But despite their obvious overlap in goals and scope, until recently, the AAPM and PS-OCs had not formally communicated or interacted. To address this disparity, the Horizons 2013 meeting brought these two groups of physicists together for the first time. Co-organized by Robert Jeraj (University of Wisconsin) and Thomas Bortfeld (Massachusetts General Hospital) for AAPM, and Jan Liphardt (Stanford University) for PS-OC, Horizons 2013: Connecting AAPM and PS-OCs aimed to identify common interests, provide roadmaps for future joint activities and identify potential new collaborators.

A meeting of minds

The Horizons 2013 meeting, held last November in Bethesda, MD, was organized around six themes, each addressed by two presentations, one from AAPM and one from PS-OC. In the first session, dedicated to cancer complexity, Soren Bentzen (AAPM) from the University of Maryland, Baltimore, summarized current knowledge regarding the vast complexity and heterogeneity of the disease.

An important lesson was that linking genomics to patient-to-patient variability requires large validation studies. Small studies often give false negative results, such as the case of single nucleotide polymorphisms' (SNPs) correlation to the late radiotherapy toxicity, for example. In fact, given the enormous complexity and heterogeneity of cancer, it is actually somewhat surprising that we are able to treat so many patients successfully, he said.

PS-OC speaker Robert Austin (Princeton University) pointed out that the heterogeneity of cancer may give rise to ecological niches, which in turn allow cancer evolution, possibly resulting in resistance to therapy. His main message concerned the application of evolutionary game theory to cancer. Based on his models and experiments with bacteria, Austin came to the provocative conclusion that there may be no cure for cancer – our best chances are to manage the disease, i.e., to keep the cancer at bay over many years, without actually eliminating it.

Progress in finding a cure for cancer was controversially discussed in this session, as well as throughout the rest of the conference.

In the second session, focused on cancer biomarkers, Stanford University's Parag Mallick (USC PS-OC) highlighted the importance of multi-scale modelling for understanding cancer and response to therapy. Multi-scale modelling was picked up by speakers in other sessions as well, and became a recurring theme of the conference. Mallick discussed the many scales relevant for cancer development, starting from the gene and protein level, moving to cells, tissues, organs and, finally, the entire organism.

Speaking for AAPM, Jeff Siewerdsen (Johns Hopkins University) examined a different side of modelling – related to imaging. He presented a number of exciting and clinically relevant innovations in imaging technology and image-guided interventions. While the modelling goals might be different (model-based image reconstruction versus model-based tumour evolution), the modelling approaches are often remarkably similar.

The clinical take

The next session – looking at treatment response and outcome modelling – had a more clinical focus. Shelley Hwang (Stanford PS-OC), a breast cancer surgeon from Duke Medical Center, reported on her interactions with physicists from the clinician's perspective. She mentioned the importance of "traditional" medical physics approaches in advancing imaging (specifically tomosynthesis) and radiation therapy. She also discussed her work with Stanford PS-OC, namely measurements of the mechanical properties (stiffness) of breast tissue and its relation to breast cancer risk, as well as measurements of circulating tumour cells (CTCs) and their prognostic value.

AAPM speaker Randy Ten Haken (University of Michigan) provided a fantastic example of a several-decade-long joint effort between physicists and radiation oncologists to refine our understanding of the induction of radiotherapy toxicity in liver. He explained how modelling the response of liver tissue to radiation has had a substantial practice-changing impact in radiation oncology.

Ten Haken pointed out that this modelling and model refinement effort, along with corresponding clinical trials, is a long-term process that may take decades. Like several other speakers, he quoted George Box: "Essentially, all models are wrong but some are useful". The liver dose-response model belongs to the group of extremely useful models, he noted, but only after putting a great deal of effort into it.

The final session of the first day was devoted to the AAPM and PS-OC visions. AAPM president John Hazle (MD Anderson Cancer Center) summarized the 55 year long history of AAPM and its focus on clinical and translational work, mainly in imaging and radiation therapy. He pointed out that only 10% of AAPM members are primarily involved in research and education. However, with 8000 members in total, those 10% constitute a significant workforce. Part of Hazle's vision is that some non-traditional "wild physics" should be promoted.

The PS-OCs, represented by Office of Physical Sciences Oncology programme director Larry Nagahara, have a much shorter history. But unlike AAPM, they were created to do the "wild physics" from the start. Twelve centres taking a fresh look at the cancer problem from many different (but all physical sciences related) angles are currently supported by the NCI. The programme announcement for the next PS-OC funding cycle will come out soon and will highlight two suggested themes: the physical dynamics of cancer, and spatio-temporal organization in cancer.

The second day of the meeting began with a session on advanced technologies. Starting with "small technologies", PS-OC speaker Peter Kuhn (Scripps Research Institute) discussed fluid biopsies – new techniques for measuring CTCs in the bloodstream. One of his research team's goals is to better understand the fluid dynamics of blood and its implications for cancer metastasis.

These experimental methods investigating metastasis are matched with the modelling efforts in a form of a Markov chain model to derive metastatic pathways from a large number of autopsies. Their simple "spreaders and sponges" model, based on the algorithm employed in the Google search engine, led to results that agreed surprisingly well with common knowledge about where certain tumours tend to metastasize.

At the other end of the spectrum, Rock Mackie (AAPM) from the University of Wisconsin talked about "large technologies". He gave several examples of successful projects and reported specifically on his own experience with TomoTherapy, in which he has been intimately involved as an inventor, developer and entrepreneur. One of Mackie's key points was the importance of project management for the success of any big project. An active discussion evolved around the question of how important project management is to win the war against cancer, and the related question about the balance between goal-driven research/engineering and curiosity-driven research.

The last session was dedicated to precise targeting. The University of Toronto's David Jaffray (AAPM) presented advances in targeting radiation, in particular, the development of innovative technologies for image-guided therapy. Jaffray also gave an outlook into the use of gold nanoparticles as radiation sensitizers, as well as the use of nanoparticles to improve imaging.

Mauro Ferrari (Methodist Hospital Research Institute PS-OC) rounded off the workshop with an overview of the fascinating potential of nanomedicine. Many physics-related questions remain to be answered in this field. Ferrari mentioned the idea of shaping nanoparticles for optimal targeting, for example. It turns out that spherical particles are the least effective, because they do not remain attached to the target; other shapes such as disk-shaped particles are much preferred.

Looking ahead

Based on the successful exchange of ideas between PS-OCs and AAPM at this workshop, a few action items were agreed upon that will bring the two closer in the future. First of all, a joint working group between AAPM and PS-OC will be formed to coordinate future joint activities. Second, participation of PS-OC investigators at the Annual AAPM Research Meeting and AAPM researchers at the Annual PS-OCs' Network Investigators' Meeting will be enhanced.

In light of this collaboration, the topic selected for the prestigious Science Council session at the 2014 AAPM annual meeting is the "Physics of Cancer", a subject of clear interest to members of both groups. Similarly, both groups will actively participate in the next Horizons meeting, anticipated for 2014 and devoted to "Bridging the Scales". Finally, and most importantly, research collaboration between the two groups is foreseen through exchange of multiple types of data and models.

The Horizons 2013 meeting clearly achieved its main objective: expanding horizons for every one of the 75 participants. All are excited about the future potential of synergies between AAPM and PS-OC to jointly attack one of the most pressing problems of our time – how to effectively fight and defeat cancer.

• The Horizons 2013 presentations are available to access online.

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