CIC-DUX4 Sarcoma of the Skin: A Rare Case Report and Literature Review.

CIC::DUX4 fusion sarcoma represents a rare and aggressive subtype of undifferentiated small round blue cell tumors. We report on a 23-year-old African male who developed a rapidly enlarging inferolateral left buttock nodule with ulceration. After debulking excision of the lesion, histologic sections demonstrated sheets and lobules of atypical round blue cells with significant cytologic atypia. Prominent foci of atypical mitotic figures and tissue necrosis were present. Tumoral cells stained strongly and diffusely using MDM2, vimentin, WT1 and CD99 immunohistochemical (IHC) markers. Molecular testing was performed and highlighted CIC::DUX4 gene fusion positivity, making the diagnosis of a CIC::DUX4 sarcoma (CDS). Post-surgical excision, the patient showed no disease on imaging and underwent five cycles of adjuvant chemotherapy with no recurrence observed at the eight-month follow-up. With fewer than 200 cases reported in the literature and somewhat nonspecific clinicopathologic characteristics, CIC::DUX4 sarcoma presents a diagnostic challenge. This case underlines the importance of molecular diagnostics in undifferentiated sarcomas and presents a rare primary cutaneous manifestation of CIC::DUX4 fusion sarcoma. Additionally, we provide a review of the literature to aid in recognition, diagnosis, and treatment of this rare entity.