In this study, the tumor grade defined using immunopathological tests was the most significant predictor for metastasis in patients with 10–20-mm sized rectal NETs. The mitotic count and Ki-67 index itself were also meaningful; however, it was less significant than the tumor grade. The EUS tumor size was significant in univariate analysis. Pathological tumor size, lymphovascular invasion, and perineural invasion showed no statistically significant differences.
The tumor grade was the most significant factor to predict metastasis in the present study. Several previous studies and guidelines have also stated that mitotic count and the Ki-67 index were significant in predicting metastasis risk and prognosis [10, 13]. In contrast, some other studies have reported that the tumor grade was a significant predictor, assessed by combining the mitotic count and Ki-67 index [6, 11, 14]. In the present study, we focused on patient with on 10–20-mm sized rectal NETs, and the grade was an independent predictive factor for metastasis in multivariate analysis. In particular, 71.4% of the patients with metastasis had tumor grade 2. The remaining two patients with tumor grade 1 were suspected of lymph node metastasis based on lymphovascular and perineural invasion, or lymph node enlargement on EUS. Therefore, it seems that the proper treatment could be determined when the tumor grade has been used as the main factor to predict metastasis in post-operative biopsy while referring to the results of imaging and other pathological tests.
It is surprising that the tumor size, as determined by EUS, was a more significant factor than the pathological tumor size in predicting metastasis. There was a possibility that the biopsy results may not have accurately reflected the original tumor properties, as tissue deformation may occur during and after resections. Conversly, it can be thought that EUS retains the original shape before the manipulation and better reflects the tumor’s original size. This reasoning seems to be more conspicuous, as the accuracy of the measurements obtained with the EUS machine has improved with technological advancement [1]. Moreover, the risk of metastasis can be assessed before treatments, such as endoscopic resection. Thus, the tumor size defined by EUS could be considered more significant to determine radical resection than the pathological tumor size.
Lymphovascular and perineural invasion had no statistical significance in the present study; however, they were observed in higher percentage (28.6% and 42.9%, respectively) in patients with metastasis (vs. 5.9% in those in the non-metastatic group). A meta-analysis showed that lymphovascular invasion was a risk factor for lymph node metastasis [15]. Concerning the small sample size in the present study, type II error might occur, and an increase in the sample size may demonstrate a statistical significance for these factors. As aforementioned, one case with metastasis was determined as tumor grade 1, in which lymphovascular and perineural invasions were identified. The significance may be lower than that of the tumor grade, but lymphovascular and perineural invasions may certainly be considered as clinically meaningful tests.
Immunohistochemical methods for rectal NET diagnosis include chromogranin A, B, synaptophysin, CD56, CD57, p53, and neuron-specific enolase [4]. There is no consensus regarding the need for immunochemical examinations in all cases of NET. However, immunochemical testing is encouraged when the tumor presents with histologically unclear characteristics. Tests for chromogranin A and synaptophysin are considered as standards [4]. Other tests are not recommended for routine staining. p53 may be used as a marker for hypodifferentiated tumors, but it is not recommended as part of the routine test [16]. In the present study, tests for chromogranin A, synaptophysin, and CD56 were performed. In addition, this study also investigated whether immunohistochemical tests can be used for diagnosis and for determining the treatment method. However, none of these immunochemical tests for NET diagnosis could be identified to have an association with lymph node metastasis. Synaptophysin and CD56 were expressed in all patients in the metastatic and non-metastatic groups. Chromogranin A was expressed in only a few cases, and no statistically significant difference was observed between the groups.
This study focused on the controversy in the appropriate treatment choice for 10–20-mm sized rectal NETs. As previous studies have focused on various sizes of rectal NETs, the predictors of metastasis of 10–20-mm sized rectal NETs could not be directly identified. The current study suggested that the tumor grade is the most important factor in determining the radical treatment of 10–20-mm sized rectal NETs. In addition, it is considered essential to examine the tumor size, lymph node enlargement, and muscularis propria involvement using EUS and to examine lymph node enlargement using CT. Thereafter, when the tumor size defined with EUS is ≥13 mm, muscularis propria has been infiltrated, and lymph node enlargement has been identified based on EUS and CT findings, radical resection should be considered as the first option. In other cases, endoscopic resection may be considered at first choice; afterward, radical resection is suggested for tumor grade 2 or higher in pathological examination.
This study had a few limitations. First, there was small numbers of patients included in the study. The incidence of 10–20-mm sized rectal NETs was low and that of metastasis was even lower, which led to further limitation of the sample size. To compensate for this limitation, we extended the study period to 11 years. Another limitation was the retrospective design of this study; therefore, the possibility for selection bias existed. In the future, prospective studies could obtain more significant results. Moreover, lymph node metastasis was not pathologically confirmed in all patients. To exclude potential errors that could not be identified with imaging, despite lymph node metastasis, we only included patients who were followed-up for at least 30 months in case of receiving endoscopic resection with no pathologically identified lymph node metastasis. Patients who underwent only endoscopic resection with a follow-up duration of <30 months were excluded.