Intramuscular Myxoma: Results from the Largest European Single-Center Study—Clinical and Pathological Findings and Syndromal Associations

<b>Objectives:</b> Intramuscular myxomas (IMMs) are rare benign soft tissue tumors arising within large skeletal muscles. Their etiology is incompletely understood, but they are frequently associated with mutations (e.g., GNAS) and may occur in syndromic conditions such as Mazabraud and McCune-Albright syndromes. This study retrospectively analyzed clinical, radiological, histopathological, and molecular features of IMMs, including syndromic associations. <b>Methods:</b> A retrospective analysis was performed on 41 patients diagnosed with IMM who underwent biopsy or surgical resection between September 2011 and September 2022. Clinical, imaging, histopathological, and molecular data were evaluated using descriptive statistics. <b>Results:</b> The cohort included 27 females and 14 males with a mean age of 52.8 years. The most common tumor location was the quadriceps femoris, followed by other thigh muscles. Most patients presented with mild symptoms due to slow tumor growth. MRI was performed in all but one case, with additional imaging in 12 patients. Radiological differential diagnoses commonly included soft tissue sarcoma and hematoma. Complete tumor resection was achieved in 90.2% of patients. Immunohistochemistry was performed in 78.0%, most frequently showing CD34 positivity. GNAS mutation analysis was conducted in 85.4% of cases and was positive in 57.1%. Complications occurred in 9.8%, and no recurrences were observed during follow-up. <b>Conclusion:</b> IMMs are rare benign tumors that can be reliably diagnosed using histology supported by immunohistochemistry and GNAS mutation analysis. Complete surgical resection provides excellent outcomes with a low risk of complications or recurrence. Mazabraud syndrome should be considered in patients with multiple or recurrent IMMs and GNAS mutations.