My partner’s previous post illustrated several issues which
might change the number of detected CTCs in the bloodstream. While the study of
CTCs holds many possibilities, my partner made a fair point to discuss that
they have a fair number of limitations. One such limitation that he described
was entrapment, in which CTCs sometimes get clogged within the capillaries,
causing them to be underrepresented in blood samples. In my research I also stumbled
upon similar phenomena in which the rate of CTC flow seemed to fluctuate. One particular study looked into the rate of CTC flow in both large and small blood
vessels for both melanoma and breast cancer. The figures below display the
results for these tests.
In this test, a set of mice was inoculated with melanoma cells
in the flank in order to quickly develop metastatic skin melanoma. The CTCs
were then periodically monitored by PA flow cytometry (PAFC) for 20 minutes in large
blood vessels in the skin (150-250 μm
in diameter). The figure above displays a rather curious outcome. The
rate of CTCs appears to fluctuate wildly. In the observations after the first
week, the CTC rate rose to their maximum at 6 CTCs per minute within 6 minutes
and then quickly fell to 1 CTC per minute and remained at that level until the
end of the 20 minute mark. Furthermore, there were several gaps in which the
blood flow was CTC free for 1 to 3 minutes at a time. The rate also fluctuated
at week 2 after inoculation. After 8 minutes, the CTC rate rose to its maximum
level at 9 CTCs per minute, but then gradually decreased over the 20 minutes.
Furthermore, while not as pronounced as the first week, there were also 3 CTC
free gaps observed in the second week. Each of them was only 1 minute long.
The figure above displays the tests done on mice inoculated
with breast cancer. In these tests, CTCs were continually monitored with
fluorescence flow cytometry (FFC) in the small blood vessels of the ear. At
week 2 the CTC rate would quickly rise and fall. Within 15 minutes, the CTC
rate gradually approached its maximum value of 4 CTCs per 5 minutes. However,
this was immediately followed by a 20 minute CTC free gap after which the rate
fell to 1 CTC per 5 minutes. The rate would then fluctuate wildly between these
gap periods. Furthermore there were 4 of these CTC free periods which each
ranged from 15 to 25 minutes. When observed at week 5, the CTC rate appeared
to gradually rise to its maximum value at approximately 55 CTCs per 5 minutes
within 45 minutes and then fall back to 30 CTCs per 5 minutes in 15 minutes. At
this time there was only one 5 minute CTC free period after 35 minutes of
observation.
Most studies comparing the number of CTCs in the blood with
the survivability of cancer utilize a blood draw. Therefore, most go under the
expectation that the concentration of CTCs is constant throughout the bloodstream.
However, both the small and large blood vessel tests implied that the CTCs are
not uniformly distributed within circulating blood and that their amount is not
consistent over a period of time. This is most exemplified by the appearance of
CTC free gaps and the rapidly fluctuating nature of the CTC rate. Nonetheless
both these traits were suppressed over time for both blood vessel types. At the
end of both tests, the CTC rate appeared to fluctuate much more gradually and
the number of CTC free periods decreased. Because these traits also appeared in
several types of cancer, it is implied that they could be universal to CTCs
rather than dependent on one specific type of cancer. However further tests
must be done in order to properly enforce this conclusion. Nonetheless, these
fluctuations imply that either the CTCs are arrested at certain points in the
circulatory system or there are fluctuations in the shedding of CTCs from the
primary tumor. These issues could present a problem with CTC studies as they
have the potential to skew the number of observable CTCs in a blood draw.
However it is possible for this problem to be solved by utilizing larger blood
draws. Nevertheless, more tests must be taken in order to validate these
claims.
Sources
Juratli, Mazen A., Mustafa Sarimollaoglu, Dmitry A. Nedosekin,
Alexander V. Melerzanov, Vladimir P. Zharov, and Ekaterina I. Galanzha. “Dynamic
Fluctuation of Circulating Tumor Cells during Cancer Progression.” Cancers. 6.1
(2014): 128-142. Web. 27 May 2014.
