BioXmark for high-precision radiotherapy in an orthotopic pancreatic tumor mouse model : Experiences with a liquid fiducial marker

Development of a PTV margin for preclinical irradiation of orthotopic pancreatic tumors derived from a well-known recipe for humans

In human radiotherapy a safety margin (PTV margin) is essential for successful irradiation and is usually part of clinical treatment planning. In preclinical radiotherapy research with small animals, most uncertainties and inaccuracies are present as well, but according to the literature a margin is used only scarcely. In addition, there is only little experience about the appropriate size of the margin, which should carefully be investigated and considered, since sparing of organs at risk or normal tissue is affected. Here we estimate the needed margin for preclinical irradiation by adapting a well-known human margin recipe from van Herck et al. to the dimensions and requirements of the specimen on a small animal radiation research platform (SARRP). We adjusted the factors of the described formula to the specific challenges in an orthotopic pancreatic tumor mouse model to establish an appropriate margin concept. The SARRP was used with its image-guidance irradiation possibility for arc irradiation with a field size of 10 × 10 mm2 for 5 fractions. Our goal was to irradiate the clinical target volume (CTV) of at least 90% of our mice with at least 95% of the prescribed dose. By carefully analyzing all relevant factors we gain a CTV to planning target volume (PTV) margin of 1.5 mm for our preclinical setup. The stated safety margin is strongly dependent on the exact setting of the experiment and has to be adjusted for other experimental settings. The few stated values in literature correspond well to our result. Even if using margins in the preclinical setting might be an additional challenge, we think it is crucial to use them to produce reliable results and improve the efficacy of radiotherapy.

Virtual tumor mapping and margin control with 3-D planning and navigation

Computer technology-based treatment approaches like intraoperative navigation and intensity-modulated radiation therapy have become important components of state of the art head and neck cancer treatment. Multidirectional exchange of virtual three-dimensional patient data via an interdisciplinary platform allows all medical specialists involved in the patients treatment to take full advantage of these technologies. This review article gives an overview of current technologies and future directions regarding treatment approaches that are based on a virtual, three-dimensional patient specific dataset: storage and exchange of spatial information acquired via intraoperative navigation allow for a highly precise frozen section procedure. In the postoperative setting, virtual reconstruction of the tumor resection surface provides the basis for improved radiation therapy planning and virtual reconstruction of the tumor with integration of molecular findings creates a valuable tool for postoperative treatment and follow-up. These refinements of established treatment components and novel approaches have the potential to make a major contribution to improving the outcome in head and neck cancer patients.

Feasibility and guidelines for the use of an injectable fiducial marker (BioXmark ®) to improve target delineation in preclinical radiotherapy studies using mouse models

BACKGROUND

Preclinical models of radiotherapy (RT) response are vital for the continued success and evolution of RT in the treatment of cancer. The irradiation of tissues in mouse models necessitates high levels of precision and accuracy to recapitulate clinical exposures and limit adverse effects on animal welfare. This requirement has been met by technological advances in preclinical RT platforms established over the past decade. Small animal RT systems use onboard computed tomography (CT) imaging to delineate target volumes and have significantly refined radiobiology experiments with major 3Rs impacts. However, the CT imaging is limited by the differential attenuation of tissues resulting in poor contrast in soft tissues. Clinically, radio-opaque fiducial markers (FMs) are used to establish anatomical reference points during treatment planning to ensure accuracy beam targeting, this approach is yet to translate back preclinical models.

METHODS

We report on the use of a novel liquid FM BioXmark ® developed by Nanovi A/S (Kongens Lyngby, Denmark) that can be used to improve the visualisation of soft tissue targets during beam targeting and minimise dose to surrounding organs at risk. We present descriptive protocols and methods for the use of BioXmark ® in experimental male and female C57BL/6J mouse models.

RESULTS

These guidelines outline the optimum needle size for uptake (18-gauge) and injection (25- or 26-gauge) of BioXmark ® for use in mouse models along with recommended injection volumes (10-20 µl) for visualisation on preclinical cone beam CT (CBCT) scans. Injection techniques include subcutaneous, intraperitoneal, intra-tumoral and prostate injections.

