Classification of thymic tumours

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Published: 11 Mar 2011
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Prof Michael den Bakker - Erasmus-MC University Medical Centre, Rotterdam, Netherlands

Prof Michael den Bakker talks about tumours of the thymic epithelium. These tumours range from benign to malignant with the potential for metastases and can be difficult to diagnose depending on the type. Thymic carcinoma has no histological features and is consequently can only be identified by excluding the possibility of lung metastases, however thymomas are carcinomas of the thymic gland and are more easily identified, giving thymoma patients have a much better prognosis. Prof den Bakker discusses how thymomas are typed and staged, outlines the treatment options available for thymoma patients and explains the relative merits of different classification systems for thymic tumours.

European Multidisciplinary Conference in Thoracic Oncology (EMCTO 2011) 24—26th February 2011, Lugano

Classification of thymic tumours

Professor Michael den Bakker (Erasmus-MC University Medical Centre, Rotterdam, Netherlands)

More now from the Multidisciplinary Conference in Thoracic Oncology here in Lugano. Professor Michael den Bakker, you are talking about thymoma.

That’s correct.

A tumour which has a number of different types and it’s not entirely clear to everybody what happens. What is the main thing you want to get over about thymoma; what have you discovered?

For a start, thymic tumours should be defined as tumours of the thymic epithelium so I am going to talk about those types of tumour solely, thymoma. What I am going to try and get across is that there is a controversy over the typing issues in thymoma.  They have been typed ever since the early twentieth century and the first typing scheme, I think, is 1912, 1916, and ever since then there have been multiple typing schemes and none of them have been satisfactory.

And some can be metastatic potential, can have metastatic potential, others maybe not?

Yes, thymoma, the name implies that it is a benign tumour. This is not the case, all thymomas are potentially malignant, and within the typing scheme we can see a spectrum of tumours which are essentially benign, very, very rarely may actually prove to be malignant, but on the other end of this typing scheme we have tumours which are fully malignant, can metastasise. These should be set apart from the true, always malignant, thymic carcinoma which is a separate group and these are always malignant.

How do you do the diagnosis? It has been said that diagnosis is one of exclusion.

This is true for thymic carcinoma. Thymic carcinoma does not have any histological features, microscopical features, with which you can define it as a thymic tumour. You have to rule out a metastasis from elsewhere, for instance from the lung.

So thymic carcinoma is one of exclusion and that is a bad deal?

That is a bad deal. Thymic carcinoma is a bad deal. A thymoma is a tumour which we can confidently diagnose as a tumour of the thymic gland, of the thymus, and this is a far better prognosis for the patient, to get a diagnosis of thymoma.

So when you’ve excluded thymic carcinoma, what do you then do to get accurate typing and indeed what about accurate staging with thymoma?

Yes, once we have excluded thymic carcinoma, and the distinction between thymoma and thymic carcinoma is usually straightforward, we then have to type thymoma. And to type thymoma, once again this is morphology, so looking down the microscope at a slide and using criteria which currently the most used system is that of the WHO, the World Health Organisation. There are other typing schemes which all have their merits. Typing is one thing, staging is at least and probably more important because staging in itself says far more about the prognosis of the individual patient. Staging, in part, can be done clinically, by imaging for instance, but is also the job of the pathologist to see if the tumour extends beyond the thymus gland, for instance, extends into neighbouring organs such as the lung or the heart or the great vessels. Higher stage means bad prognosis.

How much can you improve therapy for thymoma, then, because outcomes can be quite good can’t they?

Yes. I am a pathologist so I shouldn’t really speak too much about therapy, however making the right diagnosis is the start of implementing therapy. Therapy for thymoma is essentially surgical and if a thymoma has been completely removed essentially this is the cure for the patient. So the therapy is surgery at the moment.

And so what, then, is the controversy that surrounds thymoma right now?

