Utilization of polarized microscopy to differentiate deep penetrating nevus from equine type melanomas

Equine type melanoma can mimic deep penetrating nevus (DPN), making histologic diagnosis challenging. We sought to investigate if the pattern of collagen polarization could be helpful in this setting. A total of 52 specimens were reviewed with polarized microscopy to determine whether refractile collagen was present within melanocytic nests vs. surrounding but not within the nests. Seven of eight (87.5%) equine type melanomas demonstrated refractile collagen within melanocytic nests in part or all of the lesion. In contrast, DPN showed no refractile collagen within the melanocytic nests. Instead, 12 (100%) DPNs and 14 of 16 (87.5%) common combined nevi (DPN plus banal nevus) demonstrated refractile collagen only surrounding melanocytic nests. The entrapment of refractile collagen, as seen with polarized microscopy, within melanocytic nests can support a diagnosis of equine type melanoma.