Photoacoustic detection of circulating melanoma cells as a predictor of metastasis in stage III patients.

e21053 Background: Circulating tumor cells have been correlated with disease state and distant metastatic spread in cancer patients. We postulated that enumerating circulating melanoma cells (CMCs) may predict the onset of distant metastasis in Stage III patients. We detected CMCs using our photoacoustic flow cytometer, in which we irradiated enriched blood samples with nanosecond pulsed laser light. While there is no effect on non-optically active leukocytes, absorption of laser light by pigmented melanoma cells resulted in robust ultrasonic waves that indicated CMCs in the sample. Methods: We tested 32 archived samples from 9 Stage III melanoma patients using our photoacoustic flow cytometer. Each patient had between 2 and 6 serial blood samples. We used a pulsed Nd:YAG laser to irradiate mononuclear cells in suspension and under flow. The number of CMCs detected after testing was recorded, indicating the time sequence of circulating tumor cell activity. Results: The numbers of CTCs for each sample is shown in the table below. The ultimate disease state, whether the patient became metastatic or not, was blinded to the investigators who performed the photoacoustic tests. One sample for patient 3 indicated 63 CMCs, though this test was known to be contaminated and had an unknown number of false detections. Conclusions: We found that patients who had a series of more than 4 CMCs were more likely to become metastatic than those patients who tested for 4 CMCs for fewer, indicating that a sequence of CMC detections in serial blood draws provides a potentially strong predictor of metastasis in Stage III melanoma patients warranting further investigation at this and lower stages of melanoma. We are developing a more rigorous model based on time series analysis of CMCs for prediction of metastasis. [Table: see text]