CONCLUSIONS

The use of BioXmark ® can help to standardise targeting methods, improve alignment in preclinical image-guided RT and significantly improve the welfare of experimental animals with the reduction of normal tissue exposure to RT.

Comparison of 3 Positioning Techniques for Fractionated High-precision Radiotherapy in an Orthotopic Mouse Model of Pancreatic Cancer

Small-animal irradiators are widely used in oncologic research, and many experiments use mice to mimic radiation treatments in humans. To improve fractionated high-precision irradiation in mice with orthotopic pancreatic tumors, we evaluated 3 positioning methods: no positioning aid, skin marker, and immobilization devices (immobilization masks). We retrospectively evaluated the translation vector needed for optimal tumor alignment (by shifting the mouse in left-right, in cranio-caudal, and in anterior-posterior direction) on cone-beam CT from our small-animal radiotherapy system. Of the 3 methods, the skin marker method yielded the smallest mean translation vector (3.8 mm) and was the most precise method overall for most of the mice. In addition, the skin marker method required supplemental rotation (that is, roll, pitch, and yaw) for optimal tumor alignment only half as often as positioning without a positioning aid. Finally, the skin marker method had the highest scores for the quality of the fusion results. Overall, we preferred the skin marker method over the other 2 positioning methods with regard to optimal treatment planning and radiotherapy in an orthotopic mouse model of pancreatic cancer.

Dual energy CT for a small animal radiation research platform using an empirical dual energy calibration

Objective. Dual energy computed tomography (DECT) has been shown to provide additional image information compared to conventional CT and has been used in clinical routine for several years. The objective of this work is to present a DECT implementation for a Small Animal Radiation Research Platform (SARRP) and to verify it with a quantitative analysis of a material phantom and a qualitative analysis with an ex-vivo mouse measurement. Approach. For dual energy imaging, two different spectra are required, but commercial small animal irradiators are usually not optimized for DECT. We present a method that enables dual energy imaging on a SARRP with sequential scanning and an Empirical Dual Energy Calibration (EDEC). EDEC does not require the exact knowledge of spectra and attenuation coefficients; instead, it is based on a calibration. Due to the SARRP geometry and reconstruction algorithm, the calibration is done using an artificial CT image based on measured values. The calibration yields coefficients to convert the measured images into material decomposed images. Main results. To analyze the method quantitatively, the electron density and the effective atomic number of a material phantom were calculated and compared with theoretical values. The electron density showed a maximum deviation from the theoretical values of less than 5% and the atomic number of slightly more than 6%. For use in mice, DECT is particularly useful in distinguishing iodine contrast agent from bone. A material decomposition of an ex-vivo mouse with iodine contrast agent was material decomposed to show that bone and iodine can be distinguished and iodine-corrected images can be calculated. Significance. DECT is capable of calculating electron density images and effective atomic number images, which are appropriate parameters for quantitative analysis. Furthermore, virtual monochromatic images can be obtained for a better differentiation of materials, especially bone and iodine contrast agent.

Histopathological Tumor and Normal Tissue Responses after 3D-Planned Arc Radiotherapy in an Orthotopic Xenograft Mouse Model of Human Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal human cancers. Innovative treatment concepts may enhance oncological outcome. Clinically relevant tumor models are essential in developing new therapeutic strategies. In the present study, we used two human PDAC cell lines for an orthotopic xenograft mouse model and compared treatment characteristics between this in vivo tumor model and PDAC patients. Tumor-bearing mice received stereotactic high-precision irradiation using arc technique after 3D-treatment planning. Induction of DNA damage in tumors and organs at risk (OARs) was histopathologically analyzed by the DNA damage marker γH2AX and compared with results after unprecise whole-abdomen irradiation. Our mouse model and preclinical setup reflect the characteristics of PDAC patients and clinical RT. It was feasible to perform stereotactic high-precision RT after defining tumor and OARs by CT imaging. After stereotactic RT, a high rate of DNA damage was mainly observed in the tumor but not in OARs. The calculated dose distributions and the extent of the irradiation field correlate with histopathological staining and the clinical example. We established and validated 3D-planned stereotactic RT in an orthotopic PDAC mouse model, which reflects the human RT. The efficacy of the whole workflow of imaging, treatment planning, and high-precision RT was proven by longitudinal analysis showing a significant improved survival. Importantly, this model can be used to analyze tumor regression and therapy-related toxicity in one model and will allow drawing clinically relevant conclusions.

