Presentations covered a wide range of themes, from irradiation and imaging technologies that enable precision radiotherapy studies, to the development and assessment of tumour models, as well as contributions from the field of curative radiotherapy for larger animals such as dogs. Other topics included preclinical trials, immunological and abscopal effects, radio-modifiers and experimental animal facilities with proton beams. There was also a discussion on the need for standardization, common protocols and networking in this new field, to optimally exploit its mix of specialities.

Keynote speaker

The meeting opened with a keynote address from Albert van der Kogel (University of Wisconsin, Madison), who presented an overview of earlier animal irradiation studies. Van der Kogel described work using pi-mesons in Los Alamos, spinal cord irradiations with carbon ions at Heidelberg, boron neutron capture studies in Brookhaven, Ir-192 irradiations of spinal cord in Nijmegen and 150 MeV proton irradiation studies in Gröningen.

He discussed the higher dose tolerances reported when small targets are irradiated, and mentioned the bath/shower experiments in the spinal cord. He also highlighted the finding that heart-lung irradiation is important to understand late effects from lung radiotherapy.

The keynote presentation covered many research areas that were revisited in subsequent conference talks – including the validity of the linear-quadratic (LQ) model, effects of the very high doses used in stereotactic body radiotherapy, optimization of heterogeneous dose distributions and the effects of low dose baths in modern radiotherapy.

Award-winning work

The symposium saw three young researchers lauded for their work. Sarah Krueger (Beaumont Health System) won the Best Young Speaker award, for her presentation Use of a small animal image-guided irradiator in the development of a rat model of radiation cystitis. Although haemorrhagic cystitis is a major long-term chronic side effect of pelvic irradiation that's seen in 5–10% of patients, no high-quality clinical trials have been performed in this area and treatment options are not well established. It is, therefore, an ideal candidate for a well-controlled preclinical investigation.

Krueger and colleagues used a combination of a precision irradiator, CT-guidance and an automated system to show that compared to older studies using larger radiation fields, high doses from CT-guided small tailored fields were far better tolerated. The model will now be utilized to evaluate agents (such as immunosupressants, for example) that could mitigate radiotherapy side effects.

Robert Weersink (Princess Margaret Hospital) was the winner of the award for Second Best Young Speaker. In his presentation, Targeting accuracy tests of bioluminescence imaging integrated with cone-beam CT for image-guided small animal irradiation, he discussed the need for bioluminescent imaging, integrated with modern animal irradiation cabinets equipped with precision irradiation and CT, to image and precisely target early-stage tumours.

Weersink described his work on various optical source reconstruction algorithms, noting the high complexity of the problem, due to the dependence of optical photon scatter and absorption on the wavelength, depth and distribution of the optical source. Studies in homogenous phantoms showed good targeting capabilities for parallel-opposed radiation beams and more complex beam arrangements. He noted that source reconstruction is hampered in heterogeneous mouse geometries, but that more sophisticated reconstruction techniques may help.

Finally, Rebecca Bütof (OncoRay), in conjunction with a poster presentation by her colleague Antje Dietrich, was awarded Third Best Young Speaker, for her talk Establishment of orthotopic tumour models for small-animal image-guided radiotherapy. Bütof and colleagues have studied the implantation of orthotopic tumours (those situated in their naturally occurring site) in the lung and brain of mice. Such models are hypothesized to represent human tumours better than the commonly used human xenograft tumours implanted subcutaneously in mice. The research revealed many details regarding the practicality of cone-beam CT and optical bioluminescence imaging of orthotopic tumours and should help pave the way towards more realistic tumour preclinical studies.

Honourable mentions

Ashley Rubinstein (MD Anderson Cancer Center), winner of the Best Young Speaker award at the 2013 conference (see Winning work in small-animal radiotherapy), returned to present a Monte Carlo study on magnetic field dose effects in mice. She discussed the optimal combination of beam energy and magnetic field strength required to scale down MRI-linac radiotherapy.

