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Dermatopathology: Practical & Conceptual April - June 2008
5. New Heights: “Animal-type” melanoma and entities related to it (Part I): Evolution of a concept
François Milette, M.D.
A. Bernard Ackerman, M.D.
Contents of Part I
I. Melanosis in horses and men? (Dick, 1832)
II. A précis of equine melanotic disease (Levene, 1971)
III. Melanoma arising in “blue nevi”? (Darier, 1925)
IV. Diffuse mesodermal pigmentation? (Carleton and Biggs, 1948)
V. Melanotic disorders in horses and men? (Levene, 1979)
VI. Pilar neurocristic hamartoma? (Tuthill, Clark, and Levene, 1982)
VII. Malignant melanoma arising in a blue nevus? (Pathy, Helm, Elston, Bergfeld and Tuthill, 1993)
VIII. Cutaneous malignant melanotic neurocristic tumor arising in neurocristic hamartoma? (Pearson, Weiss, Headington, 1996)
IX. Malignant melanoma with prominent pigment synthesis: “Animal-type melanoma”? (Crowson, Magro, Mihm, 1999)
X. Animal type melanoma? (Requena, de la Cruz, Moreno, Sangueza, Requena, 2001)
XI. Animal-type melanoma? (Kazakov, Rütten, Kempf, Michal, 2004)
XII. In the textbooks?
XIII. Melanomas in horses as described in veterinary medicine literature? (Valentine, 1995; Seltenhammer, 2004)
VII. Malignant melanoma arising in a blue nevus? (Pathy, Helm, Elston, Bergfeld and Tuthill, 1993)
Pilar neurocristic hamartoma disappeared from the literature of pathology, including dermatopathology, until 1993 when Pathy et al. revived it in a report of a melanoma that, according to them, had arisen in a blue nevus with "features of pilar neurocritic hamartoma." [
"65 year-old man presented with a history of a giant blue plaque of the parietal scalp since childhood. . . ."
to wit, a congenital plaque of blue nevus of the "cellular type." which according to the authors, was
". . . observed for 10 years and thought to be unchanged until a new adjacent lesion appeared . . ."
] Then something occurred in this congenital lesion that by then had been present for 65 years:
"A wide excision revealed MM arising in a blue nevus." The authors opined that "the cellular blue nevus was originally described as a form of melanoma (Darier, 1925) but only later recognized as a variant of blue nevus . . ."
They then went on to comment thus:
"Neurocristic hamartoma describes a subset of patch- and plaque-type blue nevus with a prominent perifollicular arrangement of pigment laden spindle cells. pilar neurocristic hamartoma has a distinct histologic appearance similar to equine melanotic disease. It is possible that both represent hamartomas of neural-crest origin."
It is instructive to compare the content of those sentences to one pertinent to the issue written about by Tuthill et al. 10 years earlier, i.e., in 1982. This is what they said then:
"In 1925 Darier compared "melanosarcoma." a term used for what we now call malignant blue nevus, to the rapidly progressive form of melanotic disease of grey horses and indeed there are similarities histological and clinical."
In other words, the entities "melanosarcoma," "malignant blue nevus," and "cellular blue nevus" are "similar" to equine melanotic disease, which, in turn, "might be a hamartoma of neural crest origin" and is "remarkably similar" to "pilar neurocristic hamartoma."
These comments transport the reader to a world where, at the same time, everything is similar and different, a world where an otherwise healthy brain is expected to genuflect to the expert, ceasing entirely to think independently and skeptically, and, thereby, becoming forever that of a believer!
But Pathy and colleagues have more surprises in store for a reader unwitting, one of which is delivered succinctly in these words:
"This case [which a few lines earlier in their article was a melanoma arising in a blue nevus] fulfilled the strict criteria for malignant blue nevus."
There is a difference profound, not only linguistic but conceptual, between a melanoma arising in a congenital nevus (i.e., a blue nevus) and a malignant blue nevus, the latter being a contradiction in terms (malignant/benign) staggering. And still another surprise is in store for the reader:
"The primary blue nevus had features of what Tuthill have called pilar neurocristic hamartoma, a subset of patch- and plaque-type blue nevus with perifollicular arrangement of pigment-laden spindle cells."
Solely because of "perifollicular arrangement of pigment-laden spindle cells" (those presumably being melanocytes, but not identified as such with specificity by the authors) in the "primary blue nevus" on the scalp of the 65-year-old man, it being a feature of what Tuthill et al. earlier called pilar neurocristic hamartoma, a relationship was established by the coworkers between the lesion on the scalp in which melanoma developed and pilar neurocristic hamartoma.
What is the pertinence (other than testifying to the nature congenital of blue nevus) of that "feature" ("perifollicular arrangement of pigment-laden spindle cells") to the story told by Pathy et al.? Other than being a vehicle to calling attention to the article of 1982, there is only one, that is, to justify analogy of pilar neurocristic hamartoma to equine melanotic disease.
Concerning pilar neurocristic hamartoma, the authors said this:
"Lesions may clinically appear as beaded rows of pigmented papules. This distinct clinical appearance helps to differentiate pilar neurocristic hamartoma from ordinary blue nevi that show a perifollicular arrangement of nevus cells."
] The last two sentences add more confusion still because the lesion in their patient showed no evidence of the "distinct clinical appearance" they refer to, i.e., "beaded rows of pigmented papules." Curiously, they admit that "ordinary blue nevus [may] show a perifollicular arrangement of nevus cells." which makes injection of pilar neurocristic hamartoma into the matter superfluous and irrelevant.
The authors continue merrily on their path of violation of principles rudimentary to pathology and logic, these lines of theirs inducing wonder:
"In our case, a punch biopsy in 1981 yielded a histologic diagnosis of "atypical blue nevus" with suggestion of an element of "cellular blue nevus." It is conceivable that an excisional biopsy at that time might have demonstrated an early frankly malignant melanoma."
] Not only is it "conceivable," it certainly is possible and even probable! But what do they mean by "atypical blue nevus" and by "elements of cellular blue nevus"? And what, if any, is the relationship of those conditions to pilar neurocristic hamartoma? And what does "frankly malignant" means? Is anything less than malignant? Of course, no answer is provided.
Yet another matter bewildering is set before a reader seeking diligently to make sense of these issues:
"Initial metastases resembled common blue nevus, except for mild but definite cytologic atypia and absence of a stromal component of fibrosis."
] The theme is sustained: a statement being nullified by another statement in the same sentence. The "initial" metastases resembled blue nevus but, at the same time, they were completely different from blue nevus! Parenthetically, what is the relationship of "common" blue nevus to "cellular" blue nevus, and of them to "atypical" blue nevus (and malignant blue nevus)? And what are "initial metastases" and how are they differentiated from later ones? This mish-mash of ideas, coupled with garble, makes it impossible to understand what these authors are saying.
In summary, what Pathy and associates recorded is a melanoma that arose in that kind of congenital nevus designated conventionally "blue." Because the lesion is much like that in Darier's Patient 1, the coworkers invoked equine melanotic disease because Darier, himself, had done that in his discussion of histopathologic findings in his patient. Reference to equine melanotic disease appears suddenly in the text of Pathy et al., seeming to come from nowhere. This is the sentence in which the condition arrives abruptly and unannounced:
"Equine melanotic disease has a variable course but biopsy of older lesions often reveal foci of amelanotic tumor cells showing frequent mitoses and cytologic pleomorphism."
One can only wonder worriedly why equine melanotic disease was introduced into the discussion at all and why it is not alluded to again. Moreover, Tuthill et al. refer to equine melanotic disease in a way that is singularly at variance with that of most other authors, stating that
"biopsy of older lesions
[emphasis ours] reveals foci of amelanotic tumor cells showing frequent mitoses and cytologic pleomorphism,"
whereas most other students of the subject insist on intense pigmentation, indolent evolution, bland melanophagic nature cytologically (rather than pleomorphic melanocytic character of the cells neoplastic) except in
cases. Again things are said to be similar or different according to the message intended for delivery, rather than pertinence actual.
Although comparison of melanoma that developed after many years in the congenital nevus of the patient of Pathy et al. with a melanoma that occurred in equine melanotic disease could be justified, there are many more differences between those two circumstances than there are similarities. Among the differences are these:
Equine melanotic disease, which is never congenital and develops almost exclusively in the region of the anus, whereas the congenital nevus in this patient was positioned on the scalp.
In horses, multiple lesions continue to grow progressively, whereas in this patient the solitary lesion was stable for more than 60 years.
When melanoma arises, as it does rarely, in the setting of equine melanotic disease, the horses afflicted are aged, whereas melanoma may develop in a blue nevus of a human who still is young.
For all the razzle-dazzle of Pathy et al., their patient, in fact, had a melanoma that arose in a "blue nevus."
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