One of the most discussed and sometimes confusing subjects on forums is the staging and grading of Neuroendocrine Neoplasms (NENs). Mixing them up is a common error and so it’s important to understand the difference despite the apparent complexity. If I was to make a list of questions for my specialist/Oncologist at diagnosis, it would include “What is the stage, grade and differentiation of my cancer”. To enable me to synchronise with the documented guidance, I’m going to use the following WHO 2019 approved terms in this post and then provide an update of the key WHO 2019 changes below.

• Neuroendocrine Neoplasm (NEN) – all types of Neuroendocrine tumour of whatever grade (please note Neoplasm is another word for tumour)
• Neuroendocrine Tumour (NET) – all well-differentiated tumours, Grade 1,2 or 3 (an explanation of differentiation will be provided below)
• Neuroendocrine Carcinoma (NEC) – all poorly differentiated tumours. Neuroendocrine Carcinomas are by definition Grade 3.

Stage vs Grade

In the most basic of terms, stage is the spread or extent of cancer and grade is the aggressiveness of cancer. They are totally different things and an understanding of both is important as they are critical to predict outcome (to a certain extent) and guide therapy. There is no correlation between the two, you can have the lowest grade with the highest stage (actually very common with NETs).

As patients, we deal with many medical specialists during diagnosis and subsequent treatment. However, we rarely meet the pathologist who plays a critical role in the outcome. Precise diagnosis is what drives patient decisions and treatment. If the pathology is wrong, everything that follows could be incorrect as well. It’s a very important area.

Grading and Ki67/Mitotic Count

Grade is the aggressiveness of cancer – basically, how fast it’s growing. Neuroendocrine Neoplasms (NENs) are divided in to 3 Grades depending what range the Ki-67% measurement number lands in. With NENs, the differentiation (well or poorly) is very important when dealing with grade 3 as specialists now regard a well differentiated tumour (i.e. a NET) with a Ki67 of less than 55% as important for outcomes and therapy decisions.

So, what is KI-67? It’s a nuclear protein associated with and may be necessary for cellular proliferation. It’s therefore an excellent marker to determine the growth fraction of a given cell population. The fraction of Ki-67-positive tumour cells (the Ki-67 labelling index) is often correlated with the clinical course of cancer. Pathologists normally need to count about a thousand cells to determine the percentage of cells that are Ki67 positive – thus why you see Ki67 expressed as a percentage. Zero percent is the lowest, 100% is the highest. Often, they add greater or less than signs depending on the sample involved, i.e. >5% or <5%. There are other measurement systems in place, mainly Mitotic Count which tends to be more frequently used for Lung NENs. The Ki-67 index can usually be determined, even in cases of small biopsies but Mitotic rate counting requires a moderate amount of tumour tissue (at least 50 HPFs or 10 mm) and may not be feasible for small biopsies. The Mitotic Count method may be preferred or used in addition to Ki-67 for certain Lung NET scenarios as it is said to be more helpful in distinguishing atypical from typical Lung NET, and for small and large cell lung Neuroendocrine Carcinomas (NEC).

Please note there are major differences in how Pheochromoctyomas and Paragangliomas are graded and staged, see below.

Why is differentiation important?

To fully understand grading, you also need to understand the concept of ‘differentiation’. In the most basic of terms, ‘differentiation’ refers to the extent to which the cancerous cells resemble their non-cancerous counterparts. This is an important point for NENs because the differentiation of a NEN has an impact on both prognostics and treatment regimes.

Although there are 3 grades of NEN, there are two classifications of differentiation and these become really important at Grade 3.

Some of you may have heard the term ‘moderately differentiated’ which tended to align with an intermediate grade or Grade 2. However, please note that the term moderately differentiated as a classification was phased out in 2010 by WHO reducing from 3 differentiation levels to 2. Grade 2 is also defined as well differentiated but based on different proliferative rate (see table).

Grading – Key WHO 2019 Changes

WHO Classifications of Cancer are published in something known in medical world as “The Blue Book”. It’s important to understand the source of how all types of cancer (including Neuroendocrine) are named and graded, including any associated terms. This mainly comes from a World Health Organisation agency known as the International Agency for research on Cancer (IARC). The “Blue Book” is in fact several books. Each book covers an anatomical system with the human body and Neuroendocrine Neoplasms are covered within each particular system – this is a topical point for Neuroendocrine Neoplasms given that they can appear anywhere in the body and often this causes confusion when describing cancers (e.g. we know the issues behind Pancreatic Cancer for example). There is a proposal for a single “Neuroendocrine” blue book but that has yet to come to fruition. However, the latest publication is “Digestive System” covers most Neuroendocrine types so it’s a very useful update, and as you will see a milestone in resolving past nomenclature issues with our disease. The work behind the 2019 Digestive System follows on the from the excellent work in the 2017 Endocrine update where the significant change from 2 to 3 grades of well differentiated pancreatic NETs was a significant change. In fact, the 2017 pancreatic neuroendocrine update has been copied and pasted right into the pancreatic section of the 2019 digestive system book, effectively delivering a blue book which contains the vast majority of Gastroenteropancreatic Neuroendocrine Neoplasms (GEPNENs) including those in the hepatopancreatobiliary organs (pancreas/bile duct/liver/gallbladder). Important to also point out that a very similar structure was presented in the 2017 Endocrine Organ blue book which included (but not limited to) Neuroendocrine related tumours of the Pituitary, Thyroid, Parathyroid, Pheochromocytoma, Paraganglioma and their associated hereditary syndromes. ENETS and many other regions adopted this for all types of Neuroendocrine Cancer (as a result of a proposal to do so), so the 2019 Digestive System is really a rubber stamping of this change but it also confirms one very important difference – the word “carcinoid” has been removed from the vocabulary, not before time and something I’ve been quite vocal about since 2015.