Evaluation of computed tomography settings in the context of visualization and discrimination of low dose injections of a novel liquid soft tissue fiducial marker in head and neck imaging

Background

Intraoperative incorporation of radiopaque fiducial markers at the tumor resection surface can provide useful assistance in identifying the tumor bed in postoperative imaging for RT planning and radiological follow-up. Besides titanium clips, iodine containing injectable liquid fiducial markers represent an option that has emerged more recently for this purpose. In this study, marking oral soft tissue resection surfaces, applying low dose injections of a novel Conformité Européenne (CE)-marked liquid fiducial marker based on sucrose acetoisobutyrate (SAIB) and iodinated SAIB (x-SAIB) was investigated.

Methods

Visibility and discriminability of low dose injections of SAIB/x-SAIB (10 µl, 20 µl, 30 µl) were systematically studied at different kV settings used in clinical routine in an ex-vivo porcine mandible model. Transferability of the preclinical results into the clinical setting and applicability of DE-CT were investigated in initial patients.

Results

Markers created by injection volumes as low as 10 µl were visible in CT imaging at all kV settings applied in clinical routine (70–120 kV). An injection volume of 30 µl allowed differentiation from an injection volume of 10 µl. In a total of 118 injections performed in two head and neck cancer patients, markers were clearly visible in 83% and 86% of injections. DE-CT allowed for differentiation between SAIB/x-SAIB markers and other hyperdense structures.

Conclusions

Injection of low doses of SAIB/x-SAIB was found to be a feasible approach to mark oral soft tissue resection surfaces, with injection volumes as low as 10 µl found to be visible at all kV settings applied in clinical routine. With the application of SAIB/x-SAIB reported for tumors of different organs already, mostly applying relatively large volumes for IGRT, this study adds information on the applicability of low dose injections to facilitate identification of the tumor bed in postoperative CT and on performance of the marker at different kV settings used in clinical routine.

Oncoimmunology Meets Organs-on-Chip

Oncoimmunology represents a biomedical research discipline coined to study the roles of immune system in cancer progression with the aim of discovering novel strategies to arm it against the malignancy. Infiltration of immune cells within the tumor microenvironment is an early event that results in the establishment of a dynamic cross-talk. Here, immune cells sense antigenic cues to mount a specific anti-tumor response while cancer cells emanate inhibitory signals to dampen it. Animals models have led to giant steps in this research context, and several tools to investigate the effect of immune infiltration in the tumor microenvironment are currently available. However, the use of animals represents a challenge due to ethical issues and long duration of experiments. Organs-on-chip are innovative tools not only to study how cells derived from different organs interact with each other, but also to investigate on the crosstalk between immune cells and different types of cancer cells. In this review, we describe the state-of-the-art of microfluidics and the impact of OOC in the field of oncoimmunology underlining the importance of this system in the advancements on the complexity of tumor microenvironment.

Intraoperative marking of the tumour resection surface for improved radiation therapy planning in head and neck cancer: preclinical evaluation of a novel liquid fiducial marker

OBJECTIVE

To evaluate a novel liquid fiducial marker for intraoperative marking of the tumour resection surface in oral cancer patients to facilitate precise postoperative delineation of the interface between the tumour resection border and reconstructed tissue for intensity-modulated radiation therapy.

METHODS

A total of 200 markers were created by injecting the volumes of 10 µl, 20 µl, 30 µl, 40 µl and 50 µl of a liquid marker composed of sucrose acetoisobutyrate (SAIB) and iodinated sucrose acetoisobutyrate (x-SAIB) into the soft tissue of porcine mandible segments. Visibility of the resulting markers was quantified by threshold-based segmentation of the marker volume in CT- and CBCT imaging and by a comparison of signal intensities in MRI.

