You will be shown biomedical passages and your task is to the answer the provided question. Provide a concise and accurate response and do not include any additional text in the response other than the answer.

Question: How rare are CTCs (circulating tumour cells) in the plasma of patients?

Context: We have focused on breast cancer as most clinical studies on CTC detection so far have been done in these patients.
However, largely because of the extremely low number of CTCs (as low as 1 in 10(9) hematologic cells) in the blood of patients, effective detection and separation of the rare cells remain a tremendous challenge.
However, selective capture and quantification of CTCs from whole blood was still full of challenge due to the extremely scare number of CTCs.
Finally, the IMNs were successfully applied to the isolation and detection of CTCs in cancer patient peripheral blood samples and as low as one CTC in the whole blood was captured and identified by the ICC method.
This study reports a microfluidic-based optical sensing device for label-free detection of circulating tumor cells (CTCs), a rare cell species in blood circulation.
1 in 10 to 9th, or 1e-9, or 1 in 10^9