Intramuscular myxoma of the buttock mimicking low-grade fibromyxoid sarcoma: diagnostic usefulness of MUC4 expression

Intramuscular myxoma of the left thigh: A case report

Introduction and clinical importance

Intramuscular myxoma (IM) is a rare benign soft tissue tumor that involves the musculoskeletal system with a reported incidence of 0.1–0.13 per 100,000. The mean age of diagnosis is 40–70 years of age, with female predilection. The most common site of involvement is the thigh, however, it may present in other areas such as the upper arm, calf, and buttock.

Case presentation

A case of a 45-year-old female without a significant past medical or surgical history who presented with 3-year history of a slow-growing, painless mass in her left upper thigh. MRI scan was performed which showed intramuscular soft tissue mass lesion 9 × 6 × 4.5 cm implicating the left distal vastus medialis muscle. A fine needle aspiration was inconclusive so a core needle biopsy was performed which was suggestive of intramuscular myxoma. A complete surgical excision of the mass was done and the postoperative period was uneventful and patient was discharged home. The final histopathological examination confirmed the diagnosis of intramuscular myxoma.

Discussion

Intramuscular myxoma is a rare benign soft tissue neoplasm. 50 % of cases commonly occur in the thigh. IM has an unknown etiology, however, the literature has showed common gene mutations such as the GNAS gene mutations (Guanine nucleotide binding protein, alpha stimulating). Imaging modalities such as ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) are useful in diagnosis of soft tissue mass but not specific to intramuscular myxoma. Histopathological examination is the gold standard for diagnosis. The treatment of choice is surgical excision with clear margins to prevent recurrence, which is extremely rare.

Conclusion

Intramuscular myxomas, although benign and rare, should be in the differential diagnosis of soft tissue lesions due to the similarity they share with malignant tumors such as sarcomas. Histopathological examination is the gold standard for diagnosing a soft tissue lesion and surgical excision is the treatment of choice.

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Soft tissue masses are common but intramuscular myxoma is a rare differential diagnosis

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Clinical examination and imaging modalities are inconclusive in formulating the diagnosis of intramuscular myxoma

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Histopathological examination is the gold standard for diagnosing cases of intramuscular myxoma

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Surgical excision with a gross negative margin is the treatment of choice

Radiomic features from MRI distinguish myxomas from myxofibrosarcomas

BACKGROUND

Myxoid tumors pose diagnostic challenges for radiologists and pathologists. All myxoid tumors can be differentiated from each other using fluorescent in-situ hybridization (FISH) or immunohistochemical markers, except for myxomas and myxofibrosarcomas. Myxomas and myxofibrosarcomas are rare tumors. Myxomas are benign and histologically bland, whereas myxofibrosarcomas are malignant and histologically heterogenous. Because of the histological heterogeneity, low grade myxofibrosarcomas may be mistaken for myxomas on core needle biopsies. We evaluated the performance of T1-weighted signal intensity (T1SI), tumor volume, and radiomic features extracted from magnetic resonance imaging (MRI) to differentiate myxomas from myxofibrosarcomas.

METHODS

The MRIs of 56 patients (29 with myxomas, 27 with myxofibrosarcomas) were analyzed. We extracted 89 radiomic features. Random forests based classifiers using the T1SI, volume features, and radiomic features were used to differentiate myxomas from myxofibrosarcomas. The classifiers were validated using a leave-one-out cross-validation. The performances of the classifiers were then compared.

RESULTS

Myxomas had lower normalized T1SI than myxofibrosaromas (p = 0.006) and the AUC using the T1SI was 0.713. However, the classification model using radiomic features had an AUC of 0.885 (accuracy = 0.839, sensitivity = 0.852, specificity = 0.828), and outperformed the classification models using T1SI (AUC = 0.713) and tumor volume (AUC = 0.838). The classification model using radiomic features was significantly better than the classifier using T1SI values (p = 0.039).

CONCLUSIONS

Myxofibrosarcomas are on average higher in T1-weighted signal intensity than myxomas. Myxofibrosarcomas are larger and have shape differences compared to myxomas. Radiomic features performed best for differentiating myxomas from myxofibrosarcomas compared to T1-weighted signal intensity and tumor volume features.

18F-FDG PET/CT and MRI features of myxoid liposarcomas and intramuscular myxomas

OBJECTIVE

To examine the imaging characteristics of intramuscular myxomas (IM) and myxoid liposarcomas (MLS) on ¹⁸F-FDG PET/CT and MRI.

MATERIALS AND METHODS

With IRB approval, our institutional imaging database was searched for pathologically proven IM and MLS evaluated by ¹⁸F-FDG PET/CT and MRI. PET/CT and MRI imaging characteristics were recorded and correlated with pathologic diagnosis.

RESULTS

We found eight patients (2 M, 6 F) with IM (mean age 65.6 ± 10.4 years) and 16 patients (7 F, 9 M) with MLS (mean age 42.8 ± 16.3 years). MRI was available in 7/8 IM and 15/16 MLS patients. There was no significant difference between the two groups in SUVmax (IM 2.7 ± 0.8, MLS 3.0 ± 1.0; p = 0.35), SUVmean (1.7 ± 0.4, 1.5 ± 0.5; p = 0.40), total lesion glycolysis (101.8 ± 127.3, 2420.2 ± 4003.3 cm³*g/ml; p = 0.12), metabolic tumor volume (62.3 ± 71.1, 1742.9 ± 3308.0 cm³; p = 0.17) or CT attenuation (p = 0.70). MLS occurred in younger patients (p = 0.0015), were larger (16.4 ± 8.2 vs. 5.6 ± 2.5 cm; p = 0.0015), more often T1 hyperintense (p = 0.03), with nodular enhancement (p = 0.006), and macroscopic fat on CT (p = 0.0013) and MRI (p = < 0.001) compared to myxomas.

CONCLUSIONS

IM and MLS most commonly demonstrate low-grade FDG activity and overlapping metabolic measures on PET/CT. MRI is useful in differentiation, but MLS can present without macroscopic fat on MRI, underscoring the importance of radiologic-pathologic correlation for accurate diagnosis.

A Low-Grade Fibromyxoid Sarcoma of the Internal Abdominal Oblique Muscle

A low-grade fibromyxoid sarcoma (LGFMS) is a rare tumor, with a benign histologic appearance but malignant behavior. This report describes a 74-year-old man with an internal abdominal oblique muscle mass. The tumor appeared as a well-defined ovoid mass on computed tomography, with mild uptake on fluorine-18-fluorodeoxyglucose positron-emission tomography images. Radical resection with wide safe margins was performed. Histologically, the tumor was composed of spindle-shaped cells in a whorled growth pattern, with alternating fibrous and myxoid stroma. MUC4 expression, a highly sensitive and specific immunohistochemical marker for LGFMS, was detected. Therefore, we diagnosed the tumor as LGFMS. At the 3-month follow-up, there was no sign of recurrence or metastasis. We report the first case of LGFMS arising from the internal abdominal oblique muscle.