz test revealed maximum directional offsets between the SOC and primary collimator axes of 0.53 mm at 0° and 0.68 mm over the full 360°. Upper and lower abutting field patterns had maximum dose deviations of 18.8 ± 3.1% and 15.5 ± 2.9%, respectively, and grid patterns showed intra- and intercalibration repeatability of 93% and 91%, respectively. Extremely low midleaf (0.15 ± 0.05%) and interleaf (0.27 ± 0.22%) transmission was measured, with no significant rotational variation. The average penumbra was ~0.8 mm for all leaves at field center, with a range of 0.17 mm for all leaf positions. A highly concave test plan was delivered with a ~ 95% gamma analysis pass rate, and a realistic mouse phantom liver irradiation plan achieved a pass rate of ~98%. A highly complex dose distribution was also created with 551 SOC apertures averaging 2.4 mm in size.

CONCLUSIONS

A sparse orthogonal collimator was developed and commissioned, with promising preliminary dosimetry results. The SOC design, with its limited moving components and high dose-modulation resolution, is ideal for delivering high-quality small animal IMRT with our RAO-based treatment planning system.

A sparse orthogonal collimator for small animal intensity-modulated radiation therapy part I: Planning system development and commissioning

PURPOSE

To achieve more translatable preclinical research results, small animal irradiation needs to more closely simulate human radiotherapy. Although the clinical gold standard is intensity-modulated radiation therapy (IMRT), the direct translation of this method for small animals is impractical. In this study we describe the treatment planning system for a novel dose modulation device to address this challenge.

METHODS

Using delineated target and avoidance structures, a rectangular aperture optimization (RAO) problem was formulated to penalize deviations from a desired dose distribution and limit the number of selected rectangular apertures. RAO was used to create IMRT plans with highly concave targets in the mouse brain, and the plan quality was compared to that using a hypothetical miniaturized multileaf collimator (MLC). RAO plans were also created for a realistic application of mouse whole liver irradiation and for a highly complex two-dimensional (2D) dose distribution as a proof-of-principle. Beam commissioning data, including output and off-axis factors and percent depth dose (PDD) curves, were acquired for our small animal irradiation system and incorporated into the treatment planning system. A plan post-processing step was implemented for aperture size-specific dose recalculation and aperture weighting reoptimization.

RESULTS

The first RAO test case achieved highly conformal doses to concave targets in the brain, with substantially better dose gradient, conformity, and target dose homogeneity than the hypothetical miniaturized MLC plans. In the second test case, a highly conformal dose to the liver was achieved with significant sparing of the kidneys. RAO also successfully replicated a complex 2D dose distribution with three prescription dose levels. Energy spectra for field sizes 1 to 20 mm were calculated to match the measured PDD curves, with maximum and mean dose deviations of 4.47 ± 0.30% and 1.71 ± 0.18%. The final reoptimization of aperture weightings for the complex RAO test plan was able to reduce the maximum and mean dose deviations between the optimized and recalculated dose distributions from 10.3% to 6.6% and 4.0% to 2.8%, respectively.

CONCLUSIONS

Using the advanced optimization techniques, complex IMRT plans were achieved using a simple dose modulation device. Beam commissioning data were incorporated into the treatment planning process to more accurately predict the resulting dose distribution. This platform substantially reduces the gap in treatment plan quality between clinical and preclinical radiotherapy, potentially increasing the value and flexibility of small animal studies.

A 3D printed modular phantom for quality assurance of image-guided small animal irradiators: Design, imaging experiments, and Monte Carlo simulations

PURPOSE

The goal of this work was to develop and test a cylindrical tissue-equivalent quality assurance (QA) phantom for micro computed tomography (microCT) image-guided small animal irradiators that overcomes deficiencies of existing phantoms due to its mouse-like dimensions and composition.

METHODS

The 8.6-cm-long and 2.4-cm-diameter phantom was three-dimensionally (3D) printed out of Somos NeXt plastic on a stereolithography (SLA) printer. The modular phantom consisted of four sections: (a) CT number evaluation section, (b) spatial resolution with slanted edge (for the assessment of longitudinal resolution) and targeting section, (c) spatial resolution with hole pattern (for the assessment of radial direction) section, and (d) uniformity and geometry section. A Python-based graphical user interface (GUI) was developed for automated analysis of microCT images and evaluated CT number consistency, longitudinal and radial modulation transfer function (MTF), image uniformity, noise, and geometric accuracy. The phantom was placed at the imaging isocenter and scanned with the small animal radiation research platform (SARRP) in the pancake geometry (long axis of the phantom perpendicular to the axis of rotation) with a variety of imaging protocols. Tube voltage was set to 60 and 70 kV, tube current was set to 0.5 and 1.2 mA, voxel size was set to 200 and 275 μm, imaging times of 1, 2, and 4 min were used, and frame rates of 6 and 12 frames per second (fps) were used. The phantom was also scanned in the standard (long axis of the phantom parallel to the axis of rotation) orientation. The quality of microCT images was analyzed and compared to recommendations presented in our previous work that was derived from a multi-institutional study. Additionally, a targeting accuracy test with a film placed in the phantom was performed. MicroCT imaging of the phantom was also simulated in a modified version of the EGSnrc/DOSXYZnrc code. Images of the resolution section with the hole pattern were acquired experimentally as well as simulated in both the pancake and the standard imaging geometries. The radial spatial resolution of the experimental and simulated images was evaluated and compared to experimental data.

