Evaluation of On- and Off-Line Bioluminescence Tomography System for Focal Irradiation Guidance

Characterization of a commercial bioluminescence tomography-guided system for pre-clinical radiation research

BACKGROUND

Widely used Cone-beam computed tomography (CBCT)-guided irradiators have limitations in localizing soft tissue targets growing in a low-contrast environment. This hinders small animal irradiators achieving precise focal irradiation.

PURPOSE

To advance image-guidance for soft tissue targeting, we developed a commercial-grade bioluminescence tomography-guided system (BLT, MuriGlo) for pre-clinical radiation research. We characterized the system performance and demonstrated its capability in target localization. We expect this study can provide a comprehensive guideline for the community in utilizing the BLT system for radiation studies.

METHODS

MuriGlo consists of four mirrors, filters, lens, and charge-coupled device (CCD) camera, enabling a compact imaging platform and multi-projection and multi-spectral BLT. A newly developed mouse bed allows animals imaged in MuriGlo and transferred to a small animal radiation research platform (SARRP) for CBCT imaging and BLT-guided irradiation. Methods and tools were developed to evaluate the CCD response linearity, minimal detectable signal, focusing, spatial resolution, distortion, and uniformity. A transparent polycarbonate plate covering the middle of the mouse bed was used to support and image animals from underneath the bed. We investigated its effect on 2D Bioluminescence images and 3D BLT reconstruction accuracy, and studied its dosimetric impact along with the rest of mouse bed. A method based on pinhole camera model was developed to map multi-projection bioluminescence images to the object surface generated from CBCT image. The mapped bioluminescence images were used as the input data for the optical reconstruction. To account for free space light propagation from object surface to optical detector, a spectral derivative (SD) method was implemented for BLT reconstruction. We assessed the use of the SD data (ratio imaging of adjacent wavelength) in mitigating out of focusing and non-uniformity seen in the images. A mouse phantom was used to validate the data mapping. The phantom and an in vivo glioblastoma model were utilized to demonstrate the accuracy of the BLT target localization.

RESULTS

The CCD response shows good linearity with < 0.6% residual from a linear fit. The minimal detectable level is 972 counts for 10 × 10 binning. The focal plane position is within the range of 13-18 mm above the mouse bed. The spatial resolution of 2D optical imaging is < 0.3 mm at Rayleigh criterion. Within the region of interest, the image uniformity is within 5% variation, and image shift due to distortion is within 0.3 mm. The transparent plate caused < 6% light attenuation. The use of the SD imaging data can effectively mitigate out of focusing, image non-uniformity, and the plate attenuation, to support accurate multi-spectral BLT reconstruction. There is < 0.5% attenuation on dose delivery caused by the bed. The accuracy of data mapping from the 2D bioluminescence images to CBCT image is within 0.7 mm. Our phantom test shows the BLT system can localize a bioluminescent target within 1 mm with an optimal threshold and only 0.2 mm deviation was observed for the case with and without a transparent plate. The same localization accuracy can be maintained for the in vivo GBM model.

CONCLUSIONS

This work is the first systematic study in characterizing the commercial BLT-guided system. The information and methods developed will be useful for the community to utilize the imaging system for image-guided radiation research.

Mobile bioluminescence tomography-guided system for pre-clinical radiotherapy research

Due to low imaging contrast, a widely-used cone-beam computed tomography-guided small animal irradiator is less adept at localizing in vivo soft tissue targets. Bioluminescence tomography (BLT), which combines a model of light propagation through tissue with an optimization algorithm, can recover a spatially resolved tomographic volume for an internal bioluminescent source. We built a novel mobile BLT system for a small animal irradiator to localize soft tissue targets for radiation guidance. In this study, we elaborate its configuration and features that are indispensable for accurate image guidance. Phantom and in vivo validations show the BLT system can localize targets with accuracy within 1 mm. With the optimal choice of threshold and margin for target volume, BLT can provide a distinctive opportunity for investigators to perform conformal biology-guided irradiation to malignancy.

