DISCUSSION
Noninvasive imaging techniques are being developed and improved for
identifying and evaluating pathophysiological features of tumors in
order to assist in the planning of individual patient treatment
protocols [10, 11]. Particularly, new imaging approaches that assess
tumor vascularization have improved diagnosis and treatment prediction.
In this study, we evaluated (i) caliper and contrast enhanced
ultrasonography (CEUS) to define tumor dimensions and (ii) contrast
enhanced ultrasound (CEUS) and laser speckle contrast imaging (LSCI) to
appreciate tumor vascularization.
The murine melanoma model is based on B16F10 cells known for their
aggressiveness, high metastatic potential and high expression of the
PD-1 receptor [12]. Taken together, these data confirm the value of
evaluating the impact of anti-PD1 antibodies in limiting tumor growth.
Although the survival data did not allow us to conclude that there was a
significant benefit from anti-PD1, they showed that before 7 days of
treatment, no mortality was observed, whatever the group of animals,
justifying the use of this time frame to evaluate the different imaging
techniques.
To determine tumor dimensions, caliper and CEUS were used in this study.
Although tumor volumes are similar for both methods, as recently
described in non-melanoma skin cancer [13], our data suggested an
interest in CEUS because of various biases described with caliper.
Firstly, it has been described that caliper often overestimated tumor
volume. Furthermore, the caliper measurements were smaller for small
tumors compared to greater tumors also relatively seen. Consequently,
volume changes measured with caliper in small and large tumors are not
comparable and effects of anti-cancer drugs can easily be missed as
tumors will tend towards being determined with a greater bias as they
grow larger [14].
The value of CEUS has recently been confirmed by Makouei et al.
[15]. In a pilot study conducted on mice with a soft tissue tumor,
they confirmed that ultrasonography is a feasible and accurate imaging
method to assess the tumor volume. However, these authors pointed out a
number of limitations. Among these is the failure to take account of
vascularization, which is an important factor in the diagnosis and
treatment of metastatic tumors. To overcome these limitations, our study
was supplemented by microcirculation analyses through LSCI in comparison
with CEUS technique.
While CEUS analysis showed a reduction in microcirculation at treatment
initiation, no difference was observed after 7 days of treatment
suggesting that CEUS is not sufficiently robust for long-term studies.
On the other hand, LSCI analysis confirmed a decrease in tumor perfusion
over time in mice treated with anti-PD1 compared with control mice.
Compared to CEUS, LSCI measurement did not need a direct skin contact
which is an advantage in these types of analyses. Indeed, a strongly
hold of the ultrasound probe can create a local ischemia or reduce the
microcirculatory vascularization and create biased results. Besides,
LSCI measure results from local microvascular analysis without a blank
analysis, which minimize artefacts. Last, if using the same experimental
conditions, LSCI showed a good reliability and reproducibility. These
data suggest the potential clinical value of LSCI in improving the
diagnosis of metastatic melanoma.