Approach to Diagnosis

J Clin Pathol 2000;53:409–418 409 Leaders Problematic pigmented lesions: approach to diagnosis S L Edwards, K Blessing Abstract General guidelines A number of pigmented lesions are diY- Before looking at a pigmented lesion it is vital cult to classify and raise the possibility of a to know certain clinical factors, particularly the melanoma diagnosis. Care should be exer- age of the patient, the anatomical location of cised to exclude non-melanocytic lesions, the lesion, and if there has been previous and benign melanocytic entities, both of surgery or trauma. The age is important which can mimic melanoma histologi- because melanomas are more common with cally. In addition, the possibility of the increasing age and are extremely rare (although lesion being a melanoma variant or epi- they do occur) in children.2 Spitz naevi occur dermotropic metastasis should be consid- predominantly in children and young adults, ered. There will still be some cases that and are increasingly uncommon in the older 3 are diYcult to resolve. These usually fall age groups. Some features related to an into one of three categories: atypical junc- actively growing lesion are more acceptable in tional melanocytic lesion versus early children and adolescents than they would be in melanoma; naevus versus naevoid an adult. For example, a lentiginous junctional melanoma; and atypical Spitz, cellular growth pattern in an adult would be a pointer blue, and deep penetrating naevi versus to in situ melanoma, whereas it would not infer thick melanoma. These will pose problems the same in a child. even for experts. The atypical Spitz lesions It is also essential to know the site of the are perhaps the most important category biopsy. This is important for two reasons. First, because they tend to be from younger diVerent anatomical locations have diVerent individuals, the diVerential diagnosis is qualities of skin, which cause both benign and malignant melanocytic lesions to grow in char- thick melanoma, and there is no single acteristic patterns. This is particularly perti- discriminating histological feature. nent to the in situ component of melanomas, (J Clin Pathol 2000;53:409–418) which aids their recognition. For example, len- tigo maligna occurs on sun exposed skin of Keywords: diYcult diagnosis; pigmented lesions; melanoma older individuals and presents as a lentiginous proliferation of atypical melanocytes in an atrophic epidermis. Benign melanocytic lesions at these sites and in this age group are Pigmented skin lesions comprise a large predominantly intradermal naevi.4 Acral len- component of most histopathologists’ work- tiginous melanoma in situ occurs on acral skin load, and although there is a wide spectrum of and histologically grows as a subtle lentiginous histological appearances, most can be reported proliferation on the background of an epider- as simple benign naevi. A small number will be mis with psoriasiform hyperplasia, whereas typical malignant melanomas. However, some acral naevi are typically nested and can show pigmented skin lesions will be more diYcult to florid junctional activity (figs 1 and 2). Second, classify when there are unusual features present such as asymmetry of the silhouette, excessive junctional activity, deep extension into the subcutis, cellular pleomorphism, or mitotic activity. These latter cases, although small in Department of number, are very important clinically because Pathology, Aberdeen University, of the time and attention they require and the Foresterhill, Aberdeen potential for litigation, as a result of both under AB25 2ZD, UK and over diagnosis of melanoma.1 Therefore, it S L Edwards is important to approach all melanocytic and K Blessing pigmented lesions in a ritualistic and consistent Correspondence to: manner to avoid potential serious diagnostic Dr Blessing errors. Our review attempts to present a logical email: [email protected] approach to this problem and highlights the Accepted for publication problematic diagnostic areas, particularly Spitz Figure 1 Acral naevus. At low power there is pronounced 20 October 1999 naevi. junctional activity. No psoriasiform hyperplasia is seen. 410 Edwards, Blessing tumours being S100 protein positive, and most but not all reports of cellular neurothekeoma being S100 negative.7 ATYPICAL FIBROXANTHOMA This tumour occurs most commonly on sun exposed skin of elderly individuals, and is composed of atypical histiocyte like cells, giant cells, and spindle cells. Mitotic figures includ- ing atypical forms are usually readily identifi- able. It is a diagnosis of exclusion with S100 protein, Melan-A, and pancytokeratin staining being negative.89 More recently, a monomor- Figure 2 Acral lentiginous melanoma in situ. At low phic spindle cell variant and a pigmented vari- power there