TNM staging of foregut (neuro)endocrine tumors: a consensus proposal including a grading system

The need for standards in the management of patients with endocrine tumors of the digestive system prompted the European Neuroendocrine Tumor Society (ENETS) to organize a first Consensus Conference, which Virchows Arch (2006) 449:395-401
was held in Frascati (Rome) and was based on the recently published ENETS guidelines on the diagnosis and treatment of digestive neuroendocrine tumors (NET). Here, we report the tumor-node-metastasis proposal for foregut NETs of the stomach, duodenum, and pancreas that was designed, discussed, and consensually approved at this conference. In addition, we report the proposal for a working formulation for the grading of digestive NETs based on mitotic count and Ki-67 index. This proposal, which needs to be validated, is meant to help clinicians in the stratification, treatment, and follow-up of patients.
Keywords Neuroendocrine tumors . Gut . Pancreas . Staging . TNM . Grading . Mitotic index . Ki-67 index Background It has been known for a long time, and was finally defined within the World Health Organization (WHO) classification of endocrine and digestive tumors, that neuroendocrine tumors (NET) arising at different anatomical sites of the digestive system represent tumor entities that differ in their biology [bib_ref] Incidence trends and risk factors of carcinoid tumors: a nationwide epidemiologic study..., Hemminki [/bib_ref]. Several recent publications focused on the application of the "new" WHO classification and proved its effectiveness, supporting the concept that the different endocrine tumor types also differ in their clinical behavior [bib_ref] Treatment of metastatic neuroendocrine carcinomas based on WHO classification, Artale [/bib_ref] [bib_ref] Is the new WHO classification of neuroendocrine tumours useful for selecting an..., Bajetta [/bib_ref] [bib_ref] Long-term clinical outcome of somatostatin analogues for treatment of progressive, metastatic, well-differentiated..., Panzuto [/bib_ref] [bib_ref] Prognostic factors and survival in endocrine tumor patients: comparison between gastrointestinal and..., Panzuto [/bib_ref] [bib_ref] Rapid and sustained relief from the symptoms of carcinoid syndrome: results from..., Ruszniewski [/bib_ref] [bib_ref] Classification of low-grade neuroendocrine tumors of midgut and unknown origin, Van Eeden [/bib_ref].
Malignant gastroenteropancreatic NETs may be fatal, though at a significantly slower pace than their exocrine counterparts. A number of retrospective papers and epidemiological data solidly support such statements . This peculiar clinical feature attracted the interest of pathologists very early and was the reason for the special designation of such tumors as "carcinoid" by Oberndorfer [bib_ref] Karzinoide tumoren des Dunndarms, Oberendorfer [/bib_ref].
As gastroenteropancreatic NETs are rare [bib_ref] Incidence trends and risk factors of carcinoid tumors: a nationwide epidemiologic study..., Hemminki [/bib_ref] [bib_ref] Current status of gastrointestinal carcinoids, Modlin [/bib_ref] [bib_ref] A 5-decade analysis of 13,715 carcinoid tumors, Modlin [/bib_ref] , it is tempting to lump them together and equate all digestive "carcinoids" with the appendiceal "carcinoid," probably the best known NET with the most benign behavior [bib_ref] A retrospective analysis of 1570 appendiceal carcinoids, Sandor [/bib_ref]. However, in recent years it has become clear that gastroenteropancreatic NETs, especially foregut NETs, are heterogeneous in their morphological and biological features. In the last two decades efforts were therefore made by the WHO to define NET features that discriminate true benign behavior (low risk) from low-grade malignant well-differentiated NETs in the different parts of the digestive system. Although the new WHO classification is an important step toward defining the diverse tumor biology of NETs, further efforts are necessary to improve the prognostic assessment of the individual NET.
The demand for standards in the stratification and treatment of patients with gastroenteropancreatic NETs prompted the recently established European Neuroendocrine Tumor Society (ENETS) to define guidelines [bib_ref] Guidelines for the diagnosis and treatment of neuroendocrine gastrointestinal tumours. A consensus..., Plockinger [/bib_ref] [bib_ref] From ENET to ENETS: a long odyssey in the land of small..., Wiedenmann [/bib_ref]. Such guidelines underwent scrutiny for consensus in the first of two meetings entitled "Consensus Conference on the ENETS Guidelines for the Diagnosis and Treatment of Neuroendocrine Gastrointestinal Tumors, Part 1: Foregut Tumors" held in Frascati (Rome, Italy) from . During this meeting the clinical need for a tumor-node-metastasis (TNM) classification of gastroenteropancreatic NETs was felt. Here we report the TNM staging classification proposal for foregut NETs that was approved at this consensus conference. In addition, we suggest a simple grading system with some pointers that may help to standardize the prognostic assessment of gastroenteropancreatic NETs.

