Anisometric cell lipoma/dysplastic lipoma: a distinctive type of subcutaneous adipocytic tumour of the RB1-gene-deleted neoplasm family. A clinicopathological study of 62 cases.

Anisometric cell lipoma (ACL), also called dysplastic lipoma (DL), was first described in 2015. Since then, less than 100 cases have been reported. Its recognition is important because of its shared histopathological features with atypical lipomatous tumour, a locally aggressive neoplasm. Whether this is a distinctive type of fatty tumour or a morphological variant of a known entity, such as the spindle cell/pleomorphic lipoma (SCPL) or atypical spindle cell/pleomorphic lipomatous tumour, remains controversial. We analysed the clinicopathological, immunohistochemical and molecular features of 72 ACL/DL of 62 patients (age 32-78 years; median 53). Fifty patients were male and 12 female (M/F 4.2:1). All lesions arose in the subcutaneous tissue, most frequently on the neck, back or shoulder (83%). Histopathologically, the tumours consisted of adipocytes with prominent variation in size and contained frequent single fat-cell necrosis surrounded by reactive histiocytes, fat cells with lochkern change, sparse lipoblasts and a small number of fat cells with atypical, hyperchromatic nuclei and immunoexpression of MDM2 (47/58 cases) and p53 (12/12). Fatty cells showed negativity for CDK4 (0/12) and loss of RB1 immunostaining (complete 39/51; partial 8/51). Fluorescence in situ hybridisation analysis demonstrated lack of MDM2 gene amplification (0/26) and loss of RB1 locus (8/9 evaluable tumours). Local recurrences occurred in five cases (8%), with the absence of distant metastases. In conclusion, ACL/DL appears as a distinctive type of subcutaneous benign adipocytic tumour of the RB1-gene-deleted neoplasm family, closely related to SCPL.