RESULTS

Even the lowest volume of SAIB-/x-SAIB investigated (10 µl) resulted in a higher visibility (CT_{Soft tissue}: 88.18 ± 13.23 µl; CT_Bone: 49.55 ± 7.62 µl; CBCT: 54.65 ± 12.58 µl) than observed with the incorporation of titanium ligature clips (CT_{Soft tissue}: 50.15 ± 7.50 mm³; CT_Bone: 23.90 ± 3.39 mm³; CBCT: 33.80 ± 9.20 mm³). Markers created by the injection of 10 µl and 20 µl could reliably be delineated from markers created by the injection of higher volumes.

CONCLUSION

SAIB/x-SAIB, which has recently become available as a Conformité Européenne (CE)-marked fiducial marker, provides an option for fast and reliable production of markers with excellent visibility in imaging modalities used in oral cancer radiation therapy (RT) planning routine.

Evaluation of a Novel Liquid Fiducial Marker, BioXmark®, for Small Animal Image-Guided Radiotherapy Applications

BioXmark^® (Nanovi A/S, Denmark) is a novel fiducial marker based on a liquid, iodine-based and non-metallic formulation. BioXmark^® has been clinically validated and reverse translated to preclinical models to improve cone-beam CT (CBCT) target delineation in small animal image-guided radiotherapy (SAIGRT). However, in phantom image analysis and in vivo evaluation of radiobiological response after the injection of BioXmark^® are yet to be reported. In phantom measurements were performed to compare CBCT imaging artefacts with solid fiducials and determine optimum imaging parameters for BioXmark^®. In vivo stability of BioXmark^® was assessed over a 5-month period, and the impact of BioXmark^® on in vivo tumour response from single-fraction and fractionated X-ray exposures was investigated in a subcutaneous syngeneic tumour model. BioXmark^® was stable, well tolerated and detectable on CBCT at volumes ≤10 µL. Our data showed imaging artefacts reduced by up to 84% and 89% compared to polymer and gold fiducial markers, respectively. BioXmark^® was shown to have no significant impact on tumour growth in control animals, but changes were observed in irradiated animals injected with BioXmark^® due to alterations in dose calculations induced by the sharp contrast enhancement. BioXmark^® is superior to solid fiducials with reduced imaging artefacts on CBCT. With minimal impact on the tumour growth delay, BioXmark^® can be implemented in SAIGRT to improve target delineation and reduce set-up errors.

In vitro and in vivo evaluation of the safety and efficacy of a novel liquid fiducial marker for image-guided radiotherapy

The true extent of a tumor is difficult to visualize, during radiotherapy, using current modalities. In the present study, the safety and feasibility of a mixture of N-butyl cyanoacrylate and lipiodol (NBCA/Lip) was evaluated in order to investigate the optimal combination for application as a fiducial marker for radiotherapy. Four combinations of NBCA/Lip injection (1:1–0.1, 1:1–0.15, 1:3–0.1 and 1:3–0.15 ml) were injected into the subcutaneous tissue of BALB/c mice. The changes in gross histopathology, body weight, skin score, marker volume, neutrophil and macrophage counts were observed to analyze the effects of the different mixing ratios and injection volumes, in order to identify the best combination. Evaluation according to the International Organization for Standardization criteria was further conducted in order to test the biocompatibility of the mixture, including an acute systematic assay with mice, cytotoxicity with L929 fibroblasts and delayed-type hypersensitivity tests with guinea pigs and an intradermal test with rabbits. The results revealed that at the seventh week, 42 markers (42/48; 87.5%) were still visible using computed tomography (CT) imaging. No serious adverse effects were observed throughout the study period; however, the combination of 1:1–0.1 ml had the lowest body weight and worst skin score. A review of the histopathological reaction to NBCA/Lip revealed a combination of acute inflammation, chronic inflammation, granulation tissue, foreign-body reaction and fibrous capsule formation. The 1:1 NBCA combination ratio resulted in the most intense tissue repair reaction and a slower degradation rate of markers. In general, the combination of 1:3–0.15 ml had a better fusion with local tissue, maintained a stable imaging nodule on CT images for 7 weeks and the final biocompatibility test demonstrated its safety. Overall, the findings of the present study demonstrated NBCA/Lip as a safe and feasible fiducial marker, when using the 1:3–0.15 ml combination.

