Primary Cutaneous Rhabdoid Squamous Cell Carcinoma: A Case Report and Review of Molecular Features.

We present a rare case of rhabdoid squamous cell carcinoma (RSCC) on the scalp of a non-immunosuppressed male patient in his late 60s. This aggressive variant of squamous cell carcinoma (SCC) is characterized by tumor cells with eccentrically located nuclei and abundant eosinophilic cytoplasm, as observed on histopathological examination. While rhabdoid morphology has been reported in various anatomical sites, its occurrence in primary cutaneous tumors is exceptionally uncommon, with fewer than 10 cases documented to date. Immunohistochemically, the neoplasm demonstrated positivity for p40, cytokeratin 5/6 (CK5/6), and vimentin, while retaining integrase interactor 1 (INI1) expression and showing negativity for muscle differentiation markers. Loss of p53 staining suggested its role in the progression to a more aggressive phenotype. The DecisionDx-SCC assay (Castle Biosciences, Friendswood, TX, US) classified the tumor as Class 2B, indicating the highest biological risk for metastasis and poor outcomes. Despite the limited number of reported cutaneous RSCC cases, this variant appears to exhibit high aggressiveness, mirroring other visceral tumors with rhabdoid features. We discuss potential molecular events underlying this transformation. This case highlights the importance of recognizing this rare variant and its implications for patient management and prognosis.