# Materials and methods
Sixty-two experts in the field of digestive endocrine tumors from 20 different countries attended the Consensus Con-ference. The attendees represented all medical branches involved in managing patients with gastroenteropancreatic NETs. They formed four working groups according to their specific clinical expertise: (1) pathology and genetics (11 participants, all listed as authors and G. Kloppel), (2) surgery (10 participants, including the coauthors H. Alhman and M. Falconi), (3) imaging and radiology (10 participants), (4) medicine and clinical pathology (31 participants, including the coauthors M. Caplin, W.W. de Herder, B. Erikssson, and B. Wiedenmann).
The Conference was divided sequentially into eight sessions devoted to specific topics on an anatomical basis (gastric NET sessions 1-2, duodenal NET, pancreatic NET sessions 1-4, and poorly differentiated endocrine carcinomas). A working booklet with the ENETS guidelines and specific queries had been prepared in advance by the Organizing Committee. The work was organized such that, after a short case presentation in a plenary session, each working group gathered separately to discuss groupspecific questions. Once agreement was reached within each group, consensus statements were discussed and approved or rejected by all participants gathered in the plenary session. This procedure was followed for all eight sessions. The TNM staging proposal was made by the Tumor invades adjacent structures For any T, add (m) for multiple tumors N-regional lymph nodes NX Regional lymph nodes cannot be assessed N0
No regional lymph node metastasis N1
RegionalPathology and Genetics working group and amended and approved by the plenary session of the consensus conference. The grading system was discussed and defined by the Pathology and Genetics working group only.

# Results and discussion
The consensus guidelines are reported elsewhere. Here, we report the TNM staging proposal for gastroenteropancreatic NETs of the foregut together with a grading system that may be relevant for the prognostic assessment by the pathologist. The foregut NETs were separated into gastric, duodenal (including ampulla and proximal jejunum), and pancreatic NETs, but were not distinguished according to specific functional activity, main tumor cell type, and specific genetic background.
TNM staging proposal (see [fig_ref] Table 1: Proposal for a TNM classification and disease staging for gastric endocrine tumors [/fig_ref] The currently published TNM format was adopted as working template.
Tumor The proposed definition of tumor in situ applies to the stomach only and adheres to the literature [bib_ref] Histopathological classification of nonantral gastric endocrine growths in man, Solcia [/bib_ref]. No definition is given for the duodenum and pancreas because none has been agreed upon in spite of recent working proposals [bib_ref] Precursor lesions in patients with multiple endocrine neoplasia type 1-associated duodenal gastrinomas, Anlauf [/bib_ref] [bib_ref] Persistent hyperinsulinemic hypoglycemia in 15 adults with diffuse nesidioblastosis: diagnostic criteria, incidence,..., Anlauf [/bib_ref]. The size limits indicated for T1 are those defined by the WHO for tumors with "benign behavior" according to site-specific clinicopathological correlations. Similarly, for T2 of the stomach and duodenum, the sizes are those indicated for tumors of "uncertain behavior." In the pancreas the size limit given for T2 needs to be validated. Deeply invasive tumors are included under the T3 and T4 definitions, taking into account sitespecific features.
Nodes N1 indicates the presence of any single or multiple metastases in regional lymph nodes, according to TNM rules. Although the presence of regional lymph-node metastases is, per se, a negative prognostic factor in gastroenteropancreatic NETs [bib_ref] Recent advances in the pathophysiology and management of inflammatory bowel diseases and..., Jensen [/bib_ref] , the prognostic significance of the number of metastatic nodes is not known. In light of this, stage 3B of Tables 1, 2 and 3 is proposed to mark the N1 status for future validation.
Distant metastasis M1 indicates the presence of any single or multiple metastases at any distant anatomical site (including nonregional nodes). Because there is evidence that extrahepatic bone metastases are a particularly ominous Tumor invades peritoneum or other organs For any T, add (m) for multiple tumors N-regional lymph nodes NX Regional lymph nodes cannot be assessed N0
No regional lymph node metastasis N1
Regionalsign [bib_ref] Bone metastases in patients with gastrinomas: a prospective study of bone scanning,..., Gibril [/bib_ref] [bib_ref] Prognostic factors and survival in endocrine tumor patients: comparison between gastrointestinal and..., Panzuto [/bib_ref] , it is recommended to specify the anatomical site of the metastasis according to the TNM classification rules (PUL, pulmonary; HEP, hepatic; OSS, osseous; etc.).
Staging The proposed staging system lists stage 0 only for the stomach because this is the only anatomical site where Tis is defined. Stage I encompasses the T1 NETs with limited growth. Stage II identifies tumors that are larger in size or more invasive, either T2 or T3, though always in the absence of metastasis. At stage III the increased malignancy refers either to invasion into surrounding structures (stage IIIa) or to the presence of regional node metastasis (stage IIIb). Stage IV always implies the presence of distant metastasis.