So, what about other areas not included in GEPNENs? Please note there are still loose ends in some of the blue books, particularly Pulmonary, Urinary and Female Reproductive Organs. Hopefully, these blue books will be updated in the same way as the others for Neuroendocrine Neoplasms (or will be subsumed into the proposed Neuroendocrine blue book). But in the meantime, experts are proposing the nomenclature above be extended in advance of the blue book updates – this is covered in Neuroendocrine neoplasm update: toward universal nomenclature, © 2020 Society for Endocrinology 2020, Guido Rindi and Frediano Inzani.  What this effectively means is that the term “carcinoid” is effectively now totally defunct other than the loose ends of Carcinoid Syndrome, Carcinoid Heart Disease and Carcinoid Crisis (problems I could resolve in 10 minutes if they would listen to me). In fact one well known pathologist has been campaigning for the removal of the terms ‘carcinoid’ since 2007

I’ll summarise the key changes from 2019 below for your convenience, but I need to cite the article here:

The 2019 WHO classification of tumours of the digestive system

Iris D Nagtegaal, Robert D Odze, David Klimstra, Valerie Paradis, Massimo Rugge, Peter Schirmacher, Kay M Washington, Fatima Carneiro, Ian A Cree, for the WHO Classification of Tumours Editorial Board. First published: 21 August 2019 – https://doi.org/10.1111/his.13975

• All well differentiated GEPNETs now include Grade 3 alongside Grades 1 and 2.
• All poorly differentiated GEPNENs are now Neuroendocrine Carcinomas and are by default Grade 3 (the inference in the guidance is that there is no need to use the term high grade or Grade 3 in reference to Neuroendocrine Carcinomas as they are they have those attributes by default).
• Neuroendocrine Neoplasm is now the formal umbrella term for the grouping of well and poorly differentiated tumours.
• The term ‘carcinoid’ is no longer used in nomenclature for Neuroendocrine Neoplasms within the Digestive System blue book.
• Pancreatic Neuroendocrine Neoplasms will now appear in the Digestive System blue book, previously they were listed in the Endocrine Organ book.
• The molecular differences of well and poorly differentiated tumours have been added to assist pathologists in identifying them, e.g. Mutations in MEN1, DAXX and ATRX are entity defining for well differentiated tumours while Neuroendocrine Carcinomas (poorly differentiated) usually have TP53 or RB1 mutations.
• Poorly differentiated tumours are divided into Small Cell type (SCNEC) and Large Cell type (LCNEC).
• Last year the term “Goblet Cell Carcinoid” was changed to “Goblet Cell Carcinoma”. However, this type of appendiceal neoplasm has effectively been removed from the Neuroendocrine family as it’s now recognised to have a minor neuroendocrine component. This cancer type is now called Goblet Cell Adenocarcinoma of the Appendix.
• As per the previous content of this article, mixed tumours were renamed from Mixed AdenoNeuroendocrine Carcinomas (MANEC) are now named Mixed Neuroendocrine-Non-Neuroendocrine Neoplasms (MiNENs) – clearly they now have variable grades, variable mitotic rate/Ki67 and each component can be graded separately.

Pheochromocytoma/Paraganglioma

Previously, Pheochromocytoma/ Paraganglioma (PHEO/PGL) did not have an official grading regime, i.e. they were just benign or malignant. However, since 2017 and as reflected in the fourth edition WHO classification of Endocrine Systems, on the basis that all PHEO/PGL have some metastatic potential and have their own grading system, there are different classification systems for Pheochromocytoma and Paraganglioma These grades are based on molecular profiles (the future way for many cancers and the first time for a NET). They have a system of grading called GAPP (grading of adrenal pheochromocytoma and paraganglioma) and they are clustered into the metastatic risk (this is the beauty of molecular methods). So, they are different than regular NETs and they now have 3 differentiations, loosely related to grade. This change is complex and is covered in depth here.

