Case presentation

The patient was a 63-year-old woman with a known primary malignant SFT arising in the left anterior mediastinum, complicated by multiple pulmonary and muscular metastases. SFTs are rare mesenchymal neoplasms characterized by NAB2-STAT6 gene fusion, with a subset demonstrating aggressive metastatic behavior [6-8]. There was no history of atrial fibrillation, significant valvular heart disease, or known structural cardiac abnormalities.

Transthoracic echocardiography revealed a giant multilobulated mobile left atrial mass (>5 cm), floating freely and prolapsing through the mitral valve during diastole, with protrusion toward the left ventricular apex, followed by complete retraction into the atrium during systole (Figure 1). No mass was observed in the right-sided cardiac chambers, and the cardiac valves were structurally normal.

Figure 1: Transthoracic echocardiography (apical four-chamber view), demonstrating a giant multilobulated left atrial mass prolapsing into the left ventricle.

Transesophageal echocardiography was performed for further anatomical characterization and confirmed a highly mobile multilobulated left atrial mass, with dynamic transmitral prolapse, absence of significant valvular pathology, and absence of right-sided intracardiac involvement (Figure 2). No dedicated computed tomography or cardiac magnetic resonance imaging was obtained prior to surgery, given the urgency of the presentation; diagnostic assessment relied on transthoracic and transesophageal echocardiography alone, a limitation discussed further below.

Figure 2: Transesophageal echocardiography (mid-esophageal two-chamber view), demonstrating a giant multilobulated left atrial mass prolapsing into the left ventricle.

The extreme mobility of the lesion and its repetitive prolapse through the mitral valve throughout the cardiac cycle placed the patient at an exceptionally high risk for systemic embolization or acute mitral valve obstruction.

Therapeutic anticoagulation was immediately initiated, given the presumed thromboembolic risk pending definitive management. Although surgical intervention carries elevated risk in patients with metastatic malignancy, the imminent threat of catastrophic embolization or acute mitral valve obstruction necessitated urgent surgical management.

A multidisciplinary discussion involving cardiology, cardiac surgery, oncology, imaging specialists, and pathology confirmed that surgical resection was indicated despite the patient's advanced oncologic disease burden. Given the tumor's transvenous extension, resection required a combined surgical approach through the aortopulmonary window, together with left atriotomy. Surgical thrombectomy/resection was successfully performed (Figure 3).

Figure 3: Surgical specimen of the extracted intracardiac mass.

Surgical pathology comprised two specimens. The first, from the aortopulmonary window, consisted of a fibro-adipose fragment measuring 1 x 0.5 x 0.2 cm, submitted in toto; an associated lymph node was without particularity. The second specimen, representing invasion of the left atrium through the left superior pulmonary vein, was a fleshy fragment measuring 7.5 x 4.5 x 2 cm, sampled in serial sections.

Histopathological examination demonstrated intravascular localization of an undifferentiated malignant neoplasm composed of epithelioid cells with nonspecific morphology, severe nuclear atypia, and numerous mitotic figures; a formal quantitative mitotic count was not performed. Tumor vascularization was abundant and exhibited a characteristic hemangiopericytoma-like appearance (Figure 4).

Figure 4: Hematoxylin and eosin staining, demonstrating intravascular localization of an undifferentiated malignant neoplasm composed of epithelioid cells (A: x2.5; B: x10; C: x20).

Immunohistochemical analysis demonstrated diffuse strong nuclear STAT6 positivity in tumor cells (Figure 5). Additional immunohistochemical markers frequently reported in SFT (CD34, BCL-2, CD99) and the Ki-67 proliferation index were not assessed, and margin status was not formally reported; this represents a limitation of the pathological workup, discussed further below.

Figure 5: STAT6 immunohistochemistry, demonstrating diffuse nuclear positivity in tumor cells.

The pathological analysis found an undifferentiated malignant tumor with intravascular localization, extending from the left superior pulmonary vein into the left atrium, whose immunohistochemical profile (STAT6+) is consistent with the patient's known history of malignant SFT.

These findings established the diagnosis of intracardiac neoplastic involvement, via direct intravascular extension through the left superior pulmonary vein, by metastatic malignant SFT rather than bland thrombus formation.