Abstract: 【Abstract】 Objective To summarize the clinical features, histopathological characteristics, and molecular findings of 5 cases of CRTC1::TRIM11 cutaneous tumors （CTCT）. Methods A case series study was conducted. Clinical data, histopathological findings, and molecular test results were collected from 5 patients with CTCT who underwent a pathological consultation at the Hospital for Skin Diseases, Chinese Academy of Medical Sciences, between January 2024 and June 2025, and were retrospectively analyzed. Results Of the 5 patients, there were 3 females and 2 males, with ages at onset ranging from 12 to 72 years and disease durations ranging from 2 months to 4 years. All lesions presented as asymptomatic nodules, with maximum diameters ranging from 11 to 18 mm （median, 11 mm）; ulceration was observed in only 1 case. Lesions were located on the upper extremities in 3 cases, on the lower extremity in 1 case, and on the face in 1 case. None of the 5 patients had lymph node involvement or systemic diseases. Histopathological examination revealed dermal or subcutaneous nodule-like or nest-like structures composed of epithelioid or spindle cells with intervening bands of fibrous tissue; tumor cells exhibited clear or slightly eosinophilic cytoplasm with mild to moderate nuclear atypia; focal necrosis was seen in only 1 case. The entity was prone to be misdiagnosed as malignant tumors such as clear cell sarcoma or melanoma due to its overlapping histopathological features. Immunohistochemically, the tumor cells were diffusely positive for SRY-related HMG box transcription factor 10 （SOX10） and tripartite motif-containing protein 11 （TRIM11） in all 5 cases, diffusely positive for S100 protein in 4 cases, and either negative or focally positive for human melanoma black 45 （HMB-45） and melanoma antigen A （Melan-A）. The mitotic count in hotspot areas ranged from 2 to 12 mitoses per 10 high-power fields, and the Ki-67 proliferation index was less than 10% in all cases. Fluorescence in situ hybridization （FISH） with break-apart probes revealed CRTC1 gene break-apart in all 5 cases, and RNA sequencing confirmed CRTC1::TRIM11 fusion in 2 cases. Additionally, EWSR1 gene break-apart was excluded by FISH in 3 cases, and melanoma-associated genetic alterations were also excluded in 3 cases. All the 5 patients underwent surgical excision, and no recurrence or metastasis was observed during a follow-up period of 6 to 9 months. Conclusions CTCT typically presents as asymptomatic cutaneous nodules with characteristic histopathological features, immunohistochemical study usually shows diffuse positivity for SOX10, TRIM11, and S100, and the detection of CRTC1 gene break-apart and molecular confirmation of CRTC1::TRIM11 fusion could confirm the diagnosis. CTCT often exhibit a relatively indolent biological behavior; however, close follow-up after surgical excision is recommended.

Key words: Melanocytic tumor, Skin manifestations, Pathologic processes, CRTC1::TRIM11 fusion, molecular pathology