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How to Spot Circulating Tumor Cells

Cancer cells that have broken away from the main tumor can spread the disease. Now scientists are developing better ways to find them.

One way that cancer spreads through the body is through circulating tumor cells. These are cancer cells that have broken away from the main tumor and begun to circulate in the blood. A new tumor can form if they become embedded elsewhere in the body and begin to grow.

So spotting circulating tumor cells is an important goal in the treatment of cancer.

Here’s the problem though. Circulating tumor cells are extremely difficult to find. In a single millilitre of blood there are usually several billion red blood cells, several million white blood cells but fewer than ten circulating tumor cells.

And there is the only one way to find them. The cells can be made to look different from normal blood cells. So you need a highly trained cell biologist with a microscope and plenty of time. The words needle and haystack don’t do this task justice.

Various groups are looking for better ways to find circulating tumor cells and their efforts fall essentially into two categories. The first is biochemical: trapping the cells using antibodies that bond to them. The second is mechanical: filtering them out.

Both of these methods have drawbacks. Antibodies can only bond to the cells if they can get close enough. And although circulating tumor cells are bigger than red blood cells, they are about the same size as white blood cells so filters have limited success..

Today, Markus Gusenbauer at St. Poelten University of Applied Sciences in Austria and a few buddies make some progress in this area. These guys have developed a computer model of the way blood flows through a bed of magnetic beads.

When a magnetic field is applied to such a bed, the beads line up into strings that together form a filter with a specific gap size. Whether a cell can pass through depends on its size and also its flexibility.

The Austrian team’s model takes into account the size and flexibility of both red blood cells and circulating tumor cells to show how this kind of switchable filter could catch the bad guys.

The idea here is that the beads would also be covered in an antibody that latches onto the circulating tumor cells, keeping them trapped even when the magnetic field is switched off. This method uses both of the current techniques to overcome their drawbacks.

The plan would be to store the beads in a chamber in a microfluidic lab-on-a-chip device. A blood sample containing a handful of circulating tumor cells but billions of other types is pumped into the chamber and the magnetic field switched on.

This causes the beads to line up in a filter that traps the biggest cells. The antibodies on the beads then latch on to their targets, trapping them for later study.

That’s the theory anyway. In reality, these guys have a lot more work to do before such a system can be made to work. For a start, circulating tumor cells come in a number of different flavours and the mechanical characteristics of each will need to be worked out.

More serious is the problem with white blood cells. Being a similar size to circulating cancer cells means they could easily clog these kinds of filters.

But the reality is that this kind of problem can only be solved by understanding what’s going on on the level of individual cells and engineering a solution that works on this scale. That’s why this kind of simulation is a useful first step.

Ref: A Tunable Cancer Cell Filter Using Magnetic Beads: Cellular And Fuid Dynamic Simulations

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