Misc Grading Issues

The proliferative rate can be diverse in NENs, so sampling issues can limit the accuracy of grading. More substantial samples of tumour are therefore preferable for grading thus why the Ki-67 index is preferred for biopsies where large amounts of tissue may not be available. The distinction of low-grade from intermediate grade can be challenging when using small samples. A couple of interesting observations about NET grading which I spotted during my research and ‘forum watching’. You can have multiple primary tumours, and these might have different Ki-67 scores. Additionally, on larger tumours, Ki-67 scores can be different on different parts of the tumour. And something I know from my own experience; secondary tumours can have different Ki-67 scores than primary – even a different grade. In my own case, my liver secondary tumours were graded higher than my primary which according to my surgeon is in keeping with a clone of the disease having become more aggressive over time. Royal Free Hospital NET Centre indicates a person’s grade should be taken from the highest biopsy grade taken. This is a fairly complex area but a recent study published by the US National Institute of Health and many anecdotal comments made by NET specialists indicates that is a fairly common scenario.

MANEC vs MiNEN

Added for completeness. This mixed and rare neoplasm type has traditionally been related to NEC but in 2017 the nomenclature change to a new term was necessary to reflect the fact that some of the tumours involved were not carcinomas or adenocarcinomas but rather were well differentiated tumours or even adenomas (i.e. benign). Previously known as Mixed AdenoNeuroendocrine Carcinoma (MANEC), they were renamed to Mixed Neuroendocrine Non-Neuroendocrine Neoplasms (MiNEN).

MiNEN are neoplasms with two distinct neuroendocrine and non-neuroendocrine cell populations. They can be morphologically classified into three entities: collision, composite, and amphicrine MiNEN. Currently, both components composing a MiNEN must represent at least 30% of the whole tumour. Diagnosis of MiNEN is usually facilitated by the presence of at least one well-differentiated component which may be the Neuroendocrine or Non-Neuroendocrine component. However, the two components may be difficult to identify with conventional morphological techniques, particularly when they are poorly differentiated, and their identification may require additional immunohistochemical techniques. MiNEN usually originate from organs that contain neuroendocrine cells and in which “classical” NENs are known to develop, such as pancreas, appendix, colon, and to a lesser degree small intestine. Other locations in my source document includes Oesophagus, Stomach, Bilary Tract and Gallbladder, Duodenum and Ampulla of Vater and Rectum.

NEC vs NET

Having researched widely, I believe there are 8 key differences between NET and NEC:

1. Grade – NETs can be Grade 1, 2 or 3. There is no requirement to use Grade 3 when talking about Neuroendocrine Carcinoma which by default is poorly differentiated high grade.
2. Differentiation – all NETs are well differentiated, NECs are poorly differentiated. Important difference at Grade 3 NET.
3. Aggressiveness – Most NETs tend to be indolent or slow growing while NECs tend to be aggressive and faster growing. However, Ki67 and/or mitotic count is an aggressiveness measurement tool. Genetic profiles can also be a guide but this is beyond the purposes of this article but may be explored in subsequent parts. It follows that NECs normally have a worse prognosis in comparison to NETs.
4. Hormone Secretion – NETs can produce peptide hormones that may be associated with hormonal syndromes. NECs usually fail to express hormones or produce hormonal syndromes.
5. Somatostatin Receptors – A NET is much more likely to express somatostatin receptors which can influence treatments such as somatostatin analogues and peptide receptor radiotherapy (PRRT)
6. Hereditary Syndromes – NETs are much more likely to be associated with hereditary syndromes such as Multiple Endocrine Neoplasia (MEN).
7. Platinum Based Chemotherapy – NETs are less likely to show a good response to platinum based chemotherapy which can often be the first line treatment for NEC.
8. Primary Locations – can be vastly different in terms of commonality and therefore provide clues to investigators. Common locations for NEC include: Lung, Esophagus, Colon, Urogential Organs and Skin – with the exception of Lung, these are very rare locations in NETs. Conversely, rare/very rare locations for NEC but common in NET include: Rectal, Small Intestine, Appendix, Stomach, Pancreas.

Staging for Pheochromocyoma/Paraganglioma

This may differ between regions but I attach the US version here – they are not normally too different.  Read here.

Summary

A complex area and I hope I have condensed it sufficiently for you to understand enough for your purposes. Despite looking very scientific, it is not an exact science. There are many variables as there always are with Neuroendocrine disease. NENs can be very challenging for a pathologist even an experienced one who may not have encountered NENs before. However, it is an extremely important part of initial diagnosis and also when needed during surveillance. It is a vital tool used by Multidisciplinary Teams (MDT) in treatment plans and for prognostic purposes. If you need to learn further, I recommend this document:

If you are interested in this subject and have one hour to spare, there is a great video here from LACNETS worth watching.

Finally – always make sure you get your pathology results at diagnosis and following any subsequent sampling.

Other references:

1. WHO Classification of Tumours of Endocrine Organs, 4th Edition, Volume 10.
2. WHO Classification of Tumours of Digestive System, 5th Edition, Volume 1.
3. The UICC TNM classification (the internationally accepted standard for cancer staging).
4. AJCC 8th Edition Cancer Staging.
5. ENETS Staging.

You may benefit from reading these associated posts:

Benign vs Malignant

Incurable vs Terminal

Carcinoid vs Neuroendocrine

10 Questions for your doctor

Looking for a needle in a haystack