Photoacoustic detection of circulating melanoma cells in human blood

Abstract

Circulating melanoma cells (CMCs) spread in the blood and lymph systems, seeking to create secondary tumors at distant sites from the primary tumor.1–3 A patient with CMCs has metastatic disease and is in the most dangerous phase of the cancer cycle. Detection of CMCs may serve as an early indicator of metastasis or relapse, and tracking their number may enable noninvasive disease monitoring. Likewise, their absence may provide important staging information for clinicians and patients. Current methods, such as computed tomography scans and positron-emission tomography imaging, can only detect metastatic disease when secondary tumors are at least a few millimeters in size, and thus composed of millions or perhaps billions of cancer cells. Detection of circulating tumor cells (CTCs) is an active research area that has met with limited success. Most methods, such as the real-time polymerase chain reaction (RT-PCR) technique, immunomagnetic separation, and fluorescence cytometry, use some sort of enhancement process to detect these cells or their biochemical precursors.4–6 These approaches are often time consuming, complex, and of limited accuracy. We have designed a rapid, label-free photoacoustic flowmetry system that can detect and count single melanoma cells in human blood samples (see Figure 1). We used nanosecond laser pulses to irradiate white blood cells (WBCs) derived from patient blood samples and separated by standard centrifugation.7, 8 If CMCs are present, they would reside among the WBCs because of their similar densities. Laser irradiation of pigmented CMCs induces distinct highfrequency acoustic transients. WBCs, on the other hand, do not have significant chromophores. By targeting melanin, a broadband optical absorber found in approximately 95% of Figure 1. (top) Schematic of the photoacoustic flowmeter for detection of melanoma cells in blood samples. PVDF: Polyvinylidene fluoride. (bottom) Laser irradiating the glass flow chamber.