Copyright OSA 2006

Particularly in the later stages of the disease, cancer cells can break away from a tumour and travel around a patient’s body through the blood or lymphatic systems. These cells then grow, or metastasize, on normal tissue, enabling the cancer to advance. Detecting circulating metastases provides an early indicator of the spread of cancer and presents doctors with a method of assessing the effectiveness of cancer treatments. The present technique, analysing lymph nodes or bone marrow, is not very accurate and does not detect tumour cells travelling in blood. Consequently, there is a need for a system sensitive enough to detect metastases at a concentration of about one tumour cell for every million normal blood cells.

Ryan Weight and his colleagues at the University of Missouri have demonstrated an optics-based system that could represent a path to achieving this1. Their method uses photoacoustics. When a cell, suspended in a liquid, absorbs a laser pulse, the resulting heat generated causes it to rapidly expand and contract producing an acoustic wave that travels through the surrounding liquid. Detection of these waves after pulsed laser illumination indicates the presence of the light-absorbing cells — in effect, laser-induced ultrasound. With this basic principle, Weight et al. fired 11–12-mJ light pulses with a wavelength of 450 nm at melanoma cells (a type of skin cancer) suspended in saline. The resulting acoustic vibrations were detected using a piezoelectric polymer film, verifying the potential of the technique. Further development of such photoacoustic systems could aid the detection, and consequently the treatment of cancer in the future.