Detectability and structural stability of a liquid fiducial marker in fresh ex vivo pancreas tumour resection specimens on CT and 3T MRI

PURPOSE

To test the detectability of a liquid fiducial marker injected into ex vivo pancreas tumour tissue on magnetic resonance imaging (MRI) and computed tomography (CT). Furthermore, its injection performance using different needle sizes and its structural stability after fixation in formaldehyde were investigated.

METHODS

Liquid fiducial markers with a volume of 20-100 µl were injected into freshly resected pancreas specimens of three patients with suspected adenocarcinoma. X‑ray guided injection was performed using different needle sizes (18 G, 22 G, 25 G). The specimens were scanned on MRI and CT with clinical protocols. The markers were segmented on CT by signal thresholding. Marker detectability in MRI was assessed in the registered segmentations. Marker volume on CT was compared to the injected volume as a measure of backflow.

RESULTS

Markers with a volume ≥20 µl were detected as hyperintensity on X‑ray and CT. On T₁- and T₂-weighted 3T MRI, marker sizes ranging from 20-100 µl were visible as hypointensity. Since most markers were non-spherical, MRI detectability was poor and their differentiation from hypointensities caused by air cavities or surgical clips was only feasible with a reference CT. Marker backflow was only observed when using an 18-G needle. A volume decrease of 6.6 ± 13.0% was observed after 24 h in formaldehyde and, with the exception of one instance, no wash-out occurred.

CONCLUSIONS

The liquid fiducial marker injected in ex vivo pancreatic resection specimen was visible as hyperintensity on kV X‑ray and CT and as hypointensity on MRI. The marker's size was stable in formaldehyde. A marker volume of ≥50 µL is recommended in clinically used MRI sequences. In vivo injection is expected to improve the markers sphericity due to persisting metabolism and thereby enhance detectability on MRI.

Injectable iodine-125 labeled tissue marker for radioactive localization of non-palpable breast lesions

We have developed a ¹²⁵I-radiolabeled injectable fiducial tissue marker with the potential to replace current methods used for surgical guidance of non-palpable breast tumors. Methods in routine clinical use today such as radioactive seed localization, radio-guided occult lesion localization and wire-guided localization suffers from limitations that this injectable fiducial tissue marker offers solutions to. The developed ¹²⁵I-radiolabeled injectable fiducial tissue marker is based on highly viscous sucrose acetate isobutyrate. The marker was readily inserted in NMRI mice and proved to be spatially well-defined and stable over a seven day period with excellent CT contrast (>1500 HU), enabling fluoroscopic visualization of the marker during placement. The radioactivity remains strongly associated with the marker during the implantation period, which limits exposure to healthy tissue. Biodistribution studies show that there is negligible radioactivity in all non-tumor tissues sampled, with the exception of the thyroid gland, where limited accumulation was observed (0.06% of injected dose after 7 days). Based on the excellent performance of the marker and the fact that it can be delivered through thin hypodermic needles (≥27G), the marker holds great promise for clinical application, since patient discomfort is reduced significantly compared to current methods.

STATEMENT OF SIGNIFICANCE

A new type of tissue marker for local administration to non-palpable breast tumors has been developed. The surgical guidance marker is based on derivatives of the biomaterial sucrose acetate isobutyrate and unlike currently used markers it is injectable in the tissue using thin needles, reducing the discomfort to the patients significantly. The marker confers CT contrast and has radioactive properties, meaning it also could find use in brachytherapy. The design of the iodine-125 labeled fiducial tissue marker enables control of dosimetry as well as a choice of iodine isotope used. The marker is anticipated to be clinical applicable due to its contrast performance in mice and its potential for enhanced flexibility in surgical procedures, compared to current methods.