Date Published: February 11, 2019
Publisher: Public Library of Science
Author(s): Hajime Ushigome, Yosuke Fukunaga, Toshiya Nagasaki, Takashi Akiyoshi, Tsuyoshi Konishi, Yoshiya Fujimoto, Satoshi Nagayama, Masashi Ueno, Norikatsu Miyoshi.
Surgical indications for rectal neuroendocrine tumors with potential lymph node metastasis remain controversial. Although accurate preoperative diagnosis of nodal status may be helpful for treatment strategy, scant data about clinical values of lymph node size have been reported. The aim of this retrospective study was to investigate the relationship between lymph node size and lymph node metastasis.
Participants comprised 102 patients who underwent rectal resection with total mesenteric excision or tumor-specific mesenteric excision and in some cases additional lateral pelvic lymph node dissection for rectal neuroendocrine tumor between June 2005 and September 2016. All lymph nodes from specimens were checked and measured.
Pathological lymph node metastasis was confirmed in 37 patients (36%), including 6 patients (5.8%) with lateral pelvic lymph node metastasis. A total of 1169 lymph nodes in the mesorectum were retrieved from all specimens, with 78 lymph nodes (6.7%) showing metastasis. Mean length (long-axis diameter) of metastatic lymph nodes in the mesorectum was 4.31 mm, significantly larger than that of non-metastatic lymph nodes (2.39 mm, P<0.01). The optimal cut-off of major axis length for predicting mesorectal lymph node metastasis was 3 mm. We could predict lymph node metastasis in only 7 patients (21%) from preoperative multidetector-row computed tomography. Metastatic lymph nodes were small, so predicting lymph node metastasis from preoperative computed tomography is difficult. Alternative modalities with a scan width less than 3 mm may be needed to predict lymph node metastasis of rectal NET with low cost and labour requirements.
Rectal neuroendocrine tumour (NET) is relatively rare disease. The annual incidence of this pathology is reportedly 0.14–0.76 cases per 100 000 population, with particularly high prevalence among Asian/Pacific Islanders, Native Americans, and African Americans . This has also been reported and recognized as a pathological manifestation of mild-to-moderate nuclear atypia and localized disease, mostly with very small rates of metastasis to regional lymph nodes (LNs) or distant organs . However, because of similar cancer-specific survival rates of rectal NET with regional or distant metastasis of rectal adenocarcinoma [3–5] according to some recent reports of long-term follow-up data, rectal NET is now considered a malignant disease [1,6]. As difficulty with chemotherapy has been reported [7–9], not only local resection but surgical resection with LN dissection around the rectum if LNs metastases are suspected must be used to achieve cure.
This study investigated the relationship between LN size and LN metastasis from rectal NET in patients who underwent TME or TSME. This study performed CT with 5 mm cuts, representing a common modality in clinical practice to evaluate metastatic lesions secondary to rectal NETs. Although MRI is well known to produce better images to assess the circumferential margin and detect existence of LNs than CT, in this study MRI would not contribute to predict metastasis, because many patients showed no findings on CT. We thus did not perform MRI in many cases. Another imaging tool, octreotide scan, is useful for determining metastatic diseases in general, although higher-grade colorectal NET lesions are often missed  and previous studies have reported CT and MRI as superior to octreotide scan for detecting metastasis .
This study achieved relatively favorable long-term outcomes, implying a reasonable treatment strategy in our institution and showed the difficulty of predicting LN metastasis on CT. Now that even quite small NET is often detected on screening colonoscopy, pathological findings if resected by endoscopy and the size over 1cm were properly important factors to predict LN metastasis, rather than preoperative CT imaging.