The study showed that US had a higher sensitivity than CT for diagnosing LNM. US could detect LNM, especially in small lymph nodes, small tumors, and tumors with shallow invasion. Moreover, the N stage, as predicted by US, was highly concordant with the pathological N stage. Thus, patients with advanced LNM, such as those with multiple LNMs or N2 stage, could be diagnosed preoperatively using US. This study is the first to investigate the usefulness of US for the preoperative diagnosis of LNM in colon cancer.
Preoperative diagnosis of LNM in colon cancer is important, as LNM is a stronger prognostic factor than the depth of invasion, and advanced LNM worsens prognosis [1–3]. Recently, neoadjuvant chemotherapy has been proven to be beneficial for patients with colon cancer with a poor prognosis. A retrospective study showed that neoadjuvant chemotherapy could improve survival in patients with clinical T4b colon cancer . The FOxTROT trial found evidence of histological regression in 59% of cases . Therefore, preoperative treatment should be recommended for patients with a poor prognosis. However, for this, preoperative diagnosis needs to be more accurate.
A recent study on multidetector row CT (MDCT) found accuracy rates of 64–77% for the N stage in colorectal cancer . FDG-PET has been shown to have accuracy rates of 63–69% for assessing lymph node involvement in the colorectum [8, 9]. Regarding MRI assessment, Nerad et al. reported that the sensitivity and specificity for detecting nodal involvement were 47–68% and 64–86%, respectively . Our findings show that the capability of US to accurately diagnose LNM in colon cancer is comparable to that of other modalities.
Furthermore, US is less invasive compared to other modalities. Most previous reports have used endoscopic ultrasound (EUS), which can accurately evaluate the primary tumor and lymph node status close to the primary tumor [28, 31, 33]. However, EUS has limitations and involves a complex technique for assessing tumors in the right side of the colon. Furthermore, EUS may be difficult to perform in cases where lymph nodes are far from the primary tumor and in cases with multiple LNMs. For such cases, the transabdominal approach could be useful. The effectiveness of the transabdominal method for determining the depth of invasion in colon cancer has been previously defined . Further, there have been reports on the usefulness and accuracy of HS for the transabdominal assessment of LNM in colorectal cancer [18–20, 38, 39]. However, while HS is highly sensitive in detecting primary tumors, it has low sensitivity in assessing the N stage [38, 39] and LNM, ranging widely from 25–82% [18–20].
In this study, we developed US diagnostic criteria for LNM in colon cancer. The criteria consisted of short axis ≥ 7 mm, S/L ratio ≥ 0.75, absence of hilar echoes, expansion appearance, and peripheral or mixed vascularity. Earlier reports have shown that these characteristics indicate lymph node malignancy [12, 30, 32]. Lymph node vascularity on EUS was also reported to be indicative of malignancy . In our US criteria, vascularity had the highest accuracy, consistent with a previous report . Furthermore, these criteria showed a higher agreement than CT. Collectively, these findings support that US can identify a patient with advanced LNM who should undergo preoperative treatment, with higher sensitivity than CT.
The current study has some limitations. First, this was a retrospective study conducted in a single center. Second, the LNM detected by either US or CT was not always concordant with pathological LNM. However, we analyzed concordance in the number of LNM between US or CT and pathological assessment, thus minimizing the effect of this limitation. Third, US assessment of LNM is not recognized as a standard examination of gastrointestinal tumors because US practitioners believe that it requires advanced technical skills. Systematic educational programs may be needed to increase the number of experienced technicians. Fourth, the interval between US and surgery was shorter than that between CT and surgery. Thus, US could evaluate lymph node status closer to the operation, as compared with CT.
However, our findings provide evidence that US may be useful for the diagnosis of colon cancer. A new vascular evaluation method, superb microvascular imaging, may be further useful for diagnosing malignant lymph nodes by US, as it yields more information about nodal vessels than power Doppler ultrasound . This technique could further enhance the diagnosis of LNM by US. Furthermore, we believe that evaluations using a contrast-enhanced mode with a higher frame rate than the current study or further utilizing the properties of Sonazoid® introduced into reticuloendothelial cells will contribute to an even more accurate assessment of lymph nodes. Further prospective studies, with a larger number of patients, should be performed to validate the role of US in the management of colon cancer.
In conclusion, US has higher sensitivity for diagnosing LNM in colon cancer than CT and may yield a more accurate preoperative diagnosis of the N stage. Thus, US may be more helpful than CT for preoperative decision-making regarding the appropriateness of neoadjuvant treatment in patients with colon cancer with advanced LNM.