ABSTRACT
Adipocytic tumours are the most common soft tissue tumours and are frequently encountered in the routine practice. Vast majority of adipocytic tumours are benign lipomatous tumours, but there are other rare heterogenous adipocytic neoplasms which can create diagnostic difficulties. It can vary from a locally recurrent tumour to a highly malignant type carrying a poor prognosis. The classification of adipocytic tumours has evolved in the last few decades due to advances in the understanding of pathogenetic basis that also provides a greater opportunity for the development of new treatment modalities. So, with the use of ancillary diagnostic tests, definite diagnostic criteria have been established, which have been described in WHO 2013 classification and recently a newer terminology have been described, which is Atypical Spindle Cell Lipomatous Tumour and it is considered as a separate entity based on its morphology and molecular studies. Lipomas are the most common benign adipocytic tumours and the less common tumours are lipoblastoma, angiolipoma, spindle cell/pleomorphic lipoma, myolipoma, chondroid lipoma, lipomatosis of nerve and hibernoma. Well differentiated liposarcomas/ atypical lipomatous tumours are locally aggressive adipocytic neoplasms that do not metastasize. More aggressive liposarcomas include myxoid liposarcoma, pleomorphic liposarcoma and dedifferentiated liposarcomas. Within a time period of 6 months we received 362 adipocytic tumours in the department of Pathology, of which 354 were benign and 8 cases were intermediate (locally aggressive) and malignant which includes atypical spindle cell lipomatous tumour, atypical lipomatous tumour/ well differentiated liposarcoma, myxoid liposarcoma and dedifferentiated liposarcoma. The cases of atypical lipomatous tumour and atypical spindle cell lipomatous tumour, its histological and molecular basis will be discussed along with this review of literature. Atypical spindle cell lipomatous tumours are seen usually in the subcutaneous location with a wide anatomic distribution and more commonly seen in the adults of 6th decade. Gross appearance ranges from yellowish to whitish depending on the amount of adipocytic and fibrous differentiation. Microscopic study shows a hypercellular to a hypocellular lesion with mild to moderately atypical spindle cells, adipocytes, lipoblast, and occasional bizarre, hyperchromatic cells. The stroma can be myxoid or collagenous. Mitotic figures are sparse without any tumour necrosis. Rare heterologous differentiation and different growth patterns have been describedincluding solitary fibrous tumour (SFT) like pattern, angiofibroma like pattern, myxoid liposarcoma like pattern and pericytic pattern. Immunohistochemistry studies show variable expression of CD34, S100 and desmin. Molecular studies identified a characteristic loss of nuclear RB gene expression with deletion of 13q14. The indolent behaviour of atypical spindle cell lipomatous tumour, with very low recurrence rate, no risk of dedifferentiation and no reported distant metastasis / death from disease, highlights the importance of distinguishing it from atypical lipomatous tumour/well differentiated liposarcoma, in order to avoid aggressive surgical resections. So atypical spindle cell lipomatous tumour is now considered as an independent entity rather than a morphological variant of atypical lipomatous tumour.