Molecular pathology as a diagnostic aid in difficult-to-classify melanocytic tumours with spitzoid morphology

Abstract

Accurate classification of melanocytic proliferations has important implications for prognostic prediction, treatment and follow-up. Although most melanocytic proliferations can be accurately classified using clinical and pathological criteria, classification (specifically distinction between nevus and melanoma) can be challenging in a subset of cases, including those with spitzoid morphology. Genetic studies have shown that mutation profiles differ between primary melanoma subtypes and Spitz nevi. These differences may aid in distinguishing benign from malignant in some melanocytic tumours. Here, we present a selection of melanocytic proliferations with equivocal histopathological criteria, wherein genetic analysis was requested to help guide classification. In two of four cases, the genetic results offered valuable insights, allowing a definitive diagnosis, indicating the diagnostic value of mutation profiling in a real-world routine clinical setting. Although histopathological assessment remains decisive in melanocytic proliferation classification, we recommend including genetic profiling in cases of borderline or atypical lesion to support accurate classification.

Keywords: Atypical spitzoid lesion; Borderline tumour; Malignant melanoma; Mutation profiling; Spitz nevus; Spitzoid melanoma.

Figure 1

Histological findings showed an exophytic dermal-based tumor composed of spindle and epithelioid cells (A, bar represents 2 mm). The tumor cells are pleomorphic with enlarged nuclei; there are irregularly distributed junctional components with isolated breakdown of the overlying epidermis (B, bar represents 400 μm). Ki-67 staining shows increased proliferative activity in the junctional and dermal components present (C, bar represents 300 μm). Melan-A staining highlights the asymmetrical silhouette of the tumor and the junctional melanocytic component (D, bar represents 300 μm). The BRAF V600E (c.1799_1800 CA>TT) mutation and the TERT promoter mutation (Chr. 5: 1,295,250 C>T) determined by targeted next generation sequencing are shown at the top with a representative wild-type sequence at the bottom. The mutation site is highlighted by the black arrow (E). The tumor was diagnosed as melanoma.

Figure 2

Clinically, multiple brownish pigmented macules are visible (A, arrow indicates biopsy site). A dermally located, predominantly nested melanocytic cell population (B, bar represents 2 mm). The epidermis shows hyperpigmentation of the basal cell layer with no melanocytic cell proliferation (C, bar represents 400 μm). The dermal melanocytic cells extend into the subcorium and show no maturation to the deep. Cytomorphologically, predominantly epithelioid cells with pale gray cytoplasm, nuclear size variation and hyperchromatic nuclei (D, bar represents 80 μm).

Figure 3

The primary tumor from the lower leg showed a predominantly dermal asymmetric melanocytic lesion (A, bar represents 2 mm) with junctional nests and single intraepidermal melanocytic cells (B, bar represents 400 μm). No epidermal ulceration was present. The dermal tumor cells did not mature with depth; the small nests grew between dermal collagen fibers. Atypical melanocytic cells distributed in the corium as small nests and cords, partly epitheloid cells with variably sized hyperchromatic nuclei with prominent nucleoli were present (C, bar represents 80 μm). Melan A staining showed positivity of junctional and dermal portions of the leg tumor (D, bar represents 500 μm) and in in the lymph node metastasis (E, bar represents mm). NRAS Q61R (c.182 T>C) and TERT Chr. 5: 1295228 C>T promoter mutations determined by targeted next generation sequencing are shown at the top with a representative wild-type sequence at the bottom. The mutation site is highlighted by the black arrow. The tumor was diagnosed as a metastasized cutaneous melanoma (F).

Figure 4

Cellular melanocytic cell population abutting the epidermis and extending into the deep corium is seen (A, bar represents 2 mm). In the overlying epidermis, there is hyperpigmentation of the basal cell layer with no significant melanocytic cell proliferation or ulceration (B, bar represents 200 μm). The dermal melanocytic cells are predominantly spindle cells and grow in strands and larger nests (B). Cytomorphologically, there are hyperchromatic nuclei with prominent nucleoli, and occasional deep mitoses (C, bar represents 200 μm).