Chemosensitivity testing of a primary tumour is possible on the one hand by examination of a tumour biopsy, i.e. material is taken from the tumour by a minor surgical procedure (usually a so-called punch biopsy cylinder approx. 3mm long and 1mm in diameter). This is not always possible as tumours may sometimes grow together with healthy tissue, occupy a very deep-seated position or envelop important blood vessels.

For this reason it makes sense to isolate the disseminated cells (CTC) of the primary tumour from the blood and perform chemosensitivity testing. The CTC contain genetic information from the primary tumour and above all the information about the "Achilles' heel" of the future metastases.

Isolating the CTC is not an easy task. One millilitre (1 ml) of blood contains millions of blood cells (red and white blood cells, immune cells etc.) amongst which one to ten circulating tumour cells are present.

As described above tumour cells differ from healthy cells through their somewhat different genetic activity and composition. This is also manifest in slightly altered structures on the outer surface of the tumour cells. So-called antibodies are able to recognize these structures; they can thus distinguish to a certain extend between healthy cells and tumour cells and bind to the tumour cells. The further procedure is ingenious: the antibodies are coupled to tiny magnetic particles (micrometre sized, μ) and the blood sample is infused with these “magnetic beads“. The antibodies then fish around for tumour cells, attach to the cells and can be drawn out of the sample by means of a larger magnet after 20-30 minutes. To make sure that these are really tumour cells and not other blood cells at least the expression of approx. 10 marker genes typical of tumours are quantified. In addition to this the isolated cells are examined microscopically for tumour cells using suitable antibodies and cell staining.

Then the expression of certain genes and their corresponding proteins (signal proteins, enzymes, receptors etc. e.g. EGFR, DCK etc.) which are used by the tumour cell is determined at the mRNA level in the CTC. The data were validated and incorporated into a therapy proposal.

The procedure is suitable not only for circulating tumour cells but is also used for biopsy specimens from solid tumours.

The number of CTC in the blood can fluctuate, so that in approx. 5 – 10% of samples at a particular time no cells can be found. After a certain time (3 – 6 days) the situation changes and CTC can be detected again. Drugs used in chemotherapy can also affect the number of CTC.