Can existing liquid biopsy instrument replace tissue biopsy?
Conventional tissue biopsy, as the key of the diagnostic pathology, can
make a definite histopathological diagnosis over the vast majority of
the cancer cases [21]. However, tissue biopsy not only needs
complex, time-consuming and expensive operation processes, but also is
difficult to obtain pathological tissue from body organ. In addition,
the conventional tissue biopsy cannot be used to monitor the treatment
response. In contrast, the liquid biopsy instrument can be easily
applied to interrogate cancer-related biomarkers sustainably with
minimal invasive, and then achieve real-time monitor for cancers.
The major challenge of existing LB instruments is
their insufficient accuracy and
sensitivity caused by the
heterogeneity and rarity of cancer biomarkers [22].
To address these issues, LB
instruments used for CTC analysis typically contain white blood cell
(WBC) depletion modules, which employ immune- or physical isolation
methods depending on the morphological, biological, or physical
properties of the CTC and WBCs. The immune-isolation method mainly
allows the positive selection of CTCs through antigens expressed in
epithelial cells, such as EpCAM. However, subpopulations of metastatic
tumor cells with low EpCAM expression or undergoing
epithelial-to-mesenchymal transition (EMT) cannot be captured or
identified using this method
[23]. Physical isolation methods
typically separate CTCs based on the differences in size, shape, or
deformability between CTCs and blood cells. However, CTC subpopulations
with physical features similar to those of WBCs cannot be differentiated
or sorted during the separation process [24]. Furthermore, the LB
instrument used for cfDNA also faces similar limitations. The ctDNA
originating from CTCs can be contaminated by those derived from normal
blood cells of the patient and might only account for
~0.01% of the total cfDNA. Various patient-related
factors, including pregnancy, smoking, exercise, and various
non-malignant conditions, also affect the cfDNA levels in blood
[13]. Therefore, the association between patient-related factors and
the performance of specific cfDNA assays may not be entirely accurate.
The second challenge of the existing LB instruments is their inadequate
automation and integration. To achieve rapid and real-time cancer
diagnosis, LB instruments have been developed to reduce biochemical
operations, lower the economic burden for training hospital personnel,
including pathologists, nurses, and physicians, and simplify the
interpretation of the test results. However, most LB instruments can
only isolate biomarkers and are unequipped to conduct necessary
subsequent analyses. For example, the
Parsortix
and ClearCell FX platforms are designed for CTC isolation [25,26],
whereas the KminTrak instrument is
used for cfDNA extraction [27]. The
Automatic Fraction Collector can
only serve as a pretreatment unit for exosome isolation [28].
Furthermore, most LB instruments require complex biochemical operations,
even during the biomarker isolation process, which further impairs their
automation levels. For example, CytoSorter requires labor-intensive
immunoaffinity operations to separate CTCs from blood cells [29].
For LB instruments without an analytical module, the analysis of
biomarkers commonly relies on complex biochemical operations, such as
fluorescent in situ hybridization (FISH), next-generation sequencing
(NGS), and digital PCR (dPCR) [30]. For instance, for cfDNA
detection, PCR technologies are
typically used to amplify the target DNA fragment and improve analytical
sensitivity. These assays are usually difficult to integrate into the
corresponding LB instrument [31].
The third challenge is the validity of the detection results.
CellCollector and KminTrak [32,33] as new-developed LB instruments
can isolate CTCs and cfDNA, separately. Although the number of CTCs in
the blood and the ctDNA content in plasma have been used as a reference
for early treatment response of cancers, they do not inform patients
about the cancer progression stage, as no additional
clinical evidence demonstrating their
diagnostic efficacy is available compared to standard
imaging-based tools [34].
Thus, CTC- and ctDNA-based LB technologies are not sophisticated enough
to monitor treatment response in the clinical setting [35]. In
addition, early-stage cancer detected via LB instrument may be indolent
or will never develop into life-threatening cancers, such as in the case
of prostate and breast cancers. This may result in the overdiagnosis of
incidental cancers that are unlikely to affect the overall health or
lifespan of the patients [36]. Notably, the overtreatment guided by
an overdiagnosis can have considerable consequences [4]. For
example, complications that include incontinence or erectile dysfunction
can be aroused due to overtreatment in prostate cancers.
To better understand the evolutionary changes in the genetic and
epigenetic landscape of tumors, multigene assays of biomarkers obtained
from LB instruments are often performed. However, these additional
testing require a considerable investment of time, knowledge, and
resources, such as state-of-the-art NGS machines, adequate storage space
for multigene sequencing data, and a wet laboratory [37].
National and international central
laboratories are better equipped to perform these assays, wherein the
large-scale sample processing would be more economical. Furthermore,
conducting multigene assays rather than single assays may not be
worthwhile for local pathology laboratories. Therefore, LB instruments
combined with multigene assays for cancer detection are difficult to
apply in point-of-care (POC) settings.
Although LB instruments can
significantly reduce the investment in manual operation and repeated
minimally invasive detection of cancers, they have been commercially
developed to a much lesser extent than conventional tissue biopsy
approaches. In addition, the rapid development of tissue biopsy- and
imaging-based diagnosis methods make them more accessible to researchers
and clinicians than LB instruments. Considering the limitations of the
existing LB instruments, an obvious challenge for broad clinical
applications should be addressed. Therefore, the current LB instrument
is not sophisticated enough to replace tissue
biopsy.