In
my last blog post, I gave a brief introduction and my own analysis of one study
done with circulating tumor cell (CTC), emphasizing some of the topics that we
covered earlier in this quarter. This week, I focused on a study described by
another paper with particular focus on the variables that may render analysis of
circulating tumor cells ineffective as a predictor for reactions experienced by
cancer patients. There were also some inconsistencies that I noticed between this
study and the one I analyzed in my last post and I will be addressing these
differences later in this post.
As a quick initial somewhat obvious note, circulating
tumor cells are mainly identified and are studied in patients who have cancer
that is at a metastasizing stage. For reasons unknown (though likely because of
the current prevalence it has in today’s society), in the majority of studies
that check the ability to predict cancer progression off of CTC concentration,
breast cancer is often the disease analyzed; however CTCs can be detected in
blood from a patient with metastatic and primary carcinomas. Generally, the
trend that has been found with multiple studies of patients with metastasizing
breast cancer is that they generally have elevated levels of CTC, such as in
the study described in "Circulating tumor cells disease progression and survival in metastatic breast cancer" and "Circulating Tumor Cells: A Novel Prognostic Factor for Newly Diagnosed Metastatic Breast Cancer". The reason why CTC analysis is often seen as a promising
and crucial area of development today is because of the fact that cancer
mortality is a result of widespread metastasis of dysfunctional/irregular cells
who have deviated from normality. The formation of CTCs in the circulatory
system is mainly a result of the invasion of the primary tumor into the
surrounding tissue. As a result of this movement from the site it was initially
localized, there is also translocation that causes entrance into the blood and
lymphatic vessels. It is because of this reason that when analyzed, tumors in
later stages have grown and invaded surrounding tissue and possibly to the
scale of invading at an organ scale, however by that point patient survival is
quite low if nonexistent. It is strongly believed that dissemination into other
areas that are distant from the location of the initial tumor site is due to CTC
presence and the concentration itself is affected by the condition of the
initial tumor site and at which state the tumor is at.
As mentioned in the comments on my first post,
there are many other factors that I did not consider that may compromise the
efficiency of actually implementing CTC analysis as a manner of monitoring
cancer growth.
I found a separate article that conducted a study,
but also focused on this issue. In the article "Circulating Tumor Cells" the issues that could compromise the accuracy of CTC concentration were addressed.
The main recurring issue observed that
compromises the accuracy of CTC count is the fact that the circulatory system
isn’t solely a defined pathway that allows for ample room for transport,
particularly for material such as CTCs that shouldn’t normally be present in
the blood. It is because of this that there may be a form of CTC blockage or
minor clogging in areas such as capillaries where the total ample volume is
minimized. There is also the possibility that the CTCs themselves can be “masked”
via platelets.
The
image above shows a brief summary of all the many factors that can affect the
circulation and, as a result, also affect detection of CTCs within the
bloodstream. The ones that I chose to focus on were filtration, clustering, and cloaking.
As mentioned earlier, CTCs are not something
normal that should not be present within the bloodstream and as such, their
size is approximately three to four times that of the capillaries. This is a
probable cause for entrapment and as such, I believe that this entrapment and
clogging results in underrepresentation of the amount of CTCs that are actually
present in the circulatory system. This means that the only circulating tumor
cells that are detected via epithelial markers are those that have evaded all
of the biophysical and strictly biological factors resulting in counts that are
much lower than what it should be. If this difference is significant enough to
cause discrepancies as well as lower concentration readings to the point where
differences are no longer notable among cancer patients, this would lead to CTC
analysis being rendered largely inefficient. However, multiple studies being
done, particularly in breast cancer research, show significant differences in
CTC concentration among patients with differing tumors in different states, as
the study ahead suggests.
Due to the large amount of studies that revolve
around the use of CTC analysis for breast cancer I decided to read through “Circulating
Tumor Cells, Disease Progression, and Survival in Metastatic Breast Cancer.” This
study took place at 20 clinical centers within the United States and was performed
in order to evaluate the actual usefulness of CTCs in predicting the patient’s
response to therapy as well as progression-free and overall survival in
patients with metastatic breast cancer.
Patients
were initially evaluated under standard imaging and had blood samples drawn.