is psoriasiform hyperplasia of the epidermis ant have been described, which have the clini- with a subtle lentiginous proliferation of melanocytes along the basal layer. cal and immunohistochemical pattern identical to typical atypical fibroxanthoma. The former benign melanocytic lesions from certain sites in variant lacks the prominent cellular pleomor- the body can, as a normal feature, exhibit phism, being composed predominantly of architectural and possibly cytological atypia spindle cells,10 whereas the cellular component (see below). Therefore, care should be exer- of the pigmented variant contains erythrocytes cised when examining naevi from these sites to and/or haemosiderin in the cytoplasm.11 prevent over diagnosing atypical naevi or melanoma. BEDNAR TUMOUR Table 1 Non-melanocytic It is also important to know whether there Pigmented dermatofibrosarcoma protuberans entities that can mimic melanocytic lesions has been a previous shave biopsy or incomplete although rare could mimic spindle cell excision, because regrowth of melanocytes in melanoma.12 Atypical fibroxanthoma scar tissue can mimic melanoma, and create a Bednar tumour 5 Clear cell dermatofibroma so called pseudomelanoma phenomenon. CLEAR CELL DERMATOFIBROMA Epithelioid histiocytoma For the remaining lesions that are not readily This is a rare variant of dermatofibroma, which Extramammary Paget’s classifiable, all the tissue should be examined, tends to occur on the lower limbs of middle disease Granular cell tumour with levels cut where appropriate. Before aged adults. Microscopically, it is a well circum- Mastocytoma agonising and spending time on these cases it is scribed dermal to subcutaneous tumour, which Metastatic lobular carcinoma important first to exclude lesions that can stains faintly, because over 90% of the constitu- Mycosis fungoides Neurothekeoma mimic melanoma and to consider melanocytic ent cells have a clear cytoplasm and stain with lesions that are recognised entities, and which PAS (periodic acid SchiV). They may mimic could account for the atypical features present balloon cell naevus and/or melanoma, but they Table 2 Benign 13 melanocytic entities that within a particular melanocytic lesion. These are S100 protein and cytokeratin negative. can mimic thick melanoma other lesions to be considered and excluded can be categorised as follows: EPITHELIOID HISTIOCYTOMA Ancient naevus Balloon cell naevus + Non-melanocytic entities that might be This is an unusual and still poorly recognised Cellular blue naevus clinically pigmented and histologically can variant of dermatofibroma that was first Combined naevus mimic melanocytic lesions (table 1). Compound Spitz naevus described by Edward Wilson-Jones et al in Deep penetrating naevus + Benign melanocytic entities that can mimic 1989, and typically presents as a nodule on the Desmoplastic naevus malignant melanoma: extremities.14 It diVers from conventional Naevus with dermal nodules (1) Those that mimic thick melanoma dermatofibroma in its relatively sharp circum- (table 2). scription, prominent vascularity, centering on Table 3 Benign (2) Those that mimic thin melanoma the papillary dermis, predominance of epithe- melanocytic entities that (table 3). can mimic thin melanoma lioid cells, and relative lack of other secondary + Malignant melanoma variants that can features such as haemosiderin and giant Acral naevus mimic benign melanocytic lesions (table 4). cells14 15 (figs 3 and 4). A deep seated cellular Genital naevus + Epidermotropic metastatic melanoma that variant has been recognised.16–19 This entity can Halo naevus Junctional/pagetoid Spitz can mimic primary melanoma and benign mimic other non-fibrohistiocytic tumours such naevus naevi. Naevus of infancy Naevus from special anatomical sites Naevus after UV irradiation Non-melanocytic lesions that can mimic Pigmented spindle cell naevus melanocytic lesions Pseudomelanoma These lesions can mostly be excluded if a thor- ough examination is undertaken and appropri- Table 4 Malignant ate special stains are performed. Melan-A and melanoma variants that S100 stains are the most useful to detect can mimic benign lesions melanocytic lineage, with S100 being more Balloon cell melanoma sensitive but less specific than Melan-A. Both Desmoplastic melanoma stain benign and malignant melanoctyic cells Regressed melanoma strongly and diVusely, with the exception of Small (non-Merkel) cell melanoma Melan-A in desmoplastic melanomas, where Spitzoid melanoma there is little or no staining.67All of the lesions Varicose melanoma in this group are Melan-A negative.7 Most are Figure 3 Epithelioid cell histiocytoma. A cellular dermal Verrucous naevoid melanoma nodule with a Grenz zone. The lesion extends into deeper S100 protein negative,

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