Deeply located low-grade fibromyxoid sarcoma with FUS-CREB3L2 gene fusion in a 5-year-old boy with review of literature

Abstract

Background: Low-grade fibromyxoid sarcoma (LGFMS) is a rare soft tissue tumor typically affecting young to middle-aged adults. Despite its otherwise benign histologic appearance and indolent nature, it can have fully malignant behavior, and recurrence and metastasis may occur even decades later.

Case history: We report a case of LGFMS in the left lower leg of a 5-year-old Japanese boy. A magnetic resonance imaging (MRI) uncovered a well-demarcated intra-gastrocnemial tumor measuring 27 × 20 mm with a slightly high intensity on T1WI and heterogeneously high intensity on T2WI. Histologically, the tumor was composed of bland spindle-shaped cells with a whorled growth pattern. The tumor stroma was variably hyalinized and fibromyxoid with arcades of curvilinear capillaries and arterioles with associated perivascular fibrosis. Although LGFMS is known to affect children under 18 years of age, it is extremely rare in infants and children under 5 years of age. Despite the young age, this patient was accurately diagnosed by the typical histology and the detection of a FUS-CREB3L2 gene fusion.

Conclusion: Although LGFMS in children tends to be located superficially, this case presented with an intramuscular tumor in the region of the gastrocnemius. To the best of our knowledge, this is the first case of deep LGFMS arising in a child younger than 5 years of age. The patient is still alive with no evidence of the disease 4 months after diagnosis.

Virtual slides: The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/13000_2014_163.

Figure 1

MRI of lower left leg of the 5-year-old patient. An axial view MRI revealed a well-defined mass measuring 27 × 20 mm in the lateral aspect of the gastrocnemius. The mass had slightly high intensity compared to the skeletal muscle on T1WI (A) and a heterogeneously high intensity on T2WI (B).

Figure 2

Tumor Histology. An admixture of myxoid and fibrous areas was observed on H&E staining (A). Higher magnification views of myxoid (B) and fibrous (C) components of the tumor. A whorled growth pattern with bland spindle-shaped tumor cells was observed. Immunoreactivity for nuclear β-catenin was absent (D). Ki67 staining was present with a proliferative index of approximately 20% (E). Tumor cells showed diffuse and strong immunoreactivity for MUC4 (F). Tumor was negative for claudin-1 (G).

Figure 3

Detection of a FUS-CREB3L2 fusion in the LGFMS tumor. RT-PCR on FFPE-derived RNA was performed. DNA sequencing revealed a fusion between FUS exon 6 and CREB3L2 exon 5.

Figure 4

Gross morphology of LGFMS from the 5-year-old patient. The resected surface of intramuscular tumor is tan to gray colored and has a glistening appearance. The tumor is located in the subcutis and skeletal muscle throughout the fascia.