Disease status was then reevaluated every 9 to 12 weeks. Quite notably their
status in terms of the disease was defined using the criteria set forth by the
World Health Organization without factoring the level of CTCs. Something that I
noted about the study was that it did not make explicitly clear as to how their
study was considered a blinded study. However it turns out the “blinding” in
this study wasn’t solely because the administrators didn’t know the difference
between the control and experimental, but rather the unknown knowledge of
variance within the control. The control was split in half in terms of their
state in respect to menopause. This was further split by those who had no known
illness or history of cancer. However, the control also included 200 other
women that had diseases that either had benign breast disease or some other
type of nonmalignant disease. When I saw that the control had this many
factors, I questioned the validity of some of their results, particularly those
that showed differences that weren’t extremely noticeable. So many other
factors were introduced to the standard to which they had to compare that this
could have caused variance within the control. Similar to the article
that I analyzed last time, blood samples that were drawn were analyzed through
the use of a CellSearch Epithelial Cell Kit and immunostaining via DAPI.
However something that was noticeably different between this study and the one
made in the separated study was that the CTC counts were reevaluated via the
CellSpotter system. Though this could have possibly reduced errors by
recounting, this made me question the reason why the other study only opted to
use one form of analysis. By evaluating using two different methods, the
chances of misreading and having the wrong number of CTCs accounted for is
minimized. Ultimately it allowed me to make my own conclusion in that even
within CTC analysis, there are multiple subdivision into different categories
that jeopardizes accuracy of one sole form of defining samples.
The above figure shows the relationship between CTC concentration and the set conditions. A part that I thought was most prevalent was the P values for the difference between those within the positive HER2 group and those in the negative HER2 group. Interestingly enough, it is clear to see that none are below the .05 level of significance meaning that there was no significant difference between the percentage of patients that constituted the two types of HER2 states at each CTC level.
I
further analyzed the study and I noticed something interesting at the section
where manners of analysis were critiqued. Paralleling my assumption that I made
with the previous study that I analyzed, this study showed a greater amount of
attention to the relevance of the type of analysis that was used when compared
to the other studies. Basically, it mentioned CellSearch and CellSpotter Assay
systems and their abilities to detect rare epithelial cells in the blood as a
whole. Overall, in this study it was concluded that CTCs can be effective
predictors for progression-free, as well as, overall survival. However, their
results did not end there since it appeared that patient outcome predictions
via CTC was not capable of being applied at all stages. Not only does the limited
predictive accuracy seem to be confined to those that had a form of CTC
analysis before a new therapy as well as in their first follow up visits, but
the data also suggested that the strength of the predictions that can be made
from CTCs further on during treatment do not give accurate representations as to
outcomes at even later stages.
Additionally,
something that caught my attention was that in part of this study where they
had to assess a cutoff point they chose to make it at when CTCs were greater
than or equal to 5 cells in the 7.5 ml of blood sample. When compared to this
study, it is clear to see there are differences in what is categorized and
defined into a certain category seeing as how the other study chose to define
the cutoff at 4 CTCs per 7.5 ml of blood. This further increased my doubt in
the universality of the standard at which conditions are defined. I don’t see
how one issue can be indiscriminately be categorized into two classes and not
have some sort of scaling that separates those with differing CTC concentration
into different levels rather than just solely splitting up the group at one
cutoff point and making two defined sets. I believe that strictly setting them
into two groups is too “static” and inflexible in categorizing the patients.
However, something else that I considered when reading through this study was that
if the manner of grouping was done via a scale, then most, if not all, patients
would have fallen into different categories, leading to no solid significant
difference/trend found among the patients leading to largely inconclusive
results. At the scale of this study and the amount of resources used, it would
seem that attaining such inconclusive results would have rendered this study
inefficient.
References
Cristofanilli, M. "Circulating Tumor Cells: A Novel Prognostic Factor for Newly Diagnosed Metastatic Breast Cancer." Journal of Clinical Oncology 23.7 (2005): 1420-430. Web. 18 May 2014.
Cristofanilli, Massimo, G. Thomas Budd, Matthew J. Ellis, Alison
Stopeck, Jeri Matera, M. Craig Miller, James M. Reuben, Gerald V. Doyle,
W. Jeffrey Allard, Leon W.m.m. Terstappen, and Daniel F. Hayes.
"Circulating Tumor Cells, Disease Progression, and Survival in
Metastatic Breast Cancer." New England Journal of Medicine 351.8 (2004): 781-91. Web. 18 May 2014.
Miller, M. Craig, Gerald V. Doyle, and Leon W. M. M. Terstappen.
"Significance of Circulating Tumor Cells Detected by the CellSearch
System in Patients with Metastatic Breast Colorectal and Prostate
Cancer." Journal of Oncology 2010 (2010): 1-8.
Plaks, V., C. D. Koopman, and Z. Werb. "Circulating Tumor Cells." Science 341.6151 (2013): 1186-188. Web. 18 May 2014.