Grading proposal (seeGrading It has been widely discussed and generally accepted that no histological grading system effectively predicts the behavior of well-differentiated endocrine tumors. The major obstacle to developing a practically effective grading system is the fact that severe cytological atypia, as, for instance, in pheochromocytomas, has no impact on the clinical behavior and malignancy of such tumors. However, recent studies in well differentiated NETs of the foregut, including the pancreas, and of the midgut have shown the usefulness of a grading system [bib_ref] Prognostic factors in pancreatic endocrine neoplasms: an analysis of 136 cases with..., Hochwald [/bib_ref] [bib_ref] ECL cell tumor and poorly differentiated endocrine carcinoma of the stomach: prognostic..., Rindi [/bib_ref] [bib_ref] Classification of low-grade neuroendocrine tumors of midgut and unknown origin, Van Eeden [/bib_ref]. Thus, well-differentiated endocrine tumors with a more solid appearance and distinct proliferative activity, which also lead to difficulties in the differential diagnosis vs poorly differentiated endocrine carcinomas, seem to have a worse prognosis than NETs without these features . It was therefore decided to introduce a grading system that could be of help in distinguishing the welldifferentiated NETs into G1 and G2 categories. As a working suggestion, we propose to apply to foregut NETs a grading system modified from that adopted by the WHO for endocrine tumors of the lung, though exclusively referring to the proliferation status. In brief (see, three tumor categories are identified: G1, <2 mitosis per Tumor limited to the pancreas and size <2 cm T2
Tumor limited to the pancreas and size 2-4 cm T3
Tumor limited to the pancreas and size >4 cm or invading duodenum or bile duct T4
Tumor invading adjacent organs (stomach, spleen, colon, adrenal gland) or the wall of large vessels (celiac axis or superior mesenteric artery) For any T, add (m) for multiple tumors N-regional lymph nodes NX Regional lymph node cannot be assessed N0
No regional lymph node metastasis N1
Regional In general, G1 and G2 should refer to well-differentiated NETs displaying diffuse and intense expression of the two general immunohistochemical neuroendocrine markers, chromogranin A and synaptophysin [bib_ref] Biological and molecular aspects of gastroenteropancreatic neuroendocrine tumors, Rindi [/bib_ref]. Punctate necrosis is, per se, indicative of a more aggressive tumor, pointing to a G2 status, which, however, has to be confirmed by the mitotic count. G3 indicates a poorly differentiated neuroendocrine carcinoma. It has high mitotic counts/Ki-67 index, is often associated with fields of necrosis, and shows significantly reduced chromogranin A expression, while maintaining intense staining for synaptophysin. It is relevant to remind here that the diagnosis of G3 carcinoma is based on a specific histologic pattern according to the current WHO criteria. In addition, the clinical behavior of G3 poorly differentiated neuroendocrine carcinomas of the gastroenteropancreatic tract does not necessarily correspond to that of small cell cancers of the lung or of any other sites.
Mitotic count and Ki-67 index We propose that mitoses should be counted on hematoxylin and eosin-stained slides in at least 40 HPFs, where possible. The mitoses should be assessed in areas where they are most frequent after a general slide survey. For Ki-67 assessment, the MIB1 antibody is recommended at the conditions that have been established at the laboratory in question. The Ki-67 index should be assessed in 2,000 tumor cells in areas where the highest nuclear labeling is observed (often but not exclusively at the tumor periphery).

## Concluding remarks
Requests for standardization in the management of patients with gastroenteropancreatic NETs recently resulted in the development of several guidelines, including those proposed by ENETS [bib_ref] Guidelines for the management of gastroenteropancreatic neuroendocrine tumours (including bronchopulmonary and thymic..., Oberg [/bib_ref] [bib_ref] Guidelines for the management of gastroenteropancreatic neuroendocrine tumours (including bronchopulmonary and thymic..., Oberg [/bib_ref] [bib_ref] Guidelines for the diagnosis and treatment of neuroendocrine gastrointestinal tumours. A consensus..., Plockinger [/bib_ref] [bib_ref] Guidelines for the management of gastroenteropancreatic neuroendocrine (including carcinoid) tumours, Ramage [/bib_ref]. However, it was never attempted to reach consensus on specific practical issues. The TNM staging system we propose here was developed to meet a clinical need, is based on the current WHO classifications of endocrine and digestive tumors, and is the result of a consensus conference held by specialists involved in the management of digestive endocrine tumor patients. Along the same line, the grading system described here attempts to close the gap between the advances of the most recent WHO classifications and the need for a better prognostic assessment of NETs. It is obvious, of course, that all our proposals have to be validated by future clinicopathological work.
[table] Table 1: Proposal for a TNM classification and disease staging for gastric endocrine tumors [/table]
[table] Table 2: Proposal for a TNM classification and disease staging for endocrine tumors of the duodenum/ampulla/proximal jejunum [/table]
[table] Table 3: Proposal for a TNM classification and disease staging for endocrine tumors of the pancreas [/table]