Stealth Pigmented Squamous Cell Carcinoma in Situ: An Undescribed Variant With Deceptive Clinical and Histological Features.

Cutaneous squamous cell carcinoma in situ is characterized by well-established architectural and cytological abnormalities that are readily appreciated at both low and high magnification. We have encountered a subset of lesions with a deceptively bland appearance at scanning magnification that demonstrated unequivocal malignant features on closer examination, which we term "stealth pigmented squamous cell carcinoma in situ." In this study, we describe 51 cases of this entity and compare its clinical, histological, and immunohistochemical features with those of its closest morphologic mimickers. These stealth lesions are most often found on the lower extremities and pigmented, thus clinically suspected to be melanocytic lesions or pigmented keratoses, not atypical squamous proliferations. At low magnification, they typically seem flat and orthokeratotic with basal hyperpigmentation above minimal solar elastosis. At higher magnification, keratinocyte atypia, dysmaturation, and numerous mitotic figures are evident throughout the epidermis. This stealth variant is as proliferative as conventional squamous cell carcinoma in situ by Ki67 but significantly less likely to overexpress p16. We believe stealth pigmented squamous cell carcinoma in situ represents a distinct variant of squamous cell carcinoma in situ. Although prognostic significance remains to be studied, recognition of this pigmented entity is important to avoid misdiagnosis and undertreatment as a benign epidermal lesion.