RESULTS

For the centered phantom images acquired in the pancake geometry, all imaging protocols passed the spatial resolution criterion in the radial direction (>1.5 lp/mm @ 0.2 MTF), the geometric accuracy criterion (<200 μm), and the noise criterion (<55 HU). Only the imaging protocol with 200-μm voxel size passed the criterion for spatial resolution in the longitudinal direction (>1.5 lp/mm @ 0.2 MTF). The 70-kV tube voltage dataset failed the bone CT number consistency test (<55 HU). Due to cupping artifacts, none of the imaging protocols passed the uniformity test of <55 HU. When the phantom was scanned in the standard imaging geometry, image uniformity and longitudinal MTF were satisfactory; however, the CT number consistency failed the recommended limit. A targeting accuracy of 282 and 251 μm along the x- and z-direction was observed. Monte Carlo simulations confirmed that the radial spatial resolution for images acquired in the pancake geometry was higher than the one acquired in the standard geometry.

CONCLUSIONS

The new 3D-printed phantom presents a useful tool for microCT image analysis as it closely mimics a mouse. In order to image mouse-sized animals with acceptable image quality, the standard protocol with a 200-μm voxel size should be chosen and cupping artifacts need to be resolved.

Evolution of the Supermodel: Progress in Modelling Radiotherapy Response in Mice

Mouse models are essential tools in cancer research that have been used to understand the genetic basis of tumorigenesis, cancer progression and to test the efficacies of anticancer treatments including radiotherapy. They have played a critical role in our understanding of radiotherapy response in tumours and normal tissues and continue to evolve to better recapitulate the underlying biology of humans. In addition, recent developments in small animal irradiators have significantly improved in vivo irradiation techniques, allowing previously unimaginable experimental approaches to be explored in the laboratory. The combination of contemporary mouse models with small animal irradiators represents a major step forward for the radiobiology field in being able to much more accurately replicate clinical exposure scenarios. As radiobiology studies become ever more sophisticated in reflecting developments in the clinic, it is increasingly important to understand the basis and potential limitations of extrapolating data from mice to humans. This review provides an overview of mouse models and small animal radiotherapy platforms currently being used as advanced radiobiological research tools towards improving the translational power of preclinical studies.

Technical Note: Manufacturing of a realistic mouse phantom for dosimetry of radiobiology experiments

PURPOSE

The goal of this work was to design a realistic mouse phantom as a useful tool for accurate dosimetry in radiobiology experiments.

METHODS

A subcutaneous tumor-bearing mouse was scanned in a microCT scanner, its organs manually segmented and contoured. The resulting geometries were converted into a stereolithographic file format (STL) and sent to a multimaterial 3D printer. The phantom was split into two parts to allow for lung excavation and 3D-printed with an acrylic-like material and consisted of the main body (mass density ρ=1.18 g/cm³ ) and bone (ρ=1.20 g/cm³ ). The excavated lungs were filled with polystyrene (ρ=0.32 g/cm³ ). Three cavities were excavated to allow the placement of a 1-mm diameter plastic scintillator dosimeter (PSD) in the brain, the center of the body and a subcutaneous tumor. Additionally, a laser-cut Gafchromic film can be placed in between the two phantom parts for 2D dosimetric evaluation. The expected differences in dose deposition between mouse tissues and the mouse phantom for a 220-kVp beam delivered by the small animal radiation research platform (SARRP) were calculated by Monte Carlo (MC).

RESULTS

MicroCT scans of the phantom showed excellent material uniformity and confirmed the material densities given by the manufacturer. MC dose calculations revealed that the dose measured by tissue-equivalent dosimeters inserted into the phantom in the brain, abdomen, and subcutaneous tumor would be underestimated by 3-5%, which is deemed to be an acceptable error assuming the proposed 5% accuracy of radiobiological experiments.

CONCLUSIONS

The low-cost mouse phantom can be easily manufactured and, after a careful dosimetric characterization, may serve as a useful tool for dose verification in a range of radiobiology experiments.