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Atypical nevi


Dr. Phillip McKee

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Everyone has his/her hobby horses and I am no exception. My [i]bête noire[/i] is use of the term atypical in melanocytic pathology. The best place to start is by defining typical:
[indent=1]1. representative[b]:[/b] having all or most of the characteristics shared by others of the same kind and therefore suitable as an example of it[/indent]
[indent=1]2. characteristic[b]:[/b] characteristic of an individual person or thing[/indent]
In the context of melanocytic nevi, the common or banal nevus (junctional, compound or dermal) is regarded as typical and therefore any nevus that departs from this nevus could in theory be regarded as atypical e.g. balloon cell nevus, signet-ring cell nevus, pseudoglandular nevus etc. Obviously no one in his/her right mind would use the term in this context. However there are numerous occasions where because of histologically worrying features, the lesion is described as atypical e.g. atypical genital nevus, atypical acral nevus, atypical nevus of the scalp, atypical Spitz nevus etc. We now know or think we know that most of these are biologically benign (atypical genital nevus, atypical acral nevus, and atypical nevus of the scalp) and include them in the category of “nevi at special sites”. Provided the dermatologist knows what the pathologist means by “nevi at special sites” and recognizes that they are thought to be of no clinical consequence then all is well!
The real problem arises with the use of the term atypical Spitz nevus. This represents a Spitz nevus which departs by greater or lesser degree from the prototypic lesion. The term has evolved over the years gaining great popularity amongst many dermatopathologists and pathologists. The reasons for this are multiple:
· Numerous papers on the subject by experts in the field of melanocytic pathology
· The increasing proportion of Spitz nevi partially sampled by shave or punch biopsy by the dermatologists
· The problem is that some of the features which would enable a confident diagnosis of benign or malignant are present in the deeper (un-sampled) portion of the lesion e.g. infiltrative versus expansile (pushing) lower border, lack of maturation, deep mitoses or atypical mitoses
· Litigation
The result of this has been a loss of confidence amongst many dermatopathologists or pathologists reporting such lesions. In addition, the diagnostic criteria are the same for both atypical Spitz nevus and spitzoid melanoma. It is all a matter of degree. To my mind (along with the late Bernie Ackerman) what this really means is that when a person makes the diagnosis of atypical Spitz nevus he/she is not really sure whether the lesion is truly benign or malignant. To put it another way, lesions that are categorized as atypical Spitz nevus comprise both Spitz nevi and spitzoid melanomas. While I do not for one moment think that I can confidently differentiate between the two in every case (far from it), I do believe that the creation of this category has resulted in an increase in the diagnosis of this “variant” to the cost of making a confident diagnosis of either Spitz nevus or spitzoid melanoma. It is just too convenient and is an easy way to opt out of making a diagnosis. Certainly over the years I have encountered many examples in consultation where the referring pathologist has wondered whether the lesion is atypical because for example it contains 2 superficial mitotic figures or something similar. I believe that the term should be used very sparingly if at all. It demonstrates diagnostic uncertainty and if used too often, will likely result in loss of confidence in the dermatopathologist by the clinicians. In addition, the clinician may be at a loss as to what to tell the patient and what further treatment to offer if any. It is essential to show all such lesions to colleagues for their help or if necessary send the case to an expert for a consultation.
The issue has been further complicated by the use of sentinel lymph node biopsy in the treatment of “atypical Spitz nevus”. There are well over 100 examples in the literature when I last counted, of “atypical Spitz nevi” with positive sentinel lymph node biopsies and yet the diagnosis has not been changed to Spitzoid melanoma. One cannot but wonder what the point actually was of doing the sentinel lymph node biopsy. In fairness, with the follow-up available (often limited) such sentinel node positive lesions appear not to have progressed and perhaps some would regard them as a “benign” metastasis. Mind you there is one series in the literature where an “atypical Spitz nevus” with systemic spread was included in the study, which is beyond belief. How on earth can a Spitz nevus with systemic spread be anything other than a melanoma?
I realize that I am an anachronism in this field but would love to hear other people’s views to see if I truly am a voice in the wilderness or whether there are others who share my concern.
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I completely agree Dr. McKee. As a dermatologist and a trying to bud dermatopathologist, I find it extremely difficult to get my head around this concept. This excellent article I think sums up my confusion [url="http://www.ncbi.nlm.nih.gov/pubmed/22262360"]http://www.ncbi.nlm.nih.gov/pubmed/22262360[/url]

You discussed a case on facebook which was microinvasive and metastasised. Similarly I am sure you have seen a number of cases where the lesion was clearly invasive and not metastasised. As a dermatologist first and a pathologist next, I find that the criteria used for the diagnosis of benign vs malignant can sometimes be very subjective and it comes down to the word of one expert vs the other. Scientific studies on this issue are difficult as once excised, who is to say how the lesion was going to behave, if left in-situ. It is impossible to tell, unless the lesion metastasizes or recurs subsequently. So our criteria for diagnosis of malignancy are biased/ based on a few cases that have metastasised. But these are just individual case reports and small series. So there is a huge selection bias, as we don't know of the hundreds of cases that were 'treated' by the excision.

My personal opinion is that in any individual case, it is difficult to call something benign or malignant because I feel that the definitions of benign and malignant themselves are flawed. I think that the entire classification of tumours should be revamped and we should classify tumors as no, very low, low, medium, high and very high risk or something similar, which will have give clinicians and patients some idea as to what they are dealing with.
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Dr. Phillip McKee

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I like your comment but the problem applies to all forms of cancer. A colonic polyp- how do we know how it will behave if we left it alone? Similarly, if a melanoma is excised, how do we know whether it will metastasize. The figures quoted in the literature are after all only guidelines. If there is an 80% mortality at 5 years, which category does the patient belong to? The problem is that medicine is not an exact science but one of probablities. If you were to revamp the classification system as you have suggested, how would you begin to define your categories? You would replace one inexact science with another which wouldn't really solve anything.
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Thank you for your comments Dr. McKee.

The reason I would prefer an inexact grading system to the inexact current classification system, is because of the fallacy of benign vs malignant. A trichoepithelioma or a trichoblastoma are considered benign while a BCC is considered malignant!

I agree that we would be replacing one inexact science with another, but I can't think of a better way of sorting this issue. The melanoma staging system is already to some extent grading tumors. In my view, we need something similar (grade/ stage) to encompass all tumours, including the 'atypical' category. For that large databases are necessary with long term follow-up etc.

As you say, existing classification systems miserably fail when we try to classify benign blue and spitz naevi with nodal metastasis!. Benign metastasis, certainly is beyond my understanding.
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Dr. Phillip McKee

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I suppose that my greatest desire is to eliminate the "atypical" category completely. Far too many people use it as a bolt-hole to get rid of cases where they cannot make up their mind. It is a huge issue in the US.
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Dr. Mona Abdel-Halim

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This blog and the comments below are very valuable to me. Thank you very much.
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Dr. Hafeez Diwan

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A superb statement and summation of the issues regarding this important topic. I don't think you are a voice in the wilderness. I know several people who agree with what you have written.
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Dr. King-Chung Lee

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Great discussion on this important topic. However, I would like to bring out another viewpoint for consideration.

While we like to know everything, we sometimes do encounter cases that have some features for both Spitz nevus and Spitzoid melanoma. I think it is appropriate to admit our uncertainty in such situation rather than forcing it to a particular entity.
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