Assessing the Accuracy of Bioluminescence Image-Guided Stereotactic Body Radiation Therapy of Orthotopic Pancreatic Tumors Using a Small Animal Irradiator

Stereotactic body radiation therapy (SBRT) has shown promising results in the treatment of pancreatic cancer and other solid tumors. However, wide adoption of SBRT remains limited largely due to uncertainty about the treatment's optimal fractionation schedules to elicit maximal tumor response while limiting the dose to adjacent structures. A small animal irradiator in combination with a clinically relevant oncological animal model could address these questions. Accurate delivery of X rays to animal tumors may be hampered by suboptimal image-guided targeting of the X-ray beam in vivo. Integration of bioluminescence imaging (BLI) into small animal irradiators in addition to standard cone-beam computed tomography (CBCT) imaging improves target identification and high-precision therapy delivery to deep tumors with poor soft tissue contrast, such as pancreatic tumors. Using bioluminescent BxPC3 pancreatic adenocarcinoma human cells grown orthotopically in mice, we examined the performance of a small animal irradiator equipped with both CBCT and BLI in delivering targeted, hypo-fractionated, multi-beam SBRT. Its targeting accuracy was compared with magnetic resonance imaging (MRI)-guided targeting based on co-registration between CBCT and corresponding sequential magnetic resonance scans, which offer greater soft tissue contrast compared with CT alone. Evaluation of our platform's BLI-guided targeting accuracy was performed by quantifying in vivo changes in bioluminescence signal after treatment as well as staining of ex vivo tissues with γH2AX, Ki67, TUNEL, CD31 and CD11b to assess SBRT treatment effects. Using our platform, we found that BLI-guided SBRT enabled more accurate delivery of X rays to the tumor resulting in greater cancer cell DNA damage and proliferation inhibition compared with MRI-guided SBRT. Furthermore, BLI-guided SBRT allowed higher animal throughput and was more cost effective to use in the preclinical setting than MRI-guided SBRT. Taken together, our preclinical platform could be employed in translational research of SBRT of pancreatic cancer.

Quantitative Bioluminescence Tomography for In Vivo Volumetric-Guided Radiotherapy

Several groups, including ours, have initiated efforts to develop small-animal irradiators that mimic radiation therapy (RT) for human treatment. The major image modality used to guide irradiation is cone-beam computed tomography (CBCT). While CBCT provides excellent guidance capability, it is less adept at localizing soft tissue targets growing in a low image contrast environment. In contrast, bioluminescence imaging (BLI) provides strong image contrast and thus is an attractive solution for soft tissue targeting. However, commonly used 2D BLI on an animal surface is inadequate to guide irradiation, because optical transport from an internal bioluminescent tumor is highly susceptible to the effects of optical path length and tissue absorption and scattering. Recognition of these limitations led us to integrate 3D bioluminescence tomography (BLT) with the small animal radiation research platform (SARRP). In this chapter, we introduce quantitative BLT (QBLT) with the advanced capabilities of quantifying tumor volume for irradiation guidance. The detail of system components, calibration protocol, and step-by-step procedure to conduct the QBLT-guided irradiation are described.

Quantitative Bioluminescence Tomography-guided Conformal Irradiation for Preclinical Radiation Research

PURPOSE

Widely used cone beam computed tomography (CBCT)-guided irradiators in preclinical radiation research are limited to localize soft tissue target because of low imaging contrast. Knowledge of target volume is a fundamental need for radiation therapy (RT). Without such information to guide radiation, normal tissue can be overirradiated, introducing experimental uncertainties. This led us to develop high-contrast quantitative bioluminescence tomography (QBLT) for guidance. The use of a 3-dimensional bioluminescence signal, related to cell viability, for preclinical radiation research is one step toward biology-guided RT.

METHODS AND MATERIALS

Our QBLT system enables multiprojection and multispectral bioluminescence imaging to maximize input data for the tomographic reconstruction. Accurate quantification of spectrum and dynamic change of in vivo signal were also accounted for the QBLT. A spectral-derivative method was implemented to eliminate the modeling of the light propagation from animal surface to detector. We demonstrated the QBLT capability of guiding conformal RT using a bioluminescent glioblastoma (GBM) model in vivo. A threshold was determined to delineate QBLT reconstructed gross target volume (GTV_QBLT), which provides the best overlap between the GTV_QBLT and CBCT contrast labeled GBM (GTV), used as the ground truth for GBM volume. To account for the uncertainty of GTV_QBLT in target positioning and volume delineation, a margin was determined and added to the GTV_QBLT to form a QBLT planning target volume (PTV_QBLT) for guidance.