Because the thymus is a composite organ, it has an epithelial framework from which these tumours arise, and thymomas are epithelial tumours but they have a spectrum of morphology; they have wide ranging appearance down the microscope, and the controversy comes from how you are going to code these various morphologies. You can take two views. You can say I am just going to lump the big groups together, I am going to ignore the morphology and I am going to look at the outcome for these groups. That is one way, which is not the WHO way; the WHO way actually says we are going to look at these tumours in detail, in morphological detail, we are going to try to label them as good as possible so we have a morphological classification, and then we will see how these groups perform. Now the first classification scheme is a more modern one, set forth in 1999 by two esteemed American colleagues, Dr Suster and Dr Moran, and they essentially just have three groups of thymoma. They have thymoma, they have an atypical thymoma and they have thymic carcinoma. There is no discussion about thymic carcinoma. What essentially they do is they lump all thymomas into two groups. They say we have a group which always has a very good prognosis, and we have a group which has a slightly less good prognosis but still better than thymic carcinoma. This is perfectly fair from a patient approach; you want to define patient groups. But of course all the information that we then have on the histological type of the tumour, what the cells exactly look like, is discarded, it is not taken into consideration. The WHO does give us the opportunity to describe in detail the exact microscopical image of the thymoma, of the tumour, but of course because it has a larger number of categories will result in a worse performance by individual observers.

What about this difference between thymic epithelial tumours and immature lymphocytes?

Yes, well essentially we are talking about the tumours of the thymic epithelium here, the tumours of the immature lymphocytes, the non-Hodgkin’s lymphoma, the acute lymphoblastic lymphoma; these are in the realm of the haematopathologist and the haematologist. There are also very rare the tumours which arise from the non-epithelial framework of the thymus, so the connective tissue; these are exceedingly rare and these are left out of consideration when we talk about thymoma.

So looking at thymic epithelial tumours and excluding the thymic carcinoma, what’s the main practical message that you would like cancer doctors and cancer clinicians to get out of your talk here in Lugano?

Well, listen to each other, I think, is one because the controversy has reached at times, it is not so bad now, but has reached proportions in which it became very polarised so people would promote their own typing scheme and essentially would not listen to the other group; now that is luckily down played. I think from a practical point of view it is fair to say that no typing scheme is perfect at the moment but we can use them in daily practice. So, as long as the clinicians know what we are talking about as pathologists and they understand the typing scheme we are using, that’s fine. So I think that would be the message: declare what typing scheme you are using and tell the clinician that information.

And from what you are saying, there is a bit of a learning curve here?

Absolutely, absolutely. For individual pathologists to use the system, which has to be improved, and that is what the ITMIG group is going to work on, but also for the interaction between pathologists and clinicians.

So the multidisciplinary approach is again very important here?

It is very important, yes, yes. What we really haven’t talked about is the staging aspect. There is very little discussion about the importance of staging. There is a staging system in place for thymomas which is the Masaoka system. Everybody agrees with this, it works fine, it can be translated into a WHO staging system if we wish. Of course it’s important when we receive a surgical specimen that this is worked up in the best possible way, and this is once again something which the ITMIG group is quite important for because they are setting out the guidelines for how we should handle these surgical specimens.

From your understanding of the clinical side of things, how big a pay-off might there be, in ball park figures, in improving the therapy if you get all the staging and typing correct?

I am not confident to give you an answer on that. Thymoma, as it stands today, has a fairly good
prognosis if the surgery has been done to a high standard. I think where we can gain a bit is in the more malignant side of thymoma, so in detail in the WHO B3 type thymoma for instance. I think perhaps that can be looked into from the clinical treatment point of view, but for the lower end, so the nearly benign end of the spectrum, good surgery is the key issue.

And a multidisciplinary approach and attention to the actual system that you are using and getting to know it?

Absolutely, yes, we have as a multidisciplinary group, which means the surgeon, the pathologist and the oncologist, in those cases they have to know exactly what one and each other is talking about.

Does this have to be, then, at a centre of excellence do you think?

Yes, these tumours are so rare, 0.13 per 100,000 patient years so that they are really very rare tumours, we see around 50 or 60 in The Netherlands on an annual basis. So you have to centre these tumours.

Well Michael thank you very much for joining us on

It is my pleasure, thank you very much.