Approaches to classification and management of histologically ambiguous/ borderline / primary cutaneous melanocytic tumours

Abstract

Because naevi and melanomas are tumours which exhibit considerable morphological heterogeneity, many histological features are common to both groups of tumours. As a consequence, the pathological diagnosis of some melanocytic tumours can be difficult. For some atypical tumours that do not display clear-cut features of malignancy, it may be difficult or impossible to exclude a diagnosis of melanoma and such tumours are sometimes referred to as ‘histologically ambiguous' or ‘borderline' tumours or as ‘melanocytic tumours of uncertain malignant potential' (MELTUMP). While not diagnoses <i>per se</i>, these terms are useful for conveying the diagnostic uncertainty to both the treating clinician and the patient. Examples of such tumours include some cases that show some resemblance to Spitz naevi, blue naevi, deep penetrating naevi and possible naevoid melanomas. We recommend that a second opinion be sought from one or more experienced colleagues when there is uncertainty about whether a primary melanocytic tumour is benign or malignant. If diagnostic uncertainty persists, the evidence for or against the various differential diagnostic considerations should be presented in the pathology report and a ‘most likely' or ‘favoured' diagnosis given. Recent studies have suggested that molecular testing of the primary tumour using techniques such as comparative genomic hybridisation or fluorescence <i>in situ</i> hybridisation may assist in establishing a diagnosis of melanoma if multiple chromosomal aberrations are identified. However, these tests are not widely available at present and require further independent validation. Complete excision of the lesion is probably mandatory, but plans for further management should be formulated on case-by-case basis. The safest course of action will usually be to manage the tumour as if it were a melanoma (taking into account the tumour's thickness and other prognostic variables); however, this may not always be appropriate, particularly if it is located on the face. In some cases, the clinician may consider conveying the diagnostic uncertainty to the patient and presenting them with management choices so that they can decide whether they wish to be managed aggressively (as for a melanoma) or conservatively. Whilst a sentinel lymph node biopsy may be considered on the basis of the primary tumour characteristics, the clinical significance of lymph node involvement for these tumours is not yet clear, but it probably does not have the same prognostic implications as nodal involvement from an unequivocal ‘conventional' melanoma.