Low grade fibromyxoid sarcoma: clinicopathological analysis of eleven new cases in support of a distinct entity.

Low grade fibromyxoid sarcoma is a recently recognized, uncommon soft tissue neoplasm with a tendency to develop in deep soft tissue of young adults. Diagnostic criteria have not been well defined and this tumour has not been widely accepted as a distinct entity. Eleven new cases are reported here for which reproducible histological features are described and in which the immunohistochemical profile of the tumour cells is documented for the first time. Ten of the eleven patients were male and the majority were young or middle-aged adults (median age 45 years). All except one of the tumours were situated in deep soft tissue. Lower limb (four cases) and chest wall (three cases) were the commonest primary sites; one case each arose in the groin, buttock, axilla and retroperitoneum. Follow-up (median duration 6 years) was available in nine patients. Six developed local recurrence and in five cases recurrences were multiple. Pulmonary metastasis occurred in one patient. All tumours were characterized by the presence of bland spindle cells, showing a mainly whorled or focally linear arrangement, set in alternating areas with a fibrous or myxoid stroma. Tumour cells were small, spindle to stellate, with poorly defined, palely eosinophilic cytoplasm and hyperchromatic ovoid nuclei. Most tumour cells showed strong staining with antibodies to vimentin, while occasional cells stained positively for actin, desmin and cytokeratin, in keeping with focal myofibroblastic differentiation. Ultrastructural examination in one case revealed features of fibroblasts. Careful consideration of the morphological and immunohistochemical features of these tumours permits a positive diagnosis of low grade fibromyxoid sarcoma and allows its distinction from a number of other benign and malignant soft tissue neoplasms.