Low-grade myxofibrosarcoma versus low-grade fibromyxoid sarcoma. Distinct entities with similar names but different clinical course

Low-grade myxofibrosarcoma (MFS) and low-grade fibromyxoid sarcoma (FMS) are two distinct entities in the spectrum of myxoid mesenchymal sarcomas with fibroblastic differentiation. Low-grade MFS is seen often in the extremities of elderly patients, subcutaneously more frequently than in deep soft tissues, whereas the majority of cases of low-grade FMS occur in young to middle-aged adults, commonly in deep soft tissues of the shoulder region, the extremities and the trunk. Histologically, low-grade MFS shows a multinodular growth pattern and is composed of round or polygonal tumour cells mixed with elongated, curvilinear, thin-walled blood vessels in a myxoid matrix. The tumour cell nuclei in low-grade MFS display at least moderate nuclear pleomorphism and hyperchromasia. Low-grade FMS, however, is composed of bland fusiform tumour cells arranged in a whorled or swirling pattern, or occasionally more linear, and set characteristically in an alternating fibrous and myxoid stroma. Low-grade MFS recurs frequently but has a very low metastatic potential, whereas low-grade FMS is characterised by an indolent but ultimately malignant clinical course with metastases in more than half of the cases, which underlines the importance of distinguishing between the two entities.