Cytomorphologic findings of low-grade fibromyxoid sarcoma

Low-grade fibromyxoid sarcoma (LGFMS) is a rare fibroblastic tumor characterized by a prolonged clinical course and malignant biological behavior. Given its deceptively bland cytomorphology, a diagnosis can be quite challenging notably on fine-needle aspiration (FNA). In an attempt to shed light on some of the distinctive cytomorphologic characteristics, this study was conducted to review all cases of LGFMS in our database, correlating available clinical data, immunohistochemical findings, and molecular analysis.

This series included 20 FNAs from 18 patients with a histologically confirmed LGFMS diagnosis from 3 large academic institutions. Detailed cytomorphologic analysis for each case was documented in conjunction with corresponding clinical characteristics and provided ancillary testing.

Out of 14 adequate FNA samples, 9 (64.2%) demonstrated a mixture of fibrous and myxoid pattern; the majority of cases were composed of deceptively bland tumor cells with rare nuclear pleomorphism and nuclear membrane irregularities. A MUC4 immunostain was performed on 5 specimens; all tested positive (100%). FUS rearrangement was detected in 4 out of 5 cases (80%). Follow-up information revealed 5-year recurrence in 1 case and metastatic disease in 2 cases, to the lung/pleura (8 years) and fourth rib (1 year), respectively.

The presence of bland spindle cells and associated with myxoid matrix material, in the appropriate clinical setting, can suggest LGFMS and direct additional confirmatory testing. A definitive diagnosis of LGFMS on FNA requires adequate sampling, familiarity with key cytomorphologic features, acquisition of diagnostic material for a cell block preparation and ancillary testing, and clinicoradiologic correlation.