 Melanocytic lesion

A melanocytic lesion is a localized abnormal growth or proliferation involving melanocytes, the pigment-producing biological cells of the skin. The description identifies the participating cell lineage; it does not by itself mean cancer. A common melanocytic nevus is a noncancerous clustered growth of melanocytes, whereas melanoma is malignant. The National Cancer Institute distinguishes these categories explicitly.^[ref]

A visible pigmented spot and a histological section provide different evidence. Pigment is an optical observation; a melanocytic lesion is a statement about the biological process. Microscopic examination asks how melanocytes are arranged in the sampled tissue and how that pattern relates to surrounding structures. Clinicopathological correlation connects those observations to the particular lesion that was sampled.

The sampling distinction can be made quantitative without inventing a diagnostic rule. In a constructed lesion containing $N$ relevant cells, examining a section that contains $n$ cells does not automatically inspect all $N$. If a feature is absent from the section, it may be absent from the lesion or located outside the sampled region. Increasing the number of levels or changing the sampled location changes the evidence available, rather than merely changing the name assigned to it.

Classification is also an observer task. A primary study of skin biopsy interpretations included a spectrum from nevi to invasive melanoma and distinguished agreement with a consensus reference from agreement on repeat interpretation.^[ref] This motivates precise statements of the category, sampled material and diagnostic reproducibility. A lineage label alone does not specify benignity, invasiveness or a complete clinical diagnosis.