Discussion

LGFMS is a rare tumor primarily found in younger and middle-aged adults.^{1, 2} About 20% of cases are in children less than 18 years old. They commonly occur in the proximal extremities or trunk. However, almost any region of the body can be affected.² LGFMS occurs slightly more commonly in males (M:F, 57:43).³ Although the tumor follows an indolent course, local recurrence is common and metastasis may occur, especially to the lungs.²

Children with LGFMS have a better prognosis than adults due to a decreased incidence of metastasis.² A study of 31 patients under age 21 years had a 5-year local-relapse-free survival of 76±17.6% (95% CI) and a 5-year overall survival of 100%, with one case of metastasis.² In comparison, a case series with 33 mostly adult patients had 64% local recurrence, 45% metastasis, and 58% overall survival.³ This variation may represent a difference in tumor biology.²

Radiographically, LGFMS appears as a noncalcified soft-tissue mass without adjacent bone erosion or destruction ( Figure 1 ).⁴ It is a solid hypoechoic mass on US with a more hypoechoic rim.⁴ It may have regions of nodularity and minimal internal blood flow on color Doppler ( Figure 2 ).⁴ The tumor is heterogeneous on CT, with heterogeneous enhancement and hypodense relative to muscle.^{4, 5} On MRI, the mass is hypo- to isointense compared with muscle on T1-weighted images and heterogeneous on T2-weighted images.⁵ It enhances heterogeneously after the administration of contrast media. Peritumoral edema is common.⁵ When an LGFMS is located within an intramuscular compartment, the “split fat” sign may be present.⁵ This finding is present when a rim of fat surrounds the tumor. The tumor may also have a gyriform pattern on T2-weighted or post-contrast sequences.⁴

As the name implies, LGFMS contains both myxoid and fibrous regions at histology.⁶ The tumor is comprised of fibroblastic spindle cells growing in a swirling pattern.⁶ The tumors have minimal cellularity and mitoses.⁶ Genetic analysis of LGFMS commonly reveals fusion of FUS-CREB3L2, making it a specific marker for the disease.⁷ Expression of the mucin four gene (MUC4) is also a highly sensitive and specific marker for LGFMS.^{8, 10}

Surgical resection is the treatment of choice for LGFMS.² The R classification system is used to define the absence or presence of tumor after treatment.⁹ R0 indicates complete remission, while R1 indicates microscopic residual tumor.⁹ The 5-year local recurrence-free survival is better for R0 disease compared with R1 disease (82.4% vs 53.6% in pediatrics).² Data on the usefulness of chemotherapy or radiation therapy in LGFMS are virtually nonexistent, although it is expected to be ineffective given the low rate of mitosis.²