Cancer affects nearly 50% of individuals in the UK, and accounts for around 30% of all deaths. For almost all cancers, both prognosis and treatment fundamentally depend on the degree of spread at diagnosis i.e. tumour stage. For example, early stage bowel (colorectal) cancer confined to the bowel wall has a cure rate of nearly 95% whereas it is lower than 50% by the time it has spread to the lymph nodes (LNs). Similarly, patients with LN involvement from breast cancer may require more extensive surgery to remove the affected node, and have poorer long-term survival than patients without LN disease, even with small volumes of nodal tumour. The same is true for most other solid organ malignancies. So, as a general oncological principle, patients with advanced disease often require more aggressive treatment such as pre-operative (neo-adjuvant) chemotherapy, radiotherapy or both; and usually need more extensive surgery / radiotherapy when definitive treatment is instituted.

Such therapeutic decision-making depends fundamentally on the advance knowledge of whether or not a particular patient has disease spread. This, in turn, requires a battery of tests designed to accurately pinpoint the extent and anatomical location of cancer dissemination throughout the body. This is typically achieved via a combination of medical imaging tests (such as ultrasound, computed tomography \[CT\], magnetic resonance imaging \[MRI\] and nuclear medicine techniques e.g. positron emission tomography \[PET\]) and tissue sampling (biopsy). Unfortunately, these techniques are imperfect, both for local staging of the primary cancer and its spread to local LNs. For example, in some cancers, over 50% of malignant LNs measure less than 10mm, the most commonly-used cut-off to define an abnormal lymph node using conventional medical imaging\[9\]. Therefore, for many tumours it is common practice to treat the entire regional LN group, either by surgical removal or radiotherapy. This strategy risks overtreatment for many and introduces the adverse effects associated with such extensive tissue damage. Regarding primary tumours, the precise extent of local tumour spread can determine whether or not a limited, local resection can be achieved rather than a more radical excision. A rapid, non-invasive, well-tolerated test that could improve local and regional nodal cancer staging would be of great clinical value, since it would immediately permit more accurate individualized treatment strategies.

Photoacoustic tomography (PAT) is a relatively novel technology that may be able to help address this urgent clinical need. PAT relies upon the absorption of laser-generated light of specific wavelengths (often in the infra-red spectrum) by intrinsic components of the imaged tissue. Such absorption results in emission of sound waves, which are ultrasonic (i.e. very high frequency). Images can then be reconstructed in a similar manner to that employed by a clinical ultrasound scanner. By imaging at multiple wavelengths, tissue distribution of water, lipid (fat) and haemoglobin (in red blood cells and therefore blood vessels) can be mapped with extremely high resolution (\~100 microns), raising the possibility that PAT can depict the small volume tumour that existing techniques cannot.