Atypia and differential diagnosis in cellular blue nevi: clinicopathological study of 21 cases

Cellular blue nevus differs from the classic blue nevus with characteristics such as large size, cellularity, intense pigmentation, and growing pattern with subcutaneous infiltration. It is a dermal melanocytic tumor that can be confused with melanoma due to the atypia it may contain.

Hematoxylin-eosin and MIB-1 stained slides of 21 cases diagnosed between 2000-2014 were re-evaluated. In order to attract attention to this rare lesion, 21 cases are presented with the clinical and above-mentioned histopathological findings.

Thirteen (61.9%) cases were females and eight (38.1%) were male. The mean age was 25.4 (2-73). The most frequent localization was the sacral and gluteal region (11 cases). The mean diameter was 14.4 mm (4-60 mm). From the parameters defined to assess the atypia, ulceration was identified in four cases. Prominent cellularity and subcutaneous infiltration were seen in three and 16 cases, respectively. Mitosis was seen in six tumors. Immunohistochemically, MIB-1 was present in two cases as 3% and 2% respectively, while in others it was 1% or less. Although there is no precise definition for the "atypical cellular blue nevus", five patients were assessed as atypical cellular blue nevus (a case with infiltrative development of six cm tumor diameter, two cases with two mitosis and a MIB-1 index 3% and 2%, a case with one mitosis and confluent development and a case with one mitosis in addition to focal necrosis areas). No lymph node and/or distant metastasis was observed during follow-up.

We think it is more important to rule out the possibility of conventional melanoma in cellular blue nevus with exaggerated morphological findings alongside low proliferative activity rather than to determine the atypia.