The presence of a magnetic field during radiotherapy can cause hot and cold dose spots at high–low density interfaces, with unknown radiobiological consequences. Rubinstein simulated various beam energies and field strengths to best adapt the secondary electron range and radius of curvature caused by the magnetic field to murine anatomy. She showed that investigating magnetic field-induced radiobiological effects in a preclinical setting requires higher beam energies than those currently used in preclinical radiotherapy research platforms.

One of the meeting's youngest speakers, Hwan Lee (British Columbia Cancer Center), delivered a presentation that provoked much discussion. Lee described experiments investigating the abscopal effect – a purportedly immune-mediated inhibition of distant tumours upon primary tumour irradiation. The studies used mouse models with subcutaneously implanted breast cancer cells at each shoulder, of which one was treated with a hypofractionation scheme of 4 x 12 Gy. Lee and colleagues assessed tumour response using FDG PET/CT scans and calliper measurements and saw that both irradiated and non-irradiated tumours on the same animal showed significant growth inhibition, as well as increased FDG uptake, compared with a control group.

Another eye-catching presentation was given by Niloufar Zarghami (London Health Sciences Centre, Canada) on the targeting accuracy of a 3D-printed immobilization device for mouse brain irradiation. Evaluating the device in six mice, using half-brain irradiation and staining for double-strand breaks, revealed a midline targeting accuracy of less than 200 µm, sufficiently accurate for future brain irradiation studies.

What distinguished this work is how well it highlighted the wide-ranging skills required to fulfil preclinical research goals – a mix of technology-driven fabrication and wet lab work. Moreover, to benefit the relatively small community of researchers, Zarghami and collaborators have made their design available as a stereolithography file.

Orthotopic tumours

One recurrent theme during the symposium was the use of orthotopic tumour models for image-guided radiotherapy, preferably based on bioluminescent imaging, to better mirror clinical situations and ease translation of study results into clinical trials.

The first model discussed was the luciferase-expressing brain tumour, as described in the abovementioned work by Bütof. Frank Giordano (University Hospital Mannheim) used a similar tumour model to demonstrate proof-of-principle of high-precision irradiation using three beams and a fractionated treatment schedule to spare oesophageal and tracheal structures. Jay Dorsey (University of Pennsylvania) combined gold nanoparticles with high-precision irradiation of orthotopic glioblastoma tumours and observed improved survival. He noted that the combination of gold and superparamagnetic iron oxide nanoparticle (SPION)-loaded micelles could provide an MRI contrast agent with sufficient sensitivity to detect and delineate tumours for treatment planning. Sanaz Yahyanejad (Maastricht University) demonstrated good correlation between contrast-enhanced CT and luminescence imaging and observed a dose-dependent irradiation effect on orthotopic glioblastoma growth.

The second model examined was the orthotopic lung tumour, originating from cells or tumour piece implantation, or from spontaneous genetic models. Bütof showed that tumour piece implantation is preferable to cell injections to prevent growth in the injection channel and early metastatic spread. Using mice that spontaneously develop lung tumours and integrated breath-hold techniques, Rubinstein examined whether the cone-beam CT of a small-animal irradiator can be used to investigate the relationship between tumour image features and characteristics such as aggressiveness (a study by MD Anderson's Laurence Court).

The final model presented was the metastasizing cervix tumour model established by the group of Richard Hill (University of Toronto). The team used an 8-beam protocol with daily target imaging prior to fractionated irradiation combined with various other therapeutic agents. All treatment modalities demonstrated enhanced primary tumour response and reduced growth of lymph nodal metastases when combined with radiation, without significant short-term gastro-intestinal toxicities.

All of these studies provide evidence that new clinically-relevant tumour models are available to fully exploit the capabilities of novel small-animal irradiators combined with imaging technology. Such developments will enable researchers to better mimic clinical radiotherapy practice using complex preclinical radiation delivery strategies.

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