This study examined the disease-specific factors that affected LN metastasis diagnosis of patients with CRC by PET/CT in the retrospective, single-center study. We reported that the diagnostic capability of PET/CT was better for distant LN metastasis than for regional LN metastasis. Regional LN metastasis diagnosis was affected by disease-specific CRC factors. The results demonstrated that “T category” was an independent factor for false-negative LN metastasis diagnosis, while “Tumor diameter” was significantly associated with false-positive LN metastasis diagnosis.
Preoperative diagnosis of CRC is performed using various modality such as colonoscopy, CT, MRI, and PET/CT . As for LN diagnosis, MRI has a high diagnostic capability [15,16]. However, it cannot be applied generally to preoperative CRC cases because the imaging range of MRI is limited. PET/CT was recognized as a useful imaging modality for LN metastasis diagnosis in addition to distant metastasis [9-12]. For advancing the diagnostic performance of LN metastasis, some studies focused on selecting SUV max optical cut-off values for LN metastasis and the relationship between SUV max and LN size [9,10,17]. However, it was not examined how CRC-specific characteristics impacted preoperative LN metastasis diagnosis by PET/CT.
Based on this study, “T category” was an independent factor for false-negative LN metastasis diagnosis. In general, deep penetration correlates with the frequency of LN metastasis . And LNs are near the tumor and merge with the FDG accumulation of the primary tumor, making it difficult to distinguish LN metastasis from the parimary tumor [19,20].
“Tumor diameter” was strongly linked with false-positive LN metastasis diagnosis in patients with CRC. Large tumor size is expected to provide a more intense antigenic immune challenge to the draining LN, resulting in nodal hyperplasia. Then large tumor size could cause reactive enlargement of regional LNs [18,21]. The cause of false-positive LN metastasis was considered the accumulation of FDG in response to reactive enlargement of LNs.
However, these disease-specific effects of CRC had little effect in DG. In a past study, it had been reported that the LN metastasis diagnosis at a distance from the tumor by PET/CT was high . The high diagnostic capability of LN metastasis distant from the tumor makes it possible to recognize the extent to which LN metastasis extends horizontally or vertically from the tumor. As a result, it will contribute to the determination of the extent and endpoint of LN dissection.
As for the LN diagnostic capability for each stage by PET/CT, it was useful in Stage I. As the tumor stage increases, the increased “T category” and “Tumor diameter” appeared to decrease the LN metastasis diagnostic capability. In the treatment strategy for CRC, surgery is the basis of the treatment in Stage I. In the case of Stage II and Stage III, multidisciplinary treatment, including preoperative treatment, is required [22,23]. Therefore, the diagnostic capability for Stage II and Stage III is essential in preoperative diagnosis. In this study, the LN metastasis diagnostic capability of Stage II and Stage III by PET/CT was not high, so LN metastasis diagnosis by PET/CT might not contribute to treatment selection for patients with CRC.
It was extremely difficult to diagnose LN metastasis of CRC by PET/CT. This was the same result by CT from a recent big data study . However, PET/CT was useful for LN metastasis diagnosis of DG and Stage I because of less impact on disease-specific factors in CRC. By limiting the cases, we thought PET/CT could be a useful preoperative diagnostic modality.
The limitations of this study were that it was a single-center study and used a fixed SUV max cut-off value. It has been reported that SUV max cut off value for LN metastasis changed depending on LN size . In this study, a SUV max cut-off value for LN was set to 2.5 with reference to past meta-analysis , so small lymph node metastasis cases were possibly overlooked.