RESULTS

The QBLT can reconstruct in vivo GBM with localization accuracy within 1 mm. A 0.5-mm margin was determined and added to GTV_QBLT to form PTV_QBLT, largely improving tumor coverage from 75.0% (0 mm margin) to 97.9% in average, while minimizing normal tissue toxicity. With the goal of prescribed dose 5 Gy covering 95% of PTV_QBLT, QBLT-guided 7-field conformal RT can effectively irradiate 99.4 ± 1.0% of GTV.

CONCLUSIONS

The QBLT provides a unique opportunity for investigators to use biologic information for target delineation, guiding conformal irradiation, and reducing normal tissue involvement, which is expected to increase reproducibility of scientific discovery.

In vivo bioluminescence tomography-guided radiation research platform for pancreatic cancer: an initial study using subcutaneous and orthotopic pancreatic tumor models

Genetically engineered mouse model(GEMM) that develops pancreatic ductal adenocarcinoma(PDAC) offers an experimental system to advance our understanding of radiotherapy(RT) for pancreatic cancer. Cone beam CT(CBCT)-guided small animal radiation research platform(SARRP) has been developed to mimic the RT used for human. However, we recognized that CBCT is inadequate to localize the PDAC growing in low image contrast environment. We innovated bioluminescence tomography(BLT) to guide SARRP irradiation for in vivo PDAC. Before working on the complex PDAC-GEMM, we first validated our BLT target localization using subcutaneous and orthotopic pancreatic tumor models. Our BLT process involves the animal transport between the BLT system and SARRP. We inserted a titanium wire into the orthotopic tumor as the fiducial marker to track the tumor location and to validate the BLT reconstruction accuracy. Our data shows that with careful animal handling, minimum disturbance for target position was introduced during our BLT imaging procedure(<0.5mm). However, from longitudinal 2D bioluminescence image(BLI) study, the day-to-day location variation for an abdominal tumor can be significant. We also showed that the 2D BLI in single projection setting cannot accurately capture the abdominal tumor location. It renders that 3D BLT with multiple-projection is needed to quantify the tumor volume and location for precise radiation research. Our initial results show the BLT can retrieve the location at 2mm accuracy for both tumor models, and the tumor volume can be delineated within 25% accuracy. The study for the subcutaneous and orthotopic models will provide us valuable knowledge for BLT-guided PDAC-GEMM radiation research.

In Vivo Bioluminescence Tomography Center of Mass-Guided Conformal Irradiation

PURPOSE

The cone-beam computed tomography (CBCT)-guided small animal radiation research platform (SARRP) has provided unique opportunities to test radiobiologic hypotheses. However, CBCT is less adept to localize soft tissue targets growing in a low imaging contrast environment. Three-dimensional bioluminescence tomography (BLT) provides strong image contrast and thus offers an attractive solution. We introduced a novel and efficient BLT-guided conformal radiation therapy and demonstrated it in an orthotopic glioblastoma (GBM) model.

METHODS AND MATERIALS

A multispectral BLT system was integrated with SARRP for radiation therapy (RT) guidance. GBM growth curve was first established by contrast CBCT/magnetic resonance imaging (MRI) to derive equivalent sphere as approximated gross target volume (aGTV). For BLT, mice were subject to multispectral bioluminescence imaging, followed by SARRP CBCT imaging and optical reconstruction. The CBCT image was acquired to generate anatomic mesh for the reconstruction and RT planning. To ensure high accuracy of the BLT-reconstructed center of mass (CoM) for target localization, we optimized the optical absorption coefficients μ_a by minimizing the distance between the CoMs of BLT reconstruction and contrast CBCT/MRI-delineated GBM volume. The aGTV combined with the uncertainties of BLT CoM localization and target volume determination was used to generate estimated target volume (ETV). For conformal irradiation procedure, the GBM was first localized by the predetermined ETV centered at BLT-reconstructed CoM, followed by SARRP radiation. The irradiation accuracy was qualitatively confirmed by pathologic staining.

RESULTS

Deviation between CoMs of BLT reconstruction and contrast CBCT/MRI-imaged GBM is approximately 1 mm. Our derived ETV centered at BLT-reconstructed CoM covers >95% of the tumor volume. Using the second-week GBM as an example, the ETV-based BLT-guided irradiation can cover 95.4% ± 4.7% tumor volume at prescribed dose. The pathologic staining demonstrated the BLT-guided irradiated area overlapped well with the GBM location.

CONCLUSIONS

The BLT-guided RT enables 3-dimensional conformal radiation for important orthotopic tumor models, which provides investigators a new preclinical research capability.