[Atypical spindle cell/pleomorphic lipomatous tumor: a clinicopathological and molecular analysis of nine cases].

Objective: To investigate the clinicopathological and molecular features of atypical spindle cell/pleomorphic lipomatous tumor (ASPLT). Methods: A retrospective analysis was conducted on nine cases of ASPLT diagnosed at the Fourth Affiliated Hospital of Soochow University and the First Hospital of Jilin University from February 2023 to September 2025. The clinical, histopathologic, immunohistochemical, and molecular genetic features were analyzed, supplemented by a comprehensive literature review. Results: The cohort comprised seven males and two females with an age of 59.0 (54.0, 63.0) years. Tumor arose in the neck, buttock, abdominal wall, face, and axilla. Histologically, the tumors exhibited a variable mixture of atypical spindle cells, adipocytes, pleomorphic/multinucleated cells, and lipoblasts embedded within a collagenous-to-myxoid stroma. Distinctive ropy collagen bundles were identified in seven cases. Mitotic activity was negligible, and tumor necrosis or dedifferentiation was consistently absent. Immunohistochemically, all cases showed diffuse CD34 expression and loss of nuclear RB1 expression. The Ki-67 proliferative index was 1%-2%, and p53 immunostaining demonstrated a wild-type expression pattern in all eight tested cases. Fluorescence in situ hybridization confirmed RB1 gene deletion in all five tested cases. Conclusions: ASPLT is a rare adipocytic neoplasm characterized by a broad morphologic spectrum, distinctive genetic alterations, and an indolent clinical course. Due to substantial morphological overlap with other lipomatous tumors, diagnosis can be challenging. Accurate identification relies on meticulous microscopic evaluation combined with ancillary testing, specifically, CD34 positivity, immunohistochemical loss of RB1 expression, and/or molecular detection of RB1 gene deletion, to ensure correct classification and prevent overtreatment.