Melanocytic Myxoid Spindle Cell Tumor With ALK Rearrangement (MMySTAR): Report of 4 Cases of a Nevus Variant With Potential Diagnostic Challenge

: Melanocytic tumors rarely display extensive dermal myxoid deposits except in the myxoid variant of melanoma. We describe in 4 patients the unusual association of morphologic and genetic features. All cases occurred in males and were located on the limbs or proximal girdle area. Age at diagnosis ranged from 8 to 47 years. Size ranged from 6 to 11 mm. Microscopic analysis showed compound, but mainly dermal melanocytic nevi, all presenting a deep dermal expansion with fascicules of amelanotic spindled cells floating in a myxoid background. Cytologic atypia and mitotic activity were low. The superficial portion was either of spitzoid or nevoid cytology with a limited junctional component. In the initial case, the dermal myxoid component was predominant with rare, barely visible, superficial melanocytic nests. This peculiar morphology was responsible for a delayed diagnostic, which required an extensive panel of antibodies ruling out most, potentially myxoid, soft tissue tumors. We later observed the presence of similar, but more limited, dermal morphologic features in 3 other cases. Immunohistochemistry in the deep myxoid areas was melanA, ALK, SOX10, and MiTF. Molecular studies confirmed the ALK rearrangement by an ALK break-apart fluorescence in situ hybridization technique and by RNA sequencing. The latter identified 4 different 5'-fusion partners. Two gene fusions were undescribed: FBXO28(e2)-ALK(e19) and NPAS2(e2)-ALK(e19), and 2 previously described: TPM3(e7)-ALK(e20) and PPFIBP1(e9)-ALK(e19). No relapse or metastatic evolution was seen during follow-up (3 to 24 mo). We denominated this potentially challenging new variant of compound nevus linked to a kinase fusion: Melanocytic Myxoid Spindle Cell Tumor with ALK Rearrangement.