Melanocytic tumours of uncertain malignant potential
Some cutaneous melanocytic lesions cause particular diagnostic challenges for pathologists as they have some features that do not typically fit with the diagnosis of a benign naevus, whilst also not fully reaching diagnostic criteria for melanoma. This is especially the case in children and adolescents, because the lesions may have features that would usually be indicative of melanoma in adults but can be benign in children. Furthermore, as children transition through puberty to become adults, so the significance of these features changes. Such melanocytic lesions that are difficult to classify as either benign or malignant may be categorised using various terms, including ‘borderline melanoma’, ‘minimal deviation melanoma’, ‘dermal based borderline melanocytic tumour’, ‘atypical cellular blue naevus’, ‘pigmented epithelioid melanocytoma’, ‘borderline deep penetrating naevus’ or ‘atypical Spitz naevus’. However, a commonly accepted term that encompasses all these is ‘melanocytic tumour of uncertain malignant potential’ (MelTUMP). More recently, on the basis of accumulating clinical, histopathological, molecular and follow-up data, and as originally proposed by Zembowicz and Scolyer, the World Health Organization Classification of Skin Tumours proposed the adoption of the term “melanocytoma” to encompass this group of intermediate/borderline tumours.
The diagnostic difficulties in correctly classifying MelTUMPs as either benign or malignant were investigated by a panel of 10 internationally recognised expert dermatopathologists, in a blinded study of 57 such cases with at least five years follow-up of all patients unless death occurred first. The majority of the pathologists classified only 47% of cases with a favourable outcome as benign and 73% cases with an unfavourable outcome as malignant, whilst 16% of lesions were either unclassified by the majority or equally classified as benign and malignant. Of the 14 histological features assessed, only three were found to have prognostic significance: mitoses, an inflammatory reaction and mitoses near base of lesion, although only the latter was significant after correction for multiple hypothesis testing. The study also found that MelTUMPs with a favourable outcome had a significantly younger median age (23 years) than those with an unfavourable outcome (38.5 years), however, there were patients as old as 70 years with favourable outcomes and as young as 3 years with unfavourable outcomes, therefore patient age was not prognostic in the highly selected cases used in this study.
Since MelTUMPs are not a clearly defined entity, and the interobserver reproducibility of their diagnosis and classification is poor, incidence is likely to vary amongst pathologists, between centres and over time. Furthermore, pathologists may prefer to err on the side of caution and diagnose such lesions as melanomas, leading to an apparently decreased incidence of MelTUMPs and a corresponding improvement in outcomes. Where a melanocytic tumour is assessed by a pathologist and there is uncertainty as to the diagnosis, it should be referred to a pathologist who is a recognised expert in the field of melanocytic lesions. In Australia, this is a Medicare rebatable consultation if the treating practitioner and the approved pathology practitioner who provided the original opinion on the patient specimen agree that a second opinion is reasonably necessary for diagnostic purposes. If the expert pathologist also has doubts as to whether the tumour is benign or malignant, it should be considered to be a MelTUMP.
Once a diagnosis of MelTUMP has been decided, appropriate management decisions must be made; including excision margins and whether to perform a sentinel lymph node biopsy.
This section addresses the following clinical questions:
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- Barnhill RL, Piepkorn M and Busam KJ. Pathology of Melanocytic Nevi and Malignant Melanoma. New York: Springer; 2004 Available from: https://www.springer.com/gp/book/9780387216195.
- Zembowicz A, Scolyer RA. Nevus/Melanocytoma/Melanoma: an emerging paradigm for classification of melanocytic neoplasms? Arch Pathol Lab Med 2011 Mar;135(3):300-6 Available from: http://www.ncbi.nlm.nih.gov/pubmed/21366452.
- Elder DE, Massi D, Scolyer RA, Willemze R. WHO Classification of Skin Tumours. 4th edn. Lyon, France: International Agency for Research on Cancer; 2018.