The presence of metastatic LNs in patients of PTC is an independent risk factor of local tumor recurrence,
which can be more life threatening than the original tumor.
So,
the exact assessment of nodal status is essential in preoperative assessment of PTC.
There have been some studies investigating what nodal characteristics relate to risk of nodal recurrence [1-9,
26-29] and they reported that nodal size,
number of positive nodes,
and the presence of extranodal spread were importantly related to the main nodal prognostic parameter of recurrence.
According to Randolph et al.
(2012),
the prognostic significance of nodal metastases from PTC can be stratified based on the size and number of metastatic LNs,
as well as the presence of extranodal invasion.
SWE can provides an objective information about tissue stiffness.
and here have been several studies to investigate the value of quantitative SWE in patients of PTC.
These reports provided new dimension to ultrasound evaluation of thyroid nodules,
however,
there have been only a few studies about SWE evaluation of metastatic LNs in,
the PTC patients and the reference standards of SWE indices to predict lymph node metastasis have not been established.
To our knowledge,
this is the first study to investigate the value of quantitative SWE as a prognostic implication of LNs metastasis in PTC patients.
This study showed that SWE-EI of Emean,
Emin,
Emax and Emean-m were significantly higher in metastatic lymph nodes than in benign lymph nodes (P < 0.05) and provided the cut off values of each SWE-EI for predicting lymph node metastasis.
We observed that Emin (cutoff value of 24.0 kPa) showed highest specificity (100%),
Emax (cutoff value of 57.0 kPa) showed highest sensitivity (84.3%) and Emean (cutoff value of 29.0 kPa) showed highest accuracy (72.6%).
We compared the diagnostic performances of SWE with BUS and the diagnostic performances of SWE were not significantly different from that of BUS.
Moreover,
we obtained higher diagnostic performances with a combined set of Emean and BUS (AUC 0.811 vs.
0.738,
P=0.0527) and Emin and BUS (AUC 0.812 vs.
0.738,
P=0.0329) than that of BUS only.
We investigated that quantitative SWE was correlated with the histopatholigc factors of metastatic lymph nodes and observed that the quantitative SWE EI of metastatic LNs were correlated significantly with the size,
ratio of positive LNs/dissected LNs and the presence of cortical invasion.
In this study,
metastatic LNs with cortical invasion showed significantly higher SWE EI values compared to the intranodal metastatic lesions.
The Emean of metastatic LN with cortical invasion and intranodal location was 91.91 kPa and 28.588 kPa (P<0.0001) and Emax of metastatic LN with cortical invasion and intranodal location was 107.213 kPa and 33.882 kPa (P<0.0001) and Emin of metastatic LN with cortical invasion and intranodal location was 64.751 kPa and 20.058 kPa (P<0.0001) and Emean-m of metastatic LN with cortical invasion and intranodal location was 64.751 kPa and 20.058 kPa,
respectively (P=0.0037).
Mann-Whitney U testindicated SWE EI as an independent factor for predicting pathologic cortical invasion preoperatively.
We used the ratio of the No.
of positive LNs divided by the No.
of dissected LNs to minimize the bias according to the different surgical dissection methods of lymph nodes.
And the ratio was significantly associated with Emean (P=0.0057) and Emin (P=0.0026).
The largest size of the metastatic LNs was also significantly associated with Emean,
Emax,
Emin and Emean-m.
(P<0.05).
According to the previous study reporting that the prognostic significance of nodal metastases from PTC can be stratified based on the size,
number and the cortical invasion of metastatic LNs we could suggest that quantitative SWE EI of metastatic LNs could be a reliable prognostic factor of LNs metastasis in PTC patients.
This study had several limitations.
First,
this study had a selection bias because SWE was performed for the cervical LNs with enlargement or suspicious findings with preoperative or postoperative US in patients of PTC,
and the surgical dissection method of the lymph nodes was different according the US and clinical findings suggesting lymph node metastasis.
We used the ratio of No.
of positive LNs/dissected LNs to minimize the effect of surgical dissection method,
however,
prospective randomized study is necessary to validate our results.
Second,
SWE was performed by one radiologist and the interobserver variability for SWE was not assessed.In conclusion,
SWE EI was useful in the prediction of lymph node metastasis of PTC and combined use of SWE and BUS provided higher diagnostic performance than BUS.
Also,
quantitative SWE could provide a preoperative prognostic implication of LN metastasis of PTC.