Spitz melanocytic tumours – a review

Spitz tumours comprise a spectrum of melanocytic proliferations that share a set of distinct cytological features and molecular pathways. They include benign naevi, intermediate or indeterminate tumours and rare melanomas. Spitz tumours are notorious for the difficulty of distinguishing benign neoplasms with atypical features from melanomas and the related diagnostic uncertainty. Advances in the knowledge of the molecular pathways and genomic aberrations associated with these neoplasms have permitted opportunities for a reduction in the number of uncertain diagnoses and a more objective distinction between Spitz tumours from Spitz-like neoplasms. The presence of a Spitz molecular pathway, such as Harvey rat sarcoma viral oncogene homologue (HRAS) aberrations or kinase fusions, distinguishes a bona fide Spitz neoplasm from Spitz-like naevi or melanomas with conventional driver mutations. Spitz neoplasms with benign histopathological features and, if such testing is performed, benign cytogenetic and molecular findings, are termed Spitz naevi. Spitz neoplasms with frankly malignant histopathological findings or ambiguous microscopic findings associated with genetic or genomic aberrations most in keeping with melanoma are designated as Spitz melanoma. Tumours with microscopic features and genetic/genomic aberrations in between naevi and melanomas are classified as Spitz melanocytoma.     

Spitz naevi and malignant melanomas of childhood and adolescence

We reviewed 33 cases of Spitz naevus and 19 of malignant melanoma in patients aged 20 years or less who were evaluated at our institution from 1950 to 1975 and followed-up for up to 32 years. The histological findings were studied prior to review of clinical data. There were no malignant melanomas in patients less than 9 years old; almost half (15/33) of the Spitz naevi were in this age group. Among the 25 histological criteria evaluated in the 52 lesions, the most striking differences between malignant melanomas and Spitz naevi were a higher degree of pagetoid spread, cellular pleomorphism, nuclear hyperchromasia and mitotic activity in the malignant melanomas, and a more prominent spindle cell component in Spitz naevi. We stress the importance of cytological as well as architectural criteria in distinguishing between Spitz naevi and malignant melanomas and emphasize the pitfalls that may be encountered because of overlap in histological features between the two groups.     

Allelic imbalance in the diagnosis of benign,atypical and malignant Spitz tumours

To test the diagnostic usefulness of allelic imbalance (AI) analysis based on routinely paraffin-embedded tissue, a series of 55 benign Spitz naevi, Spitz tumours with uncertain malignant potential, and malignant Spitzoid melanomas was investigated. Laser microdissection was used to ensure representative sampling of lesional cells and to investigate AI in separate tumour areas of four melanomas. AI was found in 2/12 (17%) typical Spitz naevi, 3/9 (33%) atypical Spitz tumours, 12/17 (65%) atypical Spitz tumours suspicious for melanoma and 15/17 (88%) Spitzoid melanomas. Additional immunohistochemical staining for Ki-67 using the MIB-1 antibody revealed positive deeply situated lesional cells in 0/6 (0%) Spitz naevi, 1/8 (13%) atypical Spitz tumours, 5/14 (35%) atypical Spitz tumours suspicious for melanoma, and 7/14 (50%) Spitzoid melanomas, respectively. Two of the melanomas examined for AI in separate tumour areas showed intratumoural genetic heterogeneity. In view of the finding of AI and deeply situated Ki-67 positive cells not only in melanomas but also in Spitz tumours with uncertain malignant potential, these approaches appear to have no direct diagnostic applicability for the distinction between benign and malignant Spitz tumours. Further molecular studies will be required to determine whether Spitz tumours and Spitzoid melanomas are unrelated entities, or whether there is a true spectrum of tumour progression.     

Benign atypical naevi: diagnostic difficulties and continued controversy

Benign melanocytic naevi exhibit a wide spectrum of histological appearances. Some share significant clinical and histological features and are recognized as entities. Included among these are pagetoid/junctional Spitz naevus, pigmented spindle cell naevus, halo naevus, recurrent and traumatized naevus, ultraviolet (UV) irradiated naevus, naevus in infants, acral naevus, genital naevus and naevi from other specific anatomic locations. However, there still remains a diagnostic grey area of acquired predominantly junctional naevi with architectural and cytological atypia. Only a small percentage of these will fulfil the criteria for dysplastic naevus if criteria are strictly applied. Therefore, there exists a group of otherwise ordinary acquired naevi with atypical junctional activity, mostly mild, whose biological significance is unclear. In older individuals, although junctional activity in otherwise benign naevi does occur, extra care should be exercised in order to prevent the diagnosis of melanoma in situ being overlooked.     

Nuclear pseudoinclusions in melanocytic naevi and melanomas.

Melanocytes in melanocytic naevi and melanomas can display great variation. The presence of nuclear pseudoinclusions (NPI) is said to be useful in the histological and cytological differential diagnosis of malignant melanoma. The prevalence and characteristics of NPI in a series of 493 naevi and 50 melanomas are described. NPI were found in 31% of adult naevi, 30% of congenital naevi from children, 42% of Spitz naevi, 20% of dysplastic naevi, and 56% of melanomas. The presence of NPI is not a reliable criterion for differentiating melanoma from benign melanocytic lesions, although it is useful in distinguishing melanocytic from non-melanocytic tumours.     

Histopathological characteristics of small diameter melanocytic naevi

BACKGROUND/AIMS: The clinical definition of an atypical naevus ("dysplastic naevus" or "naevus with architectural disorder and cytological atypia of melanocytes") stresses size larger than 5 mm in diameter as a major diagnostic criterion. Because malignant melanomas and their precursors may arise in smaller lesions, a histological study of melanocytic lesions smaller than 4 mm in diameter was conducted to evaluate their histological appearance. METHODS: Two hundred and sixty one naevi smaller than 4 mm in diameter were collected and characterised by histological examination into benign naevi without architectural disorder and naevi with architectural disorder and mild, moderate, and severe atypical melanocytes according to criteria used on larger lesions. RESULTS: Small melanocytic naevi covered the same complex histological spectrum from benign naevi to severely atypical naevi when compared with larger lesions. A high proportion of small naevi (72%) exhibited features diagnostic for naevi with architectural disorder and cytological atypia. CONCLUSION: There is a discrepancy between histological and clinically defined atypical naevi. The same generally accepted criteria for the histological diagnosis of atypical naevi should be used for small melanocytic naevi in addition to large ones. Thus, small naevi exhibiting atypical features on histological examination should be categorised as atypical naevi, regardless of their small diameter.     

Fluorescence in situ hybridization for the differential diagnosis between Spitz naevus and spitzoid melanoma

Requena C, Rubio L, Traves V, Sanmartín O, Nagore E, Llombart B, Serra C, Fernández‐Serra A, Botella R & Guillén C
(2012) Histopathology  61, 899–909 Fluorescence in situ hybridization for the differential diagnosis between Spitz naevus and spitzoid melanoma Aims: The differential diagnosis between Spitz naevus and spitzoid melanoma can be extremely difficult, or even impossible. In recent years, many attempts have been made to find specific histopathological or immunohistochemical markers, although none has proved successful. Because the prognosis and treatment of each are very different, it is important to distinguish between these entities. We evaluated the ability of the fluorescence in situ hybridization (FISH) assay–designed to detect the copy number of the RREB1 (6p25), MYB (6q23) and CCND1 (11q13) genes and of centromere 6 (Cep 6)–in order to distinguish between Spitz naevus and spitzoid melanoma. Methods and results: We evaluated 12 spitzoid melanomas and six Spitz naevi from our records. The diagnosis of both conditions was based on previously described histopathological criteria. We obtained valuable results for FISH in eight spitzoid melanomas and five Spitz naevi. Chromosomal aberrations were detected in seven of the eight spitzoid melanomas (FISH‐positive) and in none of the five Spitz naevi. The FISH‐negative spitzoid melanoma was the least typical in its group. Conclusions: FISH was able to distinguish between Spitz naevus and spitzoid melanoma, with a sensitivity of 87.5% and a specificity of 100%. Our findings suggest that FISH could prove a useful tool in the differential diagnosis between these entities.     

Problematic pigmented lesions: approach to diagnosis

A number of pigmented lesions are difficult to classify and raise the possibility of a melanoma diagnosis. Care should be exercised to exclude non-melanocytic lesions, and benign melanocytic entities, both of which can mimic melanoma histologically. In addition, the possibility of the lesion being a melanoma variant or epidermotropic metastasis should be considered. There will still be some cases that are difficult to resolve. These usually fall into one of three categories: atypical junctional melanocytic lesion versus early melanoma; naevus versus naevoid melanoma; and atypical Spitz, cellular blue, and deep penetrating naevi versus thick melanoma. These will pose problems even for experts. The atypical Spitz lesions are perhaps the most important category because they tend to be from younger individuals, the differential diagnosis is thick melanoma, and there is no single discriminating histological feature.     

Common and uncommon variants of melanocytic naevi

Numerous variants of melanocytic naevi have been described. Their main pathological significance lies in their distinction from melanoma, as well as being precursors and risk markers for melanoma. Various degrees of atypia such as cytological atypia, architectural disorder and pagetoid spread (pagetoid melanocytosis) may be present in naevi and need to be recognised as appropriate for the subtype. As well as the distinction from melanoma, naevi must be differentiated from atypical lesions, such as atypical Spitz tumours, which do not fulfil all the criteria of melanoma, may be benign or malignant and have been called 'melanocytic tumours of unknown malignant potential'. The diagnostic grey area also includes a group of benign atypical naevi which are difficult to subclassify into specific entities. In this paper naevi are divided into: firstly, the common acquired group with a brief discussion of junctional, compound and intradermal naevi, minor variants such as halo and balloon naevi and the major variants that may cause problems -- dysplastic naevi, naevi of special sites, recurrent naevi and Spitz naevi; secondly, congenital naevi; thirdly, blue naevi and related lesions (dermal melanocytoses); and finally, combined naevi. The emphasis is on diagnostic pathological features and the differential diagnosis with melanoma.     

The approach to the patient with a difficult melanocytic lesion

Although most histological diagnoses are made with relative ease and with great specificity and reproducibility, there is a subset of cases in which a specific and reproducible diagnosis is difficult or even impossible to render. In the melanocytic system, these cases can be divided into two broad categories. The first category, 'superficial atypical melanocytic proliferations of uncertain significance' (SAMPUS), includes predominantly junctional melanocytic proliferations, and melanocytic proliferations that are confined to the epidermis and papillary dermis, without evidence of tumorigenic proliferation or mitotic activity there. The prognosis for cure of these lesions is excellent if they are completely excised. Such lesions may include, for example, dysplastic naevi, Spitz naevi or pigmented spindle cell naevi with a few atypical melanocytes above the dermal-epidermal junction, or with greater than average cytological atypia, or with mitoses, where the differential diagnosis of melanoma in situ is difficult or impossible to rule out. The other category, 'melanocytic tumours of uncertain malignant potential' (MELTUMP), is comprised of melanocytic proliferations that form tumours in the dermis, and are therefore potentially capable of metastasis. Examples of such lesions may include atypical Spitz naevi, deep penetrating naevi, possible naevoid melanomas, or cellular blue naevi, where because of increased mitotic activity or cytologic atypia, a diagnosis of invasive or tumorigenic melanoma cannot be ruled out. In managing such lesions, we follow two main principles. The first is to manage each lesion with therapy designed to be adequate for management of the most significant consideration in the differential diagnosis. The second principle is to make clinicians and patients specifically aware of the diagnostic difficulty in their lesion, so that management can be undertaken on a true informed consent basis.     

Spitz naevi may express components of the plasminogen activation system

The plasminogen activation (PA) system is involved in the process of invasion and metastasis. Its major components are urokinase (uPA) and tissue-type plasminogen activator (tPA), plasminogen activation inhibitor type 1 and 2 (PAI-1 and PAI-2) and a receptor for urokinase (uPAR). In this study, the expression of plasminogen activation components in Spitz naevi was compared with that in common and dysplastic naevi on the one hand and primary cutaneous melanomas on the other. Spitz naevi had melanocytic positivity for uPA in 0% (0/36), tPA in 30% (6/20), PAI-1 in 10% (3/35), PAI-2 in 40% (8/21) and uPAR in 60% (13/21) of cases. This far exceeded the expression found in common (n = 25) and dysplastic (n = 15) naevi, which only showed melanocytic positivity for PAI-2 (20% and 15% respectively) and in one dysplastic naevus also for uPAR. This was much (for most components significantly) less than the proportion of primary melanomas with tumour cell positivity, which was 30% (11/38) for uPA, 80% (19/24) for tPA, 75% (28/38) for PAI-1, 80% (19/24) for PAI-2 and 80% (19/24) for uPAR. The main findings of this study are that Spitz naevi, firstly, may express plasminogen activator (tPA), inhibitors and the receptor of the PA system, but in a much smaller proportion than cutaneous melanomas; and secondly, do not express urokinase, whereas some of the melanomas do. uPA positivity may therefore be suggestive of melanoma. However, overlapping staining results imply that the PA system has limited value in the differential diagnosis between Spitz naevus and primary melanoma. As serine protease components are expressed, Spitz naevi may use this proteolytic machinery to accomplish matrix degradation, although in a more restricted, possibly transient manner than melanomas.     

MATRIX METALLOPROTEINASE-2 (72 kD TYPE IV COLLAGENASE) EXPRESSION OCCURS IN THE EARLY STAGE OF HUMAN MELANOCYTIC TUMOUR PROGRESSION AND MAY HAVE PROGNOSTIC VALUE

Matrix metalloproteinase-2 (MMP-2), a member of the matrix metalloproteinase family, participates in degradation of the pericellular and extracellular matrix during neoplastic growth and metastasis. Experimental data have substantiated its role in melanoma invasion, but there is no information at present concerning its expression in histological specimens from human melanocytic tumours. This study describes the occurrence and immunolocalization of MMP-2 in human melanocytic lesions, defining distinct steps in melanoma progression. Paraffin-embedded sections from 118 melanocytic lesions were immunostained using a specific antibody to 72 kD type IV collagenase. The material included 34 common naevocellular naevi, 14 dysplastic naevi, 21 in situ melanomas, 20 primary malignant melanomas, and 29 melanoma metastases. Intracytoplasmic MMP-2 immunoreactive protein was found in the ‘naevocytic nests’ of common naevi, in junctional naevus cells, and in melanoma cells. The surrounding normal skin stained negatively, except for occasional macrophages, sweat glands, and hair follicles. The number of MMP-2-positive cells increased with decreasing architectural organization and increasing atypia in the melanocytic lesions. The MMP-2 positivity in the primary and subcutaneous melanoma lesions correlated with later haematogenous metastasis. The data suggest that MMP-2 expression is an early event in melanocytic tumour progression, but is nevertheless prognostic for haematogenous metastasis in melanoma.     

Histopathological characteristics of malignant melanoma affecting mucous membranes: a unifying concept of histogenesis

AIMS: To investigate histopathological characteristics of melanocytic lesions affecting mucous membranes in various anatomical sites. Particular attention was paid to elucidation of morphological characteristics of early phases of mucosal melanoma in order to contribute to effective detection of this highly malignant neoplasm in the curable stages. METHODS: A total of 87 melanocytic lesions of mucous membranes were investigated histopathologically. There were 55 malignant melanomas including eight lesions of melanoma in situ, three in the radial growth phase (RGP) and 44 in the vertical growth phase (VGP), and 28 benign melanocytic lesions including four melanotic macules, 19 melanocytic naevi and five blue naevi. In addition, this series also included four equivocal lesions for which diagnoses were not definitely determined. With regard to malignant melanoma, histopathological patterns of in situ phase and RGP were intensely evaluated. RESULTS: Histopathological features of benign melanocytic lesions were essentially the same as those of the corresponding lesions of the skin. In the vast majority of mucosal melanomas, irrespective of anatomical sites, the main histopathological pattern seen in melanoma in situ and in RGP was the lentiginous pattern, which shows proliferation of atypical melanocytes in the lower layer of more or less acanthotic epithelium, though subtle variations of the pattern were detected. No association of melanocytic naevus was detected in any cases of melanoma. Based on these findings, we have proposed a unifying concept of de novo histogenesis of mucosal malignant melanoma. CONCLUSIONS: Our concept of histogenesis of mucosal melanoma assists in the identification of this highly malignant neoplasm in the early curable stages.     

Pitfalls in the evaluation of melanocytic lesions

The pathology of melanocytic tumours remains one of the most challenging and controversial fields in diagnostic histopathology, and it is one of the leading causes for litigation against pathologists. This is due largely to the wide morphological spectrum and often only subtle differentiating features with potential for both under- as well as overdiagnosis of melanoma. Particular pitfalls include the diagnosis of rare and unusual melanoma variants and melanoma resembling naevi. Furthermore, a subset of benign naevi may show concerning features associated more typically with a diagnosis of melanoma. These features include irregular junctional components with pagetoid spread, melanocyte atypia, lack of dermal maturation, dermal mitotic activity, intense melanin pigmentation, a desmoplastic stromal response and dermal regression. This paper focuses on a selected group of benign melanocytic lesions that are notoriously difficult to diagnose, and includes halo naevi, recurrent naevi, mitotically active naevi, desmoplastic naevi, clonal and deep penetrating naevi as well as cellular and plaque type blue naevi. The clinical and histological presentation is discussed with emphasis on clues to the correct diagnosis and distinguishing features from melanoma. Awareness of these entities and their distinguishing clinical and morphological characteristics is essential, as they present major diagnostic pitfalls.     

Atypical Spitz nevi/tumors: lack of consensus for diagnosis, discrimination from melanoma, and prediction of outcome.

The biological nature of Spitz nevi/tumors and their diagnostic distinction from, or relationship to, melanoma remain unresolved issues. In this report, a series of 30 melanocytic lesions removed from 28 patients, including atypical Spitz nevi/tumors and metastasizing Spitzoid tumors/melanomas, were evaluated by a panel of dermatopathologists to evaluate interobserver diagnostic concordance and to assess the prognostic power of histological criteria. For inclusion in the study, each lesion had to display some criteria for the Spitz nevus, and in addition one of the following was required: (1) definitive clinical outcome such as metastasis or death of disease, or (2) long-term follow-up if the patient remained disease free. Each lesion was reviewed independently and blinded as to the clinical data by 10 pathologists, who categorized them as (1) typical Spitz nevus/tumor, (2) atypical Spitz nevus/tumor, (3) melanoma, (4) tumor with unknown biological potential, or (5) other melanocytic lesion. There was limited discussion of criteria before the review. Evaluation of 17 Spitzoid lesions yielded no clear consensus as to diagnosis; in only one case did six or more pathologists agree on a single category, regardless of clinical outcome. Notably, however, some lesions that proved fatal were categorized by most observers as either Spitz nevi or atypical Spitz tumors. Conversely, seven or more pathologists scored 13 lesions as melanoma. These results illustrate (1) substantial diagnostic difficulties posed by many Spitz tumors, especially those with atypical features, even among experts, and (2) the lack of objective criteria for their distinction from melanoma and for gauging their malignant potential. Nevertheless, our observations do suggest that a biological relationship exists between the Spitz nevus/tumor and melanoma.     

Expression of interleukin-8 detected by in situ hybridization correlates with worse prognosis in primary cutaneous melanoma

Previous in vitro studies have demonstrated that endogenously produced human interleukin-8 (IL-8) can act as an important growth factor for human melanoma cells in vitro. The present study, has investigated whether IL-8 mRNA expression in primary melanomas may be of prognostic relevance with regard to melanoma progression and metastatic spread. In order to evaluate the clinical significance of IL-8 mRNA expression of melanoma cells in vivo, 59 melanocytic tissue specimens (37 primary melanomas and 22 melanocytic naevi) were studied using a semiquantitative in situ hybridization technique. Significant mRNA expression of IL-8 was found in 59 per cent (22/37) of melanomas. In 19 per cent (7/37) of the malignant melanomas, additional hybridization signals were noted within keratinocytes of the overlying epidermis. In contrast, paralesional normal-appearing epidermis and melanocytes in non-malignant lesions (melanocytic naevi) showed no IL-8 mRNA. Analysis of the relationship between IL-8 expression and clinico-histopathological features showed a significant association between IL-8 mRNA expression and the histological melanoma subtype (IL-8 mRNA: 14/19 in superficial spreading melanoma versus 4/12 in nodular melanoma, p< 0.05). Furthermore, IL-8 expression in primary tumours could be correlated with the patients' clinical course, with time to progression being significantly reduced in primary tumours expressing IL-8 in either the tumour cells or keratinocytes of the overlying epidermis. These results demonstrate for the first time that IL-8 expression, as detected by in situ hybridization in primary tumours, may serve as a significant prognostic factor for tumour progression in human malignant melanoma.     

Histological features of value in differentiating small congenital melanocytic naevi from acquired naevi

One hundred and twelve melanocytic naevi measuring less than 5 cm in largest diameter were excised from 85 patients referred because of minor concern about possible growth or malignant potential of these naevi. Sections from all 112 naevi were examined histologically, without access to the clinical history, and the following features noted: type of melanocytic proliferation in the basal layer area, contiguity of epidermal and dermal components, total width and depth of naevus, involvement of subcutaneous fat and of skin appendages, the presence or absence of an 'Indian file' type of naevus cell infiltration, cellular atypia and evidence of maturation of these cells. On the basis of these features, naevi were assigned to either a 'congenital type' or 'acquired type' classification, and their distribution in these two classifications then compared with the clinical history. It was found that in 107 of 112 lesions correct assignation had been made on histological grounds alone, 43 as congenital and 64 as acquired. A histological pattern not previously reported to be associated with congenital naevi was seen in 17 (34%) of the congenital lesions. This study suggests that it is possible to differentiate the majority of small congenital naevi from acquired naevi on histopathological grounds alone. This observation could be of considerable value in the examination of excised primary malignant melanomas for evidence of a pre-existing naevus of either type.