Unusual or atypical melanocytic nevi can be confused with malignant melanoma. The authors present two cases of an unusual variant of blue nevus that were misdiagnosed initially as malignancy. Both lesions were asymptomatic and characterized clinically by childhood onset, with slow enlargement during adolescence and subsequent nodule formation. One lesion, which measured 24 cm in greatest dimension, was located on the anterior chest wall of a 53-year-old woman. The other lesion, which measured approximately 15 cm in greatest dimension, was located on the lateral abdominal wall of a 20-year-old man. Both lesions were characterized by a multifocal dermal and subcutaneous proliferation of fusiform and dendritic pigmented melanocytes. The histologic appearance of individual foci ranged from dermal melanocytosis to common blue nevus and cellular blue nevus. The cellular foci were located in the subcutis and involved, in one patient, the stroma of the breast. The cells were immunoreactive for S-100 protein, gp100 (HMB-45), and Melan-A (A103). Ultrastructural analysis revealed melanocytes typical of blue nevus. The woman underwent complete excision of the lesion, and the man underwent only partial excision of the lesion. On clinical follow-up of 32 and 19 months, respectively, both patients are alive and well with no evidence of recurrence or progression. Because the lesions presented clinically as large plaques and were diagnosed histologically as blue nevi with subcutaneous foci of cellular blue nevus, we term this rare variant of blue nevus large plaque-type blue nevus with subcutaneous cellular nodules. Recognition of this lesion enhances our knowledge of the morphologic spectrum of melanocytic tumors and helps to avoid confusion with malignant melanoma.
The term blue nevus is used for a wide spectrum of melanocytic proliferations. 4 Typical blue nevi (BN) are usually confined to the dermis, and contain pigmented dendritic and fusiform melanocytes and melanophages in a usually sclerotic stroma. The proportion of the cellular components, the degree of pigmentation, the extent of stromal sclerosis, and the overall degree of cellularity may vary greatly from lesion to lesion. Since the first description of BN by Tieche in 1906, 50 several histologic types of BN have been described, including the common dermal blue nevus, 17,18,50 cellular blue nevus (CBN), 1,2,17,32,45 atypical cellular blue nevus, 3,21,51 desmoplastic cellular blue nevus, 37 combined blue nevus, 20 and compound blue nevus. 27 Unusual clinical variants have also been reported, such as eruptive BN, 23 congenital common blue nevus, 44 target blue nevus, 7 and plaque-type blue nevus. 24,42,43,52 One might also include Mongolian spot, 13,17,19,31 nevus of Ota, 28,38 nevus of Ito, 26 dermal melanocytic hamartoma, 8 and similar lesions in the spectrum of dermal melanocytoses. 12,25,40,47,53,54
In the vast majority of BN, their identification is straightforward clinically and histologically, and they rarely enter the differential diagnosis of malignant melanoma. However, unusual variants exist, in which distinction from malignant melanoma can be difficult. We present the clinical and pathologic features of two